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Textbook Notes

ASTR 101

Table of contents
  1. Chapter 1: Science and the Universe – A Brief Tour
    1. 1.1: The Nature of Astronomy
    2. 1.2: The Nature of Science
    3. 1.3: The Laws of Nature
    4. 1.4: Numbers in Astronomy
    5. 1.5: Consequences of Light Travel Time
    6. 1.6: A Tour of the Universe
    7. 1.7: The Universe on the Large Scale
    8. 1.8: The Universe of the Very Small
  2. Chapter 2: Observing the Sky – The Birth of Astronomy
    1. 2.1: The Sky Above
    2. 2.2: Ancient Astronomy
    3. 2.3: Astrology and Astronomy
    4. 2.4: The Birth of Modern Astronomy
  3. Chapter 3: Orbits and Gravity
    1. 3.1: The Laws of Planetary Motion
    2. 3.2: Newton’s Great Synthesis
    3. 3.3: Newton’s Universal Law of Gravitation
    4. 3.4: Orbits in the Solar System
    5. 3.5: Motions of Satellites and Spacecraft
    6. 3.6: Gravity with More Than Two Bodies
  4. Chapter 4: Earth, Moon, and Sky
    1. 4.1: Earth and Sky
    2. 4.2: The Seasons
    3. 4.3: Keeping Time
    4. 4.4: The Calendar
    5. 4.5: Phases and Motions of the Moon
    6. 4.6: Ocean Tides and the Moon
    7. 4.7: Eclipses of the Sun and Moon
  5. Chapter 5: Radiation and Spectra
    1. 5.1: The Behavior of Light
    2. 5.2: The Electromagnetic Spectrum
    3. 5.3: Spectroscopy in Astronomy
    4. 5.4: The Structure of an Atom
    5. 5.5: Formation of Spectral Lines
    6. 5.6: The Doppler Effect
  6. Chapter 6: Astronomical Instruments
    1. 6.1: Telescopes
    2. 6.2: Telescopes Today
  7. Chapter 7: Other Worlds: An Introduction to the Solar System
    1. 7.1: Overview of Our Planetary System
    2. 7.2: Composition and Structure of Planets
    3. 7.3: Dating Planetary Surfaces
    4. 7.4: Origin of the Solar System
  8. Chapter 8: Earth as a Planet
    1. 8.1: The Global Perspective
    2. 8.2: Earth’s Crust
    3. 8.3: Earth’s Atmosphere
  9. Chapter 9: Cratered Worlds
    1. 9.1: General Properties of the Moon
    2. 9.2: The Lunar Surface
    3. 9.3: Impact Craters
  10. Chapter 11: The Giant Planets
    1. 11.1: Exploring the Outer Planets
    2. 11.2: The Giant Planets
    3. 11.3: The Atmospheres of the Giant Planets
  11. Chapter 12: Rings, Moons, and Pluto
    1. 12.1: Ring and Moon Systems Introduced
    2. 12.2: The Galilean Moons of Jupiter
    3. 12.3: Titan and Triton
    4. 12.4: Pluto and Charon
    5. 12.5: Planetary Rings and Enceladus
  12. Chapter 13: Comets and Asteroids: Debris of the Solar System
    1. 13.1: Asteroids
    2. 13.2: Asteroids and Planetary Defense
    3. 13.3: The Long-Haired Comets
    4. 13.4: The Origin and Fate of Comets and Related Objects
  13. Chapter 14: Cosmic Samples and the Origin of the Solar System
    1. 14.1: Meteors
    2. 14.2: Meterorites – Stones from Heaven
    3. 14.3: Formation of the Solar System
    4. 14.4: Comparison with Other Planetary Systems
  14. Chapter 15: The Sun – A Garden-Variety Star
    1. 15.1: The Structure and Composition of the Sun
  15. Chapter 16: The Sun – A Nuclear Powerhouse
    1. 16.1: Sources of Sunshine – Thermal and Gravitational Energy
    2. 16.2: Mass, Energy, and the Theory of Relativity
    3. 16.3: The Solar Interior: Theory
  16. Chapter 17: Analyzing Starlight
    1. 17.1: The Brightness of Stars
    2. 17.2: Colors of Stars
    3. 17.3: The Spectra of Stars (and Brown Dwarfs)
    4. 17.4: Using Spectra to Measure Stellar Radius, Composition, and Motion
  17. Chapter 18: The Stars – A Celestial Census
    1. 18.1: A Stellar Census
    2. 18.2: Measuring Stellar Masses
    3. 18.3: Diameters of Stars
    4. 18.4: The H-R Diagram
  18. Chapter 19: Celestial Distances
    1. 19.2: Surveying the Stars
  19. Chapter 22: Stars from Adolescence to Old Age
    1. 22.1: Evolution from the Main Sequence to Red Giants
    2. 22.2: Star Clusters
    3. 22.3: Checking out the Theory
    4. 22.4: Further Evolution of Stars
    5. 22.5: The Evolution of More Massive Stars
  20. Chapter 23: The Death of Stars
    1. 23.1: The Death of Low-Mass Stars
    2. 23.2: Evolution of Massive Stars – An Explosive Finish
    3. 23.3: Supernova Observations
    4. 23.4: Pulsars and the Discovery of Neutron Stars
    5. 23.5: The Evolution of Binary Star Systems
    6. 23.6: The Mystery of Gamma-Ray Bursts
  21. Chapter 24: Black Holes and Curved Spacetime
    1. 24.1: Introducing General Relativity
    2. 24.2: Spacetime and Gravity
    3. 24.3: Tests of General Relativity
    4. 24.4: Time in General Relativity
    5. 24.5: Black Holes
    6. 24.6: Evidence for Black Holes
    7. 24.7: Gravitational Wave Astronomy
  22. Chapter 25: The Milky Way Galaxy
    1. 25.1: The Architecture of the Galaxy
    2. 25.2: Spiral Structure
    3. 25.3: The Mass of the Galaxy
    4. 25.4: The Center of the Galaxy
    5. 25.5: Stellar Populations in the Galaxy
    6. 25.6: The Formation of the Galaxy
  23. Chapter 26: Galaxies
    1. 26.1: The Discovery of Galaxies
    2. 26.2: Types of Galaxies
    3. 26.3: Properties of Galaxies
    4. 26.4: The Extragalatic Distance Scale
  24. Chapter 27: Active Galaxies, Quasars, and Supermassive Black Holes
    1. 27.1: Quasars
    2. 27.2: Supermassive Black Holes – What Quasars Really Are
    3. 27.3: Quasars as probes of Evolution in the Universe
  25. Chapter 28: The Evolution and Distribution of Galaxies
    1. 28.1: Observations of Distant Galaxies
    2. 28.2: Galaxy Mergers and Active Galactic Nuclei
    3. 28.3: The Distribution of Galaxies Through Space
    4. 28.4: The Challenge of Dark Matter
    5. 28.5: The Formation and Evolution of Galaxies and Structure in the Universe
  26. Chapter 30: Life in the Universe
    1. 30.1: The Cosmic Context for Life
    2. 30.2: Astrobiology
    3. 30.3: Searching for Life Beyond Earth
    4. 30.4: The Search for Extraterrestrial Intelligence
  27. Philosophical Relevance

Chapter 1: Science and the Universe – A Brief Tour

1.1: The Nature of Astronomy

  • Astronomy – obejcts which lie beyond Earth
  • Humanity’s attempt to formulate a history of the universe

1.2: The Nature of Science

  • Science is a method through which we acquire knowledge
  • Models as ideal approximations of nature
  • Note – astronomy has an intimate relationship with models
  • Hypothesis must be testable with experimentation
  • Astronomy as an observational and a historical science. This is interesting epistemically.
  • Science is self-correcting. (is it?)

1.3: The Laws of Nature

  • Rules by which nature plays
  • Universal laws held by induction
  • Imposition of laws across neighborhoods for consistency.
  • General relativity and black holes
  • The limitations of language necessitates mathematics

1.4: Numbers in Astronomy

  • Scientific notation
  • Light year – the distance light travels in one year. Fastest speed in the universe.

1.5: Consequences of Light Travel Time

  • Information in the universe almost exclusively comes to us in forms of light.
  • The speed of light is the limit on how quickly we can learn about the universe.
  • Now is when light reaches us on earth.
  • The old age of cosmic light allows us to better trace a cosmic history.

1.6: A Tour of the Universe

  • Moon – Earth’s satellite.
  • Light takes 1.3 seconds to travel from the moon to the earth.
  • Distance between the earth and the sun is an astronomical unit (AU).
  • Earth is one of eight planets which go around the sun.
  • The Sun is a local star
  • Galaxy – Milky Way Galaxy – a giant disk with a small ball
  • The space between stars is not completely empty, it has some gas and some small particles
  • Interstellar material is very sparse
  • Dust in space can block up light of distant stars
  • Dark matter – its gravity is exerted on stars, but we do not know what it is made of.
  • Some stars are involved in double or triple systems
  • No star lasts forever; eventually all stars run out of fuel.

1.7: The Universe on the Large Scale

  • The universe is made up of a large number of galaxies.
  • Universe: everything accessible to our observations
  • Galaxies – island universes, groups of stars in intergalactic space
  • Local group – a cluster of about 50 galaxies
  • Galaxies occur mostly in clusters, large and small
  • Superclusters – Virgo Supercluster, local group is part of this
  • Quasars – centers of galaxies. These are the brightest things we can see, including close to the big bang explosion which begins time.

1.8: The Universe of the Very Small

  • The Galaxy is mostly empty space
  • One atom per cubic centimeter in interstellar Galaxy gas
  • Dense places like the human body are very rare in the universe
  • Cosmically abundant elements: hydrogen, carbon, nitrogen, oxygen

Chapter 2: Observing the Sky – The Birth of Astronomy

2.1: The Sky Above

  • Geocentric view: humans as the central focus of the cosmos.
  • One of the great monuments of our intellectual history is to overthrow geocentrism.
  • Zenith – point directly above you; horizon – dome meets surface of Earth
  • Celestial sphere, Greeks
  • North and south celestial pole
  • The sun changes position
  • The Sun takes a ecliptic path
  • Fixed and wandering stars
  • Planets – wandering stars; planet means wanderer in ancient Greek
  • Constellations: geometric patterns formed by groupings of stars
  • Telescopes reveal millions of stars which are too small for the eye to see
  • Constellation refers to 88 sectors of the sky

2.2: Ancient Astronomy

  • Ancient astronomers were able to grasp the length of a year
  • Cosmology: our concept of the basics tructure and origin of the cosmos
  • Eastern Mediterraneans knew Earth was round; Pythagoras may have pioneered round earthers
  • Parallax: the apparent shift in the direction of an object as a result of the motion of the observer
  • Stellar parallax: shift in the apparent direction of a star due to Earth’s orbital motion. Stars however did not seem to shift in spring vs in fall, when Earth is on the opposite side of the sun.

This meant either that Earth was not moving or that the stars had to be so tremendously far away that the parallax shift was immeasurably small. A cosmos of such enormous extent required a leap of imagination that most ancient philosophers were not prepared to make, so they retreated to the safety of the Earth-centered view, which would dominate Western thinking for nearly two millennia.

  • Eratosthenes: measuring a single angle in one region allowed for determining the size of th Earth.
  • Hipparchus: measured positions of objects in the sky and provided the coordinates for understanding this world. The position in the sky over the north pole changed over the previous 1.5 century. Earth must be doing the wobbling. Precession: the direction in which Earth’s axis points
  • Ptolemy: geometric representation of the solar system which predicted the position oft he planets.
  • Epicycle: each planet revolves in a local circle as well as in a broader circle. However it was in fact that the planets travelled in ellipses; circles do not work.
  • Will the revolution be Copernican or a Ptolemization? – a Ptolemization requires increasing complex analyse sand mechanisms to make it work.

2.3: Astrology and Astronomy

  • Planets and stars as symbols of gods and the supernatural, the metaphysical (technology is the making of the metaphysical into the physical)
  • Astrology begins in Babylon
  • Natal astrology – Ptolemy, the Tetrabiblos. An ancient religion of shorts
  • Horoscopes – mappings of birth to the position of stars in the sky
  • Astrologists posit unknown forces exerted by planets – epistemic inaccess
  • Why do we identify with horoscopes? Maybe they are true in the more philosophical sense, that we feel the need to identify a particularity of ourselves, and the truth of the horoscope – and its sadness – is that this particularity is totally arbitrary. So maybe we should think about how even if we turn away from astrological horoscopes, we turn towards other horoscopes – my college major, my interests, my quirkiness, etc.

2.4: The Birth of Modern Astronomy

  • Many names for stars come from Arabic
  • Copernicus – new Sun-centered, heliocentric model of the solar system
  • If Earthw were moving, we would feel this motion – objection to Copernian revolution.
  • Copernicus could not prove the Earth revolves around the Sun, but appealed to the lack of beauty in Ptolemaic cosmology.
  • Galileo: pioneered many modern principles of science
    • Greatest contributions were to mechanics
    • Acceleration
    • Pioneered telescope
    • Interesting question – what is the believability of microscopes? What does it mean to see something as if you had a ‘small eye’?
  • Galileo confirmed the Copernican hypothesis
  • The Roman Catholic Church condmened Galileo.
  • Modernity: nature is rational and ultimately knowable through experimetns and observations

Chapter 3: Orbits and Gravity

3.1: The Laws of Planetary Motion

  • Tycho Brahe, Johannes Kepler
  • Brahe: recorded the loction of the sun, moon, and the planets
  • Kepler helped understand the planetary data; developed Kepler’s three laws
  • Orbit: path of an object through space. Orbits are ellipses
  • Major axis (widest) and semimajor axis
  • Eccentricity: ratio of distance between foci to the length of the major axis
  • First law: orbits of all planets are ellipses – no need to move things in circles.
  • Second law: planets speed up when they come closer to the sun and slow down when they get away. The area swept out in space over time along the orbit relative to the sun is always equal in equal intervals of time.
  • Third law: a planet’s orbital period squared is proportional to the semimajor axis of its orbit cubed
    • A planet’s semimajor axis is equal to its average distance from the sun
    • Orbital period: time it takes for a planet to travel once around the sun
    • When the orbital period is measured in years and the semimajor axis in AUs, \(P^2 = a^3\)

3.2: Newton’s Great Synthesis

  • Science first called natural philosophy
  • Newton’s first law: an object will continue to be in a state of rest or move at a constant speed in a straight line without external influence
    • Conservation of momentum
    • Momentum: mass times velocity
  • Newton’s second law: the change of motion of a body is proportional to and in the direction of the force acting on it
    • A force is required to change speed and/or direction
  • Newton’s third law: for every action there is an equal and opposite reaction
    • Acceleration of a body is proportional to the force being applied to it.
    • Generalization of the first law
    • Lets us define mass
    • In a system isolated from outside influences, total momentum of objects should remain constant (law 1)
    • Change in momentum has to be balanced by another change which is equal and opposite
    • There is always a pair of forces; if a force is exerted on an object, the object exerts another equal and opposite force.
    • What really is force as formalized?
  • Mass – measure of material in an object
  • Volume – amount of physical space occupied
  • Density – mass per volume
  • Angular momentum: measure of rotation of a body revolving around a fixed point
  • Kepler’s second law: conservation of anglar momentum

3.3: Newton’s Universal Law of Gravitation

  • Newton: gravity causes planets to move in ellipses, rather than straight lines (as per the first law)
  • Pre-Newton: gravity applies only to Earth.
  • Earth’s gravity can extend as far as the moon and curve the path to a straight line.
  • Universal attraction among all bodies everywhere in space
  • Newton invented calculus to deal with problems in universal attraction
  • Magnitude of the force of gravity must decrease with increasing distance between the sun and the planet in proportion to the inverse square of their separation
  • Gravity gives us our sense of weight
  • Why does the moon not fall in? Why in general would bodies orbit and not spiral out or in? It seems like equilibrium is fragile
  • Gravity is built into the concept of mass: wherever there is mass, there is attraction.
  • What constitutes a ‘body’? Because if you break it down all it really is is atoms together, right? Is it density? How are we defining an object? Isn’t that kind of a subjective imposition?
  • Gravity can never become zero, even though it can become infinitely weaker
  • Astronauts feel weightless because they are falling: you accelerate at the same rate as everything around you, and so you experience no additional forces. So you continue to fall through, but not towards earth but around it.
  • Orbital motion and mass: factoring in masses shows us that the orbital period squared times the sum of the two bodies’ masses equals the semimajor axis cubed.

3.4: Orbits in the Solar System

  • Place where planet is closest to Sun: perihelion
    • Farthest away: aphelion
    • Orbiting Earth: perigee, apogee
  • All planets have low-eccentric orbits except for Mercury (0.21)
  • Comets have larger orbit sizes and eccentiricites than asteroids

3.5: Motions of Satellites and Spacecraft

  • The behavior of an artificial satellite is the same as that of natural satellites
  • Circular satellite velocity – speed needed to fall around Earth
  • Escape speed: speed needed to move away from Earth forever
  • Spacecraft sometimes use gravity to direct spacecraft to another target, like ‘swingbys’.

3.6: Gravity with More Than Two Bodies

  • All planets exert gravity on themselves as well.
  • Discovery of Neptune
  • Uranus – originally recorded as a star. Herschel discovers that some planets are too dim to be unaided but are visible with a telescope.
  • Uranus does not move in an orbit predicted by Newtonian theory: another planet is influencing its trajectory, Neptune.
  • Proved the generality of Newton’s laws.

Chapter 4: Earth, Moon, and Sky

4.1: Earth and Sky

  • Great circle – any circle whose center is at the center of the sphere
  • Meidian
  • Longitude of the prime meridian is 0 degrees; it passes through Greenwich, England.
  • Declination and right ascension: make use of a fictitious celestial sphere.
  • Why does the Earth turn? Jean Foucault and the pendulum

4.2: The Seasons

  • There are significant variations in heat received from the Sun throughout the year
  • Seasons: different amount of sunlight.
  • Seasons are not the result of changing distance between the earth and the sun. Seasons are actually caused by the 23.5 degree tilt of the Earth’s axis.
  • Arctic and Antarctic circles – the place where sun can hit all day long
  • Hottest months are July and August because the Sun progressively heats it up even after the summer solstice

4.3: Keeping Time

  • Solar day: rotation period of Earth w.r.t. the Sun
  • Sidereal day: defined in terms of rotation period of Earth w.r.t. stars
  • **Why can’t we just measure how much Earth rotates ‘on its own’? **
  • Apparent solar time: actual position of the Sun in the sky, given by sundials.
  • Mean solar time: average value of the solar day over the course of the year. 24 hours
  • Standardization of time zones: noon is different for everyone
  • Travelers reset watches only when time change accumulates to a full hour.
  • New time zone ever 15 degrees longitude.
  • International Date Line: set at 180 degree meridian. By convention the date of the calendar is changed.

To maintain our planet on a rational system of timekeeping, we simply must accept that the date will differ in different cities at the same time.

  • What is time, really? Because I can’t observe something happening in Japan at the same time. What is the rigorous way to understand ‘at the same time’?

4.4: The Calendar

  • Day (Earth), month (based on moon cycles), and year (Sun)
  • Chinese: 12-year cycle of Jupyter, retained in the Zodiac
day, month, year, zodiac cycle
earth, moon, sun, jupiter
  • Julian calendar: year as 365.25 days
  • Julian calendar differs from the true year by about 11 minutes. Reformation by Pope Gregory XIII.
  • Gregorian calendar: only century years divisible by 400 are leap years; also move the calendar forward 10 days. The average length of the Gregorian calendar is correct to 1 day in 3300 years
  • Gregorian calendar was adopted immediately by catholic countries and later by other countries
  • Russia did not abandon the Julian calendar until the Bolshevik revolution; Russians omitted 13 days to sync with the rest of the world.

4.5: Phases and Motions of the Moon

  • The Moon glows from reflected sunlight
  • The Moon follows phases
  • The Sun moves 1/12 of its path around the sky every month
  • How much of the face we see illuminated by sunlight depends on the angle the Sun makes with the Moon
  • Moon – new when it is between the sun and the Earth – the moon is invisibile to us
  • It takes 30 days for the moon to orbit the Earth
  • lunacy – lunar
  • Sidereal month: 27.3217 days – period of revolution about Earth measured wrt the stars.
  • Moon rotates on its axis in the same time it takes to revolve around the earth; and so it always keeps the same face towards Earth. Synchronous rotation.
    • Q: This seems like too good of a coincidence to be true. Why?

4.6: Ocean Tides and the Moon

  • The Earth is not perfectly rigid
  • The moon exerts gravitational forces on the Earth
  • Earth is an oblate spheroid
  • Tide forces produce tidal bulges in the oceans; water gathers towards the moon.
  • Neap tides: when the moon is in the 1st or last quarter, tides produced byt he sun partially cancel tides of the moon
  • However, land masses stop the flow of water, the ocean has variable depth, etc.; so tides change a lot.

4.7: Eclipses of the Sun and Moon

  • An object in the solar system casts a shadow when it blocks the light of the Sun
  • Eclipse: any part of the earth or moon enters the shadow of the other
  • Moon’s shadow strikes earth: people in the shadow see the sun partially covered by the moon (solar eclipse)
  • Earth passes into the shadow of earth: people in the night side of Earth see the moon darken (lunar eclipse)
  • Umbra: cone where the shadow is darkest; penumbra: where the shadow is more diffuse
  • A lunar eclipse is visible to everyone who can see the moon
  • Even when eclipsed, the Moon appears red (why?) Q
  • Corona: the thn outer atmosphere of the Sun visible during a total solar eclipse.

Chapter 5: Radiation and Spectra

5.1: The Behavior of Light

  • A lot of information can be found in light from objects in the universe and how they work. They are messages ‘to be deciphered’!
  • “Light and other radiation” – light is just one type of radiation
  • Radiation in astronomy – general term for waves which radiate outwards from a source
  • Maxwell’s theory of electromagnetism
    • A typical atom has protons, electrons
    • Opposites attract
    • If charges are in motion, we experience magnetism – moving charged particles, alignment of motion
    • Filed: action of forces one object exerts over another
    • Stationary electric charges produce electric fields
    • Moving electric charges produce magnetic fields
    • Changing magnetic fields can produce electric currents
    • Electric and magnetic field changes trigger each other
    • Waves result from a pattern of electric and magnetic fields spreading through space – oscillaiton of electric charges
    • Waves generated by charged particles do not need water or air (sound needs air; sound is pressure disturbances). Electromagnetic waves: fields genrate each other and move through a voccum
    • Aether: an invented substance so light could trvel through it
    • All electromagnetic waves move at the same speed; this is the fastest possible speed in the universe.
    • Wave length, frequency
    • Speed is frequency times wavelength
  • Light as a photon
    • Photon – a packet of electromagnetic energy
    • Light behaves sometimes more like a particle
    • Electromagnetic radiation behaves sometimes like a wave and sometimes like a particle.
    • Wave-particle duality of light
    • Inver square law of light propagation

5.2: The Electromagnetic Spectrum

  • Gamma, X-ray, UV, Visible, Infrared, Microwave, Radio
  • Types
    • Gamma rays – no longer than 0.01 nanometer
    • Gamma rays ahve a lot of energy
    • X-rays – 0.01 to 20 nm – can penetrate soft tissues but not bones, so we can make images of bones inside us
    • Ultraviolet – ‘black light’ – our eyes cannot see it.
    • Visible light – 400 to 700 nm
    • Infrared: heat radiation
    • Heat lamps mainly emit infrared radiation, or also the microwave
    • Radio waves
  • What determines the type of electromagnetic radiation? Often the answer is temperature. The hotter the mateiral, the more particles vibrate, the more energetic, the higher frequency
  • Radiation laws
    • Blackbody: relationship between temperature and electromagnetic radiation, absorbs all electromagnetic radiation
    • Begins to radiate electromagnetic waves until absorption and radiation are balanced
    • Object at higher temperature emits more power at all wavelengths than a cooler one; hotter ones also tend to give off more energy
    • Color as a rough theromemter
    • Hot stars shine a fuck ton of energy

5.3: Spectroscopy in Astronomy

  • Properties of light
    • Light can be reflected from a surface
    • light is refracted when it passes through different kinds of transparent material
    • Sunlight is actually a mixture of various parts of the visible light spectrum
    • Spectrometer – an instrument which disperses light and forms a spectrum
  • The value of stellar spectra
    • Colors are not just spread out uniformly – some ranges of color re missing
    • By passing light through different substances (thin gasses), the gases are not transparent in all colors – the gas absorbs a few colors of light and not others
    • Spectral signatures – the color of gasses when heated are the same colors they absorb
    • Unique pattern of colors helps us identify the composition of the gas
  • Types of spectra
    • Continuous spectrum – all wavelengths of visible light
    • Absorption spectrum – the serious of dark lines on the continuous spectrum
    • Emission spectrum – light only where discrete wavelengths are present

5.4: The Structure of an Atom

  • Bohr: only orbits of certain sizes are possible for the electron
  • If the electron switches from orbits, it radiates energy
  • Each orbit is an energy level – requires a change in energy levels to change

5.5: Formation of Spectral Lines

  • Each atom has its unique pattern of electron orbits
  • Lowest possible energy state – ground state
  • Movement to higher energy level – excitation
  • Energy levels of an ionized atom are totally different. Ionized hydrogen produces no absorption lines because it has no electron.

5.6: The Doppler Effect

  • Motion affects waves
  • Radial velocity – movement towards or away from an observer
  • Sideways motion does not produce an effect
  • Redshift and blueshift

Chapter 6: Astronomical Instruments

6.1: Telescopes

  • Telescope is a bucket for collecting visible light
  • Telescopes sort incoming radiation by wavelength
  • Detector – senses radiation and permanently records observations
  • The same part of the sky look sdifferent depending on different sensitivity to the spectrum
  • Celestial objects send a lot of light that we can’t catch
  • Telescopes collect faint light and also focus all the light to a point of image
  • Aperture – light-gathering power by diameter of opening through which light travels
  • Lens – bends light passing through it. Lenses focus light together to form an image. Refraction towards convergence. At the focus, an image of the light source appears
  • Chromatic aberration

6.2: Telescopes Today

  • The telescope must have a motorized system to move from east to west as Earth rotates west to east
  • Limitations to telescope sites
    • Weather conditions
    • Sky above telescope needs to be dark
    • Dry sites with less water vapor
    • Steady air
  • Resolution – precision of detail in an image
  • Our planet is turbulent; we get blurred images. “Twinkling of stars”. Resolution of image measured in angle of the sky
  • Adaptive optics

Chapter 7: Other Worlds: An Introduction to the Solar System

7.1: Overview of Our Planetary System

  • Sun and smaller objects – planets, moons, rings, debris
  • Objects formed with the Sun about 4.5b years ago
  • The sun is brighter than 80% of stars in the galaxy
  • The sun is the largest member of the solar system, accounting for 99.80% of total mass
  • Most material is concentrated in JUpiter
  • Most planets follow orbits in a concentric plane
  • Four planets closest to the sun – terrestrial planets
    • Sometimes the moon is included here
  • Jupiter through Neptune – the jovial or giant planets
  • Only Mercury and Venus move through space without a moon
  • Giant planets have rings made up of many small bodies in orbit around the equator of the planet.
  • Asteroids – rocky bodies which orbit the sun like minitiar planets
  • Comets: small bodies composed mostly of ice, frozen gases, carbon dioxide, carbon monoxide
  • When grains of brkoen rock (cosmic dust) enter the atmosphere, they burn up and produce meteors; when they hit the ground, they are meteriorites

7.2: Composition and Structure of Planets

  • There are two distinct kinds of planets, so it is likely that they formed under different conditions.
  • The giant planets – the two largest planets (Jupiter and Saturn) have the same chemical makeup as the Sun – hydrogen and helium, 75% mass hydrogen and 25% helium.
  • Jupiter and Saturn are so large that their gas is compressed until hyodrgne becomes a liquid. They really should be called liquid planets.
  • Jupyiter and Saturn have heavy rock, metal, and ice regions
  • Giant planets have hydrogen and many other compounds.
  • Hydrogen-dominated composition: reduced
  • Terrestial planets: the most abundant rocks (silicates) are made of silicon and oxygen; most common metal is iron. Mercury has the greatest proportion of metals
  • Densest metals are in the center; lighter silicates are near the surface.
  • Differentiation: gravity helps separate a planet’s interior into different layers.
  • Earth’s moon is like terrestrial planets
  • Most moons have a similar composition tot he bodies they orbit
  • THe farther a body from the Sun, the cooler the surface.
    • Temperature decreases in proportion with the square root of distance from the sun
  • Earth is the only planet where surface temperatures lie between freezing and boiling points of water; the only planet to support life.
  • Geological activity – crusts of all terrestrial planets have been modified over time by internal and external forces.
  • All planets are subject to impacts – geological activity is the result of a hot interior
  • The Moon is geologically dead – Mercury ceased most volcanic activity about the same time as the Moon.
  • Primordial heat ‘powered’ bodies – the larger the body, the better it retains its internal heat
    • Is there any regeneration of heat in these bodies? Where does the heat from dead planets go to?

7.3: Dating Planetary Surfaces

  • How do we know how old planets and moons are?
  • Counting the craters – count ght enumber of impact craters; we can assume a certain uniform rate of craters
  • You can also track radioactive rocks
  • Natural radioactivity: some atomic nuclei are not stable but can spontaneously decay into smaller nuclei, which produces gamma rays
  • Decay is random in nature but radioactive substances generally have a half-half

7.4: Origin of the Solar System

  • Much of astronomy is motivated by a desire to understand origins
  • Superearths – planets in between terrestrial and giant planet sizes
  • Some exoplanet systems have giant planets close to a star.
  • We look for patterns among our planets. ALl planets lie on the same plan and spin around its own access. The Sun and planets formed together in a solar nebula.
  • The sun has the same hydrogen composition as Jupiter and Saturn. Processes which led to planet formation in the inner solar system likewise excluded lighter materials
  • Many stars in space are younger than the sun; solar nebulas which can resemble our own solar system.
  • Theoretical calculations can help us see how solid bodies might form from gas and dust
  • Precursors of planets: planetesimals.
  • Violentness of planetesimals
  • Solar nebula model can explain many regularities in the solar system

Chapter 8: Earth as a Planet

8.1: The Global Perspective

  • Earth’s orbit is almost circular
  • Earth is warm enough to support liquid water
  • In many ways we know less about our own planet 5 km below us than the surfcaes of Venus and Mars: we have to determine composition indirectly.
  • Seismic studies measure difference in refraction of waves traveling through different compositions
  • Crust (6 km thick, basalt); mantle (2900 km, ~solid, possibly molten); core (7000 diameter; probably solid – high density iron, nickel, sulfur)
  • Differentiation: sorting of components by density: at one time it was warm enough for the interior to melt
  • Earth behaves like a magnet aligned with north and south generated by moving liquid metal
  • Magnetosphere: Earth’s magnetic field dominates interplanetary magnet field.
  • Solar wind – provides particles for Earth’s magnetic sphere to trap.

8.2: Earth’s Crust

  • Oceanic basalt and continental granite: both igenous rock, or rock which cools from a molten state
  • Sedimentary rocks – fragments of igneous rock deposited by wind or water without melting
  • Metamorphic rocks – produced when high temperature alters igenous/sedimentary rock chemically
  • Primitive rock: escapes chemical modificiation by heating; is made of the original material of Earth. We can’t find this on Earth though
  • Plate tectonics – slow convection motions in the mantle move large segments of crust, resulting in gradual drifting.
  • Rift zones – upswelling currents in the mantle; molten rock rises to fill space in between receding plates
  • Ocean basins are one of the yougnest features of the planet
  • Subduction zone – where one place is forced below another one. Thin oceanic planets can be thrust into the upper mantle. This forms an ocean trench. The plate eventually melts. Amount of crust destroyed at subduction zones \(\approx\) amount of crust created at rift zones.
  • Faults – plate boundaries where crustal plates slide against each other.
  • Faults may form mountains – plates rubbing against each other
  • Planets without moving ice or water like the moon or mercury have smooth mountains
  • Volcanoes: lava rises tot he surface

8.3: Earth’s Atmosphere

  • We live at the bottom of an ‘ocean of air’
  • 1 bar: pressure of the atmosphere at sea level.
  • Humans have evolved to live at a bar of pressure.
  • Most of the atmosphere is concentrated near the surface of the earth (troposphere)
  • Temperature decreases rapidly with increasing elevation
  • Stratosphere: cold, no clouds
  • On top of the stratosphere is ozone; it protects the surface from Sun’s radiation.
  • The atmosphere is very thin above 100km; satellites can pass with little fraction
  • Continuous leaking of the atmosphere of lightweight atoms
  • The atmosphere is mostly nitrogen and oxygen, with some argon.
  • Weather – circulation of the atmosphere; energy from sunlight.
  • Climate: effects of the atmosphere through decades and centuries; changes in climate can accumulate
  • “Climate is what you expect, and weather is what you get”

Chapter 9: Cratered Worlds

9.1: General Properties of the Moon

  • The moon’s mass and gravity is too low to retain an atmosphere
  • Lunar geology
  • Lunar exploration cut off due to political and economic pressures
  • The composition of the Moon – mainly silicate rock; delted in iron and other metals.
  • Water ice detected in permanently shadowed craters near the lunar poles.
  • Polar water presumably carried to moon by comets and asteroids
  • Possible human habitation near the lunar poles?

9.2: The Lunar Surface

  • ‘the man in the moon’ – large areas of dark lava flows
  • Dark ‘seas’
  • Rocks on the moon are substantially older than rocks on Earth
  • Lunar highlands – crust with silicate rocks
  • Highlands do not have sharp folds, but rather low, rounded profiles
  • On the lunar surface
  • The surface of the moon is buried under a fine-grained soil of tiny, shattered rock fragments; upper layers are porous
  • Lunar surfaces have more extreme temperatures than the erth due to not having air.

9.3: Impact Craters

  • Moon is an important becnhmark for understanding the history of the planetary system – solid bodies leave behind the traces of impacts
  • Cratering process: impact, vaproziation, ejection of material, most ejected material falls back in the crater but some fall outside and make an ejecta blanket.
  • Crater – Greek, ‘bowl’
  • By estimating crater creation rates, we can roughly date the moon

Chapter 11: The Giant Planets

11.1: Exploring the Outer Planets

  • Giant planets hold most of the mass in the planetary system
  • Jupiter exceeds the mass of all other planets combined
  • Gases, ices, rocks
  • Challenges in exploring the giant planets - flight times measured in years to decades
  • Messages take hours to pass between Earth and the spacecraft even at the speed of light
  • Spacecraft sent to the outer solar system must be highly reliable and autonomy; also must carry their own energy sources
  • Voyagers 1 and 2 – farthest spacecraft
  • Every 175 years, planets are in a position so that a single sapcecraft can visit them all with gravity-assisted flybys (‘the grand tour’)
  • Galileo, Cassini, Juno orbiters

11.2: The Giant Planets

  • Uranus and Neptune have a mass about 15 times as much as the Earth
  • Clouds of Jupiter and Saturn are made of ammonia crystals
  • Neptune – clouds of methane
  • Uranus – no visible cloud layer
  • Jupiter has the shortest day of a planet – rotation period of almost 10 hours
  • Earth and Mars have seasons because of tilted spin axes. Jupiter does not have much of a tilt, but Saturn and Neptune both do. Uranus orbits essentially on its side.
  • Uranus has dramatic seasons
  • Jupiter and Saturn – mostly hydrogen and helium interior – only possible materials
  • Planets are so big; gasses are extraordinarily compressed
  • Hydrogen turns from gaseous to liquid state in JUpiter and Saturn; deeper, it can act like a metal
  • Jupiter is the hottest; largest internal energy source. Cross between a star (internal energy source) and a normal atmosphere (energy from Sun)
  • Saturn is still differentiating
  • Uranus and Neptune – no internal energy and some, respectively
  • Magnetospheres – regionsa round a planet where the planet’s magnetic field dominates the general one.
  • Jupiter was a source of radio waaves that get more intense at longer rather than shorter wavelengths – synchotron radiation (Jupiter has a strong magneitc field)

11.3: The Atmospheres of the Giant Planets

  • Atmospheric gases leave fingerprints in the spectrum of light
  • Different gasses freeze at different temperatures
  • Clouds we see around planets are frozen ammonia crystals.
  • Saturn: hexgonal wave pattern in North Pole
  • Uranus is entirely featureless – methane rather than ammonia clouds.
  • Neptune has convection currents in its atmosphere, which cause cloud movement
  • Many regions of high pressure
  • Heat from the insdie contributes as much energy to the atmosphere as unlight to hte outside except Uranus.
  • Oval shaped highh-pressure regions on Jupiter; Great Red Spot, a giant storm in the atmosphere
  • Jupiter has no solid surface to slow down atmosphered isturbances
  • Photochemistry: chemical changes caused by electromagnetic radiation

Chapter 12: Rings, Moons, and Pluto

12.1: Ring and Moon Systems Introduced

  • Rings and moons in the outer solar system have different composition than those in the inner solar system; this is given by different cooling procedures during formation
  • 1/3 of moons in the outer solar sytem are in direct orbits – west to east direction along the equator.
  • Retrograde: east to waste or high eccentricity or high inclination – located far from the planet
  • Jupiter has 79 known moons and a faint rings
    • 4 large moons – Callisto, Ganymede, Europe, Io, discovered in 1610 by Galileo – these are Galilean moons
    • Most of Jupiter’s moons are retrograde orbits very very far from Jupiter
  • Saturn has > 82 known moons and many rings
    • Titan – only moon with a substantial atmosphere and lakes / seas of liquid hydrocarbons
    • Enceladus – active water geysers
    • Saturn’s rings are collections of ice fragments orbiting Saturn in a traffice pattern
  • Uranus: Ring and moon system is tilted at 98 degrees
    • 11 rings
    • 27 known moons
  • Neptune
    • 14 known moons
    • Triton: large moon in retrograde, which is unusual

12.2: The Galilean Moons of Jupiter

  • From 1996 to 1999, Galileo spacecraft went through the jovian system and had close encounters with Galilean moons
  • Callisto
    • Outermost moon
    • Large distance from Jupiter
    • Callisto rotates in the same period as it revolves; keeps the same face towards jupiter
    • Callisto’s day is its month
    • Low surface temperature; permannet water ice
    • Callisto is not fully differentiated
    • Callisto covered with impact craters
  • Ganymede
    • Largest moon in the solar system
    • Has a lot of cratering
    • Ganymede is a differentriated world
    • Rock sinks to form a core about the size of the moon
    • Ganymede has a magnetic field and intermittent geological activity
    • Tidal heating – gravitational flexing due to the gravity grap of Jupiter
  • Europe
    • Has an ocean, wau
    • Acquired the majority share of ice; infrared radiation vaporized ice near Jupiter, leaivng Europa and Io with compositions similar to inner planets
    • Europa is more geologically active than Earth
    • Europa has a very complicated surface, with quasi straight lines. Form if it floats without much friction on liquid water.
    • A global ocean on Europa
    • Europa – only palce in the solar system other than Earth with large amounts of liquid water
    • Ocean must be warmed by heat escaping the interior of Europa
    • Life derives its energy from minimeral laden water. Maybe there is life under Europa?
  • Io
    • A close twin of the moon
    • Has very high volcanism; plumes which extend several 100 kms out in space
    • A lot of volcanism on Io has hot silicate lava
  • Tidal heating
    • Effect of gravity through tidal heating
    • If Io always kept the same face towards Jupiter, the bulge would not generate heat; but because of its uncircularity, it twists back and forth, which generates heat

12.3: Titan and Triton

  • Saturn’s largest moon Titan has similarities to Earth’s moon
    • Titan, seen in 1655
    • First moon discovered after the Galilean moons
    • Titan has a thick atmosphere and lakes and rivers and falling rain
    • Only moon or planet other than Earth with bodies of surface liquids
    • Atmospheric methane can condense and fall as rain
  • Triton
    • Largest of Neptune’s moons
    • 75% rock, 25% water ice
    • Coldest temperature of any visited body
    • Small atmosphere
    • Long history of geological evolution

12.4: Pluto and Charon

  • Is Pluto a planet?
  • Uranus has slight departures from its predicted orbit
  • Pluto could not have exerted any measurable pull on Uranus; the reported small anomalies were in fact not real
  • Pluto is not a giant like the 4 outer solar system planets
  • Pluto has a moon Charon and four much smaller moons.
    • Maybe Pluto and Charon are double worlds
  • Pluto is an odd cousin – Pluto is not unique: Eris, Makemake. A dwarf planet – planet much smaller than the terrestrial planets
  • Asteroid Ceres initially hailed as a planet
  • Pluto’s surface is highly reflective; partial sublimation
  • Pluto is not geologically dead, even though it is so small. Remarkable geological activity
  • Charon keeps the same size towards Plato and vice versa – the two constantly face each other. A double tidal lock

12.5: Planetary Rings and Enceladus

  • All 4 giant planets have rings
  • Each ring system has billions of moonlets / small particles
  • Rings have a complicated structure related to interactions between ring particles and larger moons
  • A ring – collection of many many particles, each like a tiny moon which follow’s Kepler’s laws
  • The ring does not rotate as a solid body – a ring is not really a thing rotating
  • Rings of Saturn and uranus; particles are close enough to exert gravitational influence; sometimes rub together and bounce off each other
  • How did rings come to be?
    • Breakup hypothesis: rings are remains of a shattered moon broken by a passing comet or asteroid; tidal forces disperse it into a disk
    • Breakin hypothesis: rings are particles who are unable to come together to form a moon
  • Gravity is important – tidal forces can drastically change the dynamics
  • Saturn’s rings are broad and very thin
  • Enceladus: possible subsurface ocean of water feeding the geysers which is conveniently escapign into space
  • Uranus has narrow and black rings discovered by occultation
  • Neptune’s rings do not have evenly distributed rings
  • Planetary rings have intricate structures, which is why they are interesting.
    • Without moons, there would be no rings, because the small particles would dissipate
  • Gaps in Saturn’s rings are gravitational resonances from smaller innoer moons; resonance: ratios of gravity

Chapter 13: Comets and Asteroids: Debris of the Solar System

13.1: Asteroids

  • Most asteroids are found between Mars and Jupiter, in the asteroid belt
  • Largest asteroid – Ceres, diameter of 1k km, but reclassified as a dwarf planet
  • Pallas and Vesta
  • Asteroids revolve around the Sun in the same direction as the planets
  • Asteroid belt – between Mars and Jupiter
  • Asteroids are not dense; typical spacing between objects is several million kilometers; this allows spacecrafft to travel through the asteroid belt without collision
  • Some asteroids fall into families
  • Asteroids are generally very dark – primitive bodies which change little since the beginningo f the solar system – C-type asteroids
  • S-type asteroids: stony or silicate; chemically primitive also
  • M-type asteroids: metalic, much better reflector; probably came from a parent body shattered in a collision
    • There is enough metal in a 1-km M-type asteroid to supply the world with iron and industrial metals for the forseeable future
  • Vesta – the surface is covered with basalt, meaning it is differentiated and that it once must have been volcanically active even though it is small (500km diameter)
  • Gaspra and Ida – 1955 Galileo encounter
    • Ida has a moon (Dactyl) in orbit around the asteroid
  • Large objects are pulled by their own gravity into spherical shapes
  • Discovery of an interstellar asteroid on a larger orbit – dscovered in late 2017 leaving the inner solar system

13.2: Asteroids and Planetary Defense

  • Asteroids which stray away from the main belt are interesting
  • Asteroids which come inwards are of interest to people on Earth
  • Comets which come close to the planet and asteroids are near-Earth objects (NEOs)
  • A 1 km or higher asteroid could cause global damage; blocking sunlight; etc.
  • None of the NEAs found so far will impact earth soon.
  • We have about a 5-second warning that an asteroid orbit will hit earth
  • Orbits of Earth-approaching asteroids are unstable over a long time
  • Will humans defend themselves against an asteroid impact? Simple ways – crash spacecraft into an asteroid
  • A spacecraft impact can have a greater effect on certain types of asteroids than others

13.3: The Long-Haired Comets

  • Comets have an icier composition, which makes them brighten a lot more when they approach the sun
  • ‘Hairy stars’
  • Comet – icy material which develops an atmosphere as it appraches the sun; later there will be tail extending several million km away from the main body of the comet
  • MOst comets appear at unpredictable times
  • Comet Halley has been observed and recorded on every passage near the Sun since 239 BCE every 74 - 79 years
  • Halley’s commet will return in 2061
  • Short-period comets have orbits changed by coming too close to the giant planets
  • Temporary atmosphere of gas and dust – atmosphere is the comet’s head / coma. The atmosphere is escaping all the time and needs a new source; this is the nuclear, which is the real comet – the icy material
  • Dirty snowball model, Fred Whipple (1950)
  • The visual profile of comets come from the evaporation of cometary ices heated by sunlight
  • Beyond the asteroid belt, comet ice is frozen
  • A tail is an extension of the atmosphere; tails always point away from the sun, not along the comet’s orbit. Comet tails are formed by the repulsive force of sunlight driving particles away from the head.
    • Dust tail: curves a little bit
    • Ion tail: pushed more directly outward by the Sun
  • Comet 67P’s strange surface and features
    • Highly porous comet material
  • In the outer solar systme, objects contain a lot of water ice
  • Pholus – reddest surface of any object
  • Centaurs – distinction between asteroids and comets breaks down in the outer solar system
  • Beyond the orbit of Neptune – trans-Neptunian objects (TNOs), e.g. Pluto
  • TNOs are part of the Kuiper belt
  • Oort cloud: resevoir of ancient icy objects from which comets are derived
  • Comets from the Oort cloud help us sample material very far from the Sun
  • The fate of comets
    • Any comet we see today will have spent most of its existence in the Oort cloud or the Kuiper belt
    • May collide with the Sun or survive the passage around the Sun and rejoin its origins
    • Sometimes a comet interacts with a planet; it can impact the planet, speed up and be ejected out of the solar system, or come into orbit with a shorter period – its lifetime begins being phased out.

Chapter 14: Cosmic Samples and the Origin of the Solar System

14.1: Meteors

  • Meteors are created as tiny solid particles which enter the atmosphere from interplanetary space
  • Friction in the air vaporizes meteors
  • ‘Shooting stars’
  • Most meteors which strike the Earth are associated with the dust particles of a comet; when Earth crosses a dust stream we experience a meteor shower
  • Perseid shower
  • Comet dust is fluffy and burnsa way quickly

14.2: Meterorites – Stones from Heaven

  • Any interplanetary debris which survives the fall through Earth’s atmosphere is a meteriorite
  • Meterorite finds
  • A lot of meteriorites landed in Antarctic
  • There are irons, stones, and stony-irons
  • Irons and stony irons are extraterrestrial
  • The average age for most primitive meteriorites is 4.56b years
  • Primitive vs differentiated (irons) meteriorites
  • To understnad early solar system history, look at primitive meteriorites. Primitive stones
  • Amino acids found on meteriorites – maybe meteriorites brought amino acids to Earth

14.3: Formation of the Solar System

  • Basic properties of the planetary system: motion, chemical, and age constraints.
  • Retrograde reotation of Venus
  • Progression from metal-rich and rocky planets to ice-dominated compositions
  • Solar nebula
    • The solar systme formed 4.5bya from a rotating cloud of vapor and dust; collapsed under its own gravity
    • Forms a rotating disk
  • Grains condensed in the solar nebula for planetsimals
  • Accretion – planetesimals grow large enough to attract neighbors gravitationally and grow from it
  • Protoplanets continue to grow by accretion of planetesimals
  • Planetary differentiation
  • With ice and rocks, protoplanets grew to become much larger – gradually cool to present state
  • Collapse of gas explains hydrogen rich composition
  • VIsible comets are on the tip of the ‘cosmic icebirg’

14.4: Comparison with Other Planetary Systems

  • Stars like our Sun are formed in dense regions in a molecular cloud feel extra gravitational force and collapse – this is ‘runaway’ – as the cloud collapses, the material get scondensed into a protostar
  • 1/2 the time, the protostar will form binary or multiple star systems
  • Circumstellar disks – have internal structures, dont shaped – gaps closed to the star.
  • Planets form gaps as they form in disks
  • As the mass of protoplanets increases, they orbit faster and sweep up material in a gap along the disk.
  • Exoplanets – difficult to find because exoplanets are faint given the glare of the orbiting star
  • Stellar sepctroscopy
    • Doppler effect: astronomers can measure radial velocity – speed of star from us
    • Measurements about radical velocity give us mass and orbital period; radial velocities can be disentangled and an entire planetary system can be decipherable
    • Planet transits – detect transits from changes in brightness
    • These techniques have been very successful for identifying planets
    • One quarter of stars have exoplanet systems: at least 50b planets in our galaxy
  • Multipleplanet systems are typical for planet formation
  • Most known planetary systems don’t resemble the current solar system.
  • Hot Jupiters – planets of jovian mass closer to stars than the orbit of Mercury. We don’t know about how a giant planet can be formed without condensation of water ice.
  • Most exoplanets have large-orbital eccentricity – not expected for disk foring planets

Chapter 15: The Sun – A Garden-Variety Star

15.1: The Structure and Composition of the Sun

  • The Sun is a very large ball of very hot, mostly ionized gas
  • 73% of Sun’s mass is hydrogen; 25% is helium
  • Sun’s outer layer is very different from Earth’s crust
  • The Sun has similar compositions to most other stars
  • The Sun is so hot that no matter can survive as liquid or a solid
  • Many atoms in the sun are ionized; hot ionized gas is plasma
  • Corona – a spectral line due to highly ionized iron, and how we found that the Sun’s atmosphere has a temperature of more than a million degrees
  • The Sun has an extremely dense core; nuclear energy is being released
  • Above the core – radiative zone, high-density region
  • Convective zone – outermost layer of the interior, transports energy from the radiative zone to the surface with large convection sells
  • Photosphere: the Sun becomes opaque; past it, we cannot see into the sun
  • A cloud look slike it has a sharp surface, but you do not feel it as you fall into it
  • Granulation – mottled appearance of the photosphere
  • Chromosphere – part of atmosphere which lies immediately above the photosphere, composed of hot gasses
  • Corona – hottest part of the solar atmosphere – very low in density
  • Solar wind – the Sun’s atmosphere produces a stream of charged particles which fly into the solar system very quickly; gases move so fast they can’t be contained by solar gravity
    • Comet tails blow in the solar breeze
  • Coronal holes – magnetic field lines stretch away instead of looping back. Solar wind comes mainly from coronal holes
  • Auroras – conflict between solar wind and the atmosphere

Chapter 16: The Sun – A Nuclear Powerhouse

16.1: Sources of Sunshine – Thermal and Gravitational Energy

  • Energy exists in many different forms
  • Watt: unit of power, joules per second
  • Conservation of energy
  • Gravitational contraction as a source of energy
    • Sun stays hot as a result of contracting
    • Contraction cannot be the primary source of energy of the Sun because the Earth is much older than the predicted time period, although contraction does apply otherwhere

16.2: Mass, Energy, and the Theory of Relativity

  • Theory of relativity: matter can be a form of energy and converted into energy; energy can be converted into matter.
  • \[E = mc^2\]
  • We regularly convert mass into energy in nuclear arms – convresion of a small amount of mass results in a massive quantity of energy
  • Conversion of mass into energy is the Sun’s heat and lgiht
  • Fundamental elements of atoms: protons, neutron, electron
  • Every particle has an antiparticle which caries an opposite charge
  • Antimatter: annihilates original particles and produces substantial amounts of energy in the form of light
  • Antimatter is created in the core of the sun and other stars
  • Pauli: energy seems to disappear when some types of nuclear reactions take place. Neutrino – carries away ‘missing energy’; neutrinos are particles with zero mass
  • Gravitational energy is released if a star shrinks under the force of gravity – binding energy of the nucleus when particles come together to form an atomic nucleus
  • Fusion – joining nuclei into a heavier one; mass is lost and energy is released
  • Fission – breaking atomic nuclei into lighter ones
  • Tremendous heat speeds up protons enough to overcome electrical forces which keep protons apart; hydrgogen is common in the sun, protons
  • Steps for solar energy
    1. Form one helium nucleus from four hydrogen nuclei; positron emerges from the reaction and carries away the positive charge
    2. Positron collides with a nearby electron; both are annihilated and release gamma-ray photons
    3. Gamma ray collides with particles of matter and transfers its energy
    4. By the time this process reaches the surface, photons have given up enough energy to be ordinary light
    5. Add a proton to the deuterium nucleus to creeate a helium nucleus
  • Step #1 also produces a neutrino

16.3: The Solar Interior: Theory

  • Proton fusion can only occur if the Sun has very hot temperature
  • The Sun is a plasma – all material in it is the form of ionized gas (plasma)
    • Plasma acts like a hot gas
  • The sun is stable
    • Neither expanding nor contracting – in equilibrium
    • Gravitational attraction between masses of regionsa round the Sun cause collapse of Sun towards the center; but it has managed to resist it for a long time; inside gas pressure resists collapsing. Just from the fact that the Sun is not contracting, we can conclude the temperature internally is high enough for protons to undergo fusion
  • The sun is not cooling down

Chapter 17: Analyzing Starlight

17.1: The Brightness of Stars

  • Luminosity: total amount of energy at all wavelengths emitted per second
  • Apparent brightness: amount of a star’s energy which reaches a given area each second on Earth
  • Photometry – measuring the apparent brightness of stars
  • Magnitude – categories of brightness. Stronomers have other ways to describe brightness but this is also used for some reason.

17.2: Colors of Stars

  • Color – different temperatures and doppler shifts
  • Wien’s law – stellar color linked to stellar temperature – blue colors dominate visible light for hot stars and cool stars emit most of visible light energy at red wavelengths
  • Color does not depend on distance
  • Apparent brightness measured through filters
  • Difference between two magnitudes – color index
  • B-V index: “blue minus red color”
  • Color index implies temperature – and this is what astronomers are actually measuring

17.3: The Spectra of Stars (and Brown Dwarfs)

  • Spectrograph: measure light in a spectrum
  • Stellar spectra look different because stars have different temperatures
  • Hydrogen is abundant in most stars
  • At sufficiently high temperatures, hydrogen becomes ionized and produces spectra emissions, does not produce absorption lines. At cooler stars, hydrogen atoms still have electrons attached and can produce lines by switching energy levels
  • Hydrogen lines in the visible light part of the spectrum (Balmer lines) are strongest in stars with intermediate temperatures
  • Spectral classes – measure of surface temperature: O, B, A, F, G, K, M – L, T, Y
  • Astronomers call elements heavier than helium metals
  • Objects with low masses cannot become hot enough for hydrogen fusion to happen – brown dwarfs, faint and cool

17.4: Using Spectra to Measure Stellar Radius, Composition, and Motion

  • A star with a low-pressure photosphere shows narrower spefctral lines than a higher-pressure photosphere
  • Metallicity – fraction of star’s mass composed of hydrogen and helium
  • Radial velocity method
  • Proper motion / transverse – across our line of sight
  • Space velocity – speed and direct moving thorugh space relative tot he sun
  • Doppler effect can help us measure how fast a star toates

Chapter 18: The Stars – A Celestial Census

18.1: A Stellar Census

  • Light-year – the distance that light travels in 1 year.
  • ONly 3 of the stars in our local neighborhood are significantly more luminous than the sun
  • Stars which appear the brightest are not the closest ones – they emit so large a quantity of energy that they do not need to be nearby.
  • Selection effect

18.2: Measuring Stellar Masses

  • Mass is an important property of a star
  • Half of stars are binary stars – stars that orbit each other
  • First binary star discovered in 1650
  • Castor
  • Visual binary – visible binary star system on a telescope
  • Another class of binary stars where only one of the stars can be seen directly – found that dark absorption lines of the birther star’s spectrum is double
  • Spectroscopic binary – a single star when photographed but spectroscopy shows is a double star really
  • MIzar – good example of how complex binary star systems can be
  • Mizar and Alcor – an optical double, a pair of stars which appear close but do not orbit each other – Mizar is a visual binary. Mizar is a quadruple system of stars in reality.
  • Gravity is a mutual attraction
  • Radial velocity curve: velocity of star change over time
  • \(D^3 = (M_1 + M_2) P^2\) for semimajor axis \(D\), masses \(M_1, M_2\), \(P\) measured in years
  • Many systems are known to be double only by carefully studying their spectra
  • Radial velocity curves allow us to determine the masses of stars in a spectroscopic binary. Measure the speeds of stars from the Doppler effect
  • Stars bigger than the sun are rare – none of the stars in 30 light years of the sun have a mass greater than 4 times it
  • There are some stars with masses 100x to 250x the Sun’s mass; but most have smaller masses
  • A true star can have 1/12 the mass of a sun at the minimum by theoretical calculations.
  • A star is defined here as being hot enough to fuse protons to form helium.
  • Mass-luminosity relation: more massive stars are more luminous generally

18.3: Diameters of Stars

  • It is easy to measure the diameter of the sun
  • Angular diameter is 1/2 degree
  • From this and knowing its distance we can calculate its true diameter
  • We can observe the dimming of light that observes when the moon passes in front of a star; we can observe the time required for the star’s luminosity to drop to zero as the moon moves around the star’s disk; we know the moon’s orbit so we can calculate the angular diameter of the star. Method only works for fairly bright stars and the moon can pass directly over them
  • Eclipsing binary star systems – wen viewed from Earth, stars pass over each other every revolution.
  • Making a light curve of an eclipsing binary – plotting how brightness changes over time.
  • Speed times time between first and second contact gives the diameter of the smaller star; speed times time between first and third contact gets the diameter of the larger star
  • Faint stars are generally smaller than more luminous stars
  • There are some supergiant/giant stars

18.4: The H-R Diagram

Surface temperature
1. Determine the color (very rough).
2. Measure the spectrum and get the spectral type.

Chemical composition
Determine which lines are present in the spectrum.

Measure the apparent brightness and compensate for distance.

Radial velocity
Measure the Doppler shift in the spectrum.

Measure the width of spectral lines.

Measure the period and radial velocity curves of spectroscopic binary stars.

1. Measure the way a star’s light is blocked by the Moon.
2. Measure the light curves and Doppler shifts for eclipsing binary stars.
  • Temperature and luminosity are related
  • Hertzsprung-Russell diagram: temperature / spectral class against luminosity. As temperature decreases, luminosity decreases.
  • Red dwarfs – low temperature, low luminosity
  • White dwarfs – high temperature, low luminosity
  • Red giatns, low temperature, high luminosity
  • Super giants, even higher luminosity
  • The structure of stars that are in equillibrium and derive all of their energy from nuclear fusion is determined by total mass and composition
  • Largest stars have the most gravity and compress their centers most, so they are hottest inside and they shine with the greatest luminosity and ht ehottest surface temperatures.
  • White dwarf – Sirius B, binary system with Sirius A. Very high densit; matter does not exist usually b/c it is so dense.
  • White dwarfs are dying stars

Chapter 19: Celestial Distances

19.2: Surveying the Stars

  • Traingulation
  • Our depth perception fails for objects more than a few tens of meters away; to see the shift of an object much farther away, our eyes need to be spread out a lot further.
  • Parallax – change in location of a remote object due to a change in vantage point
  • Parallax angles are only a fraction of a second of arc (arcsec is 1/3600 of a degree)
  • The distance of a star in parsecs is the reciprocal of its parallax in arcseconds.
  • Parsec – the distance at which we have a parallax of one second – about 3.26 light years.
  • No known star is in 1 parsec of Earth

Chapter 22: Stars from Adolescence to Old Age

22.1: Evolution from the Main Sequence to Red Giants

  • HR diagram – once a star has reached the main-sequence stage of its life, it gets is energy entirely from the conversion of hydrogen into helium via nuclear fusion. Hydrogen is very abundant so this process can go on for a very long time.
  • Zero-age main sequence – zero age: the time when a star strops contracting, settles onto the main sequence, and begins to fuse hydrogen
  • As hydrogen depletes and helium increases, the luminosity, temperature, size, interior structure, etc. changes; when luminosity and temperature change, the star on the H-R diagram moves away from the zero-age main sequence
  • Temperature and density slowly increase with accumulated helium. The rate of fusion goes up as temperature to the fourth
  • How long a star remains int he main sequence depends on its mass
  • The most massive main-sequence stars are also the most luminous, but also they burn it so quickly that they have shorter lifespans than less massive stars.
  • G-type star: human beings developed on a planet around a G-type star, but it would be a waste of time to search around O- or B- type stars because they remain stable for so short periods of times
  • When all of the hydrogen in a star’s core is used up, the core contains only helium. Energy can no longer be generated. The star’s energy is partially supplied by gravitational energy, and the core begins to contract; the star’s ocre shrinks and energy is converted into heat
  • Most stars generate more energy each second they are fusing hydrogen in a shell outside of a hleium core than in the center; the star expands and increases in luminosity. Cores are contracting but outside is expanding.
  • The more massive as tar is, the faster it goes through each stage in its life

22.2: Star Clusters

  • No star completes its evolution into a red giant quickly enough for us to observe
  • However we can look at a group or cluster of clouds; we can assume that the stars formed at approximately the same time if they orbit the same center.
  • We can find clusters in which massive stars have already finished main-sequence evolution and become red giants while lower-mass bodies are still in the main sequence
  • Globular clusters
    • Symmetrical, round ssystems of hundreds of thousands of stars
    • Very dense areas
  • Open clusters
    • Found in the disk of the Galaxy – wide range of ages
    • Stars in an open cluster remain only for a few million years; the speed of some stars may be faster than the escape vcelocity for instance
  • Stellar association
    • Group of extremely young stars
    • Associations are found in gas- and dust-rich regions open clusters, stellar associations.

22.3: Checking out the Theory

  • At every stage of evolution, massive stars evolve more quickly than lower-mass counterparts
  • Main-sequence turnout, place in the H-R diagram where stars begin to leave the main sequence

22.4: Further Evolution of Stars

  • Stars begin as a contracting protostar, live most of their life as a main stable-sequence star, then move off it into the red giant region
  • Some star histories diverge

Helium fusion

  • Red giants start out with a helium core where no energy generation happens surrounded by a fusing hydrogen shell. However once the temperature reaches 100m K, helium attoms beginf using into a carbon nucelus (triple-alpha process), which releases a helium flash. Heating, followed by more nuclear reactions, etc. – runaway generation of energy
  • Stellar evolution is a history of constant struggle against gravitational collapse; it cna do so if it can tap into energy sources
  • Onion structure as different fusion processes take place
  • The fusion of helium nuclei needs temperatures which cannot be reached; so the formation of a carbon-oxygen core is the death of the star

Mass Loss from Red-Giant Stars and the formation of Planetary nebulae

  • Stars turning into red giants have large radii and low escape velocity (so not very massive).
  • Red giants lose a lot of their mass into space; aging stars are surrounded by a fuck ton of expanding shells of gas
  • When nuclear energy is used up, the core shrinks and heats up as it gets compressed; it shrinks and gets very hot; stellar winds and ultraviolet radiation sweep outwards and heat ejected shells, ionizing them, and making them glow, forming planetary nebulae
  • The star continues to lose mass
  • ‘Last gasp’ of low-mass (< 2 sun masses) star evolution

Cosmic recycling

  • Death of stars is important part of cosmic recycling

22.5: The Evolution of More Massive Stars

  • Everything starts from the two simplest elements – hydrogen and helium
  • However, there are many heavier elements – where did they fuse together? The only place hot enough is inside stars.
  • Massive stars can start additional types of fusion in their cores – they can compress carbon-oxygen until it starts to fuse carbon nuclei – forming oxygen, neon, magnesium, silicon, etc. Iron is the most that can be built – the process requires more energy to fuse than it releases
  • Iron can be built up through nucleosynthesis in the centers of massive red giants
  • We can predict relative abundances in which elements occur in nature; stars that build up elements during nuclear reactions can help explain the differences in elements

Elements in Globular Clusters and Open Clusters Are Not the Same

  • Elements are made in stars over time, which explains differences in globular vs open clusters
  • Abundances of elements heavier than helium are very different
  • Open cluster stars have 1-4% of matter as heavy elements; globular clusters are only 1/10 to 1/100 as much as the sun
  • First stars contain only hydrogen and helium and created heavy elements in their interiors; then they eject matter to create a new generation of stars
  • Globular clusters are much older than open clusters, so they have fewer abundance of elements heavier than hydrogen and heium

Approaching Death

  • The events that characterize the end of stellar evolution happen very quickly; luminosity increases and nuclear fuel gets consumed much faster
  • Hydrogen is exhausted and other fuel sources die out

Chapter 23: The Death of Stars

23.1: The Death of Low-Mass Stars

  • Stars whose final mass is less than 1.4 \(M_{Sun}\) – most stars in the universe are like this

A Star in Crisis

  • The core of a star undergoes an energy crisis in which helium is exhausted and there is no source of pressure to counterbalance gravity
  • Star mass is low and it cannot push its core temperature high enough to begin fusion. The core continues to shrink; at extreme density, the star takes on new behavior and achieves final equilibrium as a white dwarf
  • White dwarfs are very very dense. Gravity is very strong and shrinks the star further, but electrons resist being pushed closer together.
  • No two electrons can be in the same place at the same time doing the same thing
  • When a star’s core contracts, it becomes so dense that it would require two or more electrons to violate the Pauli exclusion principle
  • Electrons in a degenerate gtas resists further crowding
  • Degenerate electrons do not require an input of heat to maintain the pressure they exert; it can last essentially forever
  • The dying star has atomic nuclei; nuclei must be squeezed to much higher densities. So nuclei do not exhibit degeneracy pressure.

White Dwarfs

  • White dwarfs are stable, compact objects with electron-degenerate cores which cannot contract further
  • White dwarfs are the likely end state of low-mass stars
  • The radius of a white dwarf shrinks as mass increases; a white dwarf with a mass of 1.4 the sun has a radius of zero; even the force of degenerate electrons cannot stop the collapse of such a star. 1.4 times the mass of the Sun is the Chandrasekhar limit.

The Ultimate Fate of White Dwarfs

  • The birth of a mains-equence star is defined by the onset of fusion reactions
  • End of fusion reactions is the time of a star’s death
  • White dwarf – ceasing of nuclear fusion in its interior
  • A stable white dwarf’s only energy source is the motions of atomic nuclei in its interior, and it slowly radiates all of its heat into space.
  • It eventually becomes a black dwarf – a “cold stellar corpse”

23.2: Evolution of Massive Stars – An Explosive Finish

  • Stars beginning with masses at least 8 times the mass of the sum end up as white dwarfs
  • what about 150x \(M_{Sun}\)?
  • After helium is exhausted, a massive star’s outer layers can force the carbon core to contract until it fosues carbon into oxygen, neon, magnesium; then another contraction into silicon, sulfur, calcium, argon; then finally iron; this happens very quickly, in months or even days
  • When nuclear reactions stop, the core is supported by degenerate electrons.
  • NO energy is generated in the white dwarf core of a star but fusion still occurs in surrounding shells
  • Iron reaches a mass eventually so large that even degenerate electrons cannot support it; electrons are squeezed into the atomic nuclei and form neutrons and neutrinos from protons
  • The core shrinks rapidy; neutrons are squeezed out of the nuclei; the collapsing core becomes a crushed ball mae of neutrons – a neutron star
  • Might collapse into a black hole
  • The core is saved by degenerate neutrons but the rest of the star blows apart; electrons are absorbed into the nuclei, the star rapidly contracts
  • A shock wave travels outwards, which is absorbed by atomic nuclei; protons form with electrons to make neutrons, releasing a neutrino, which carries away nuclear energy –
  • Supernova – resulting explosion

Supernova giveth and taketh

  • The fused elements are recycled into space to form new stars and planets
  • Neutrons can be absorbed by iron, where they turn into protons, and can build up to much more massive nuclei.
  • Supernovae importantly provide the galaxyw ith heavier elements which will become the building blocks of life
  • Supernovae probably produce high-energy cosmic ray particles, which may have contributed to mutations in genetic material on earth
  • Lif eofrms near supernovae would perish from high radiation

23.3: Supernova Observations

  • Nova – new
  • No supernova explosion has been observable since the invention of the telescope
  • The most luminous supernovae have 10b times the luminosity of the sun and so it might outshine the entire galaxy

Supernova 1987A

  • Supernova which could be seen with the unaided eye 160k light years away
  • The star was a blue supergiant before the explosion

Synthesis of Heavy Elements

  • SN 1987A helped us confirm theories about heavy element production
  • Decay of radioactive nickel and cobalt

23.4: Pulsars and the Discovery of Neutron Stars

  • After a type 2 supernova explosion, all that is left behind is a neutron star or a black hole
  • Neutron stars are the densest objects in the universe
    • Interior: 95% neutrons
    • A neutron star is a giant atomic nucleus
  • Diameter of a neutron star is the size of a small town
  • Neutrons make their presence clear even across large areas of sapce
  • 1967 discovery of reglar pulses of radio radiation
  • Pulsars: pulsating radio sources

A Spinning Lighthouse Model

  • Pulsars are spinning neutron stars – you see a pulse of light every time the beam points in your direction; radiation sweeps across space
  • Neutron stars can turn very rapidly due to conservation of angular momentum
  • Neutron stars can complete a full spin in a fraction of a second
  • A magnetic field becomes highly compressed
  • MIsalignment of rotational axis with the magnetic axis
  • Crab Nebula pulsar is gradually slwoling down

The evolution of pulsars

  • One new pulsar is born in the galaxy every 25-100 years
  • Lifetime of a pulsar is 10m years
  • Pulsar beams might miss us

23.5: The Evolution of Binary Star Systems

  • ‘Single star chauvinism’
  • At least half of stars develop in binary systems
  • The presence of an orbiting star can have a large influence on evolution
  • Stars can exchange material, especially during swelling – mass transfer can be dramatic

White Dwarf Explosions: The Mild Kind

  • Fresh hydrogen accumulates on the surface of a hot white dwarf from a star system; the new layer reaches a demperature which cuases fusion and blasts the material away
  • White dwarf becomes very bright
  • White dense dwarf is itself quite unaffectde by the explosions on its surface

White Dwarf Explosions: The Violet Kind

  • If a white dwarf accumululates mass from a c ompanion star much more quickly, it can get pushed over the Chandrasekhar limit; nuclear reactions begin in the degenerate core; a bunch of fusion happens at once and there is a huge explosion; the white dwarf is gone
  • This explosion coutns as a supernova – produces a lot of energy in a very short time.
  • There is no remnant unlike explosions of high-mass stars – these are type 1a supernovae
  • Type II supernovae originate from the death of massive stars
  • a means strong silicon absorption lines – silicon results from carbon and oxygen and means there was a sudden fusion of carbon and oxygen
  • How else can 1a supernovae emerge? Carbon and oxygen meat absence of hydrogen in the spectrum

Neutron Stars with Companions

  • A binary system can survive the explosion of one its members as a type II supernova, in which case an ordinary star shares a system with a neutron star
  • The radius of a neutron star decreases as more mass is added
  • Milisecond pulsars can form from material being transferred to the neutron star
  • Some binary systems have two neutron stars and alter each other’s orbit


  • Core mass less than 1.4 \(M_{Sun}\) end up as white dwarfs
  • Core mass between 1.4 and 3 \(M_{Sun}\) become neutron stars
  • Core mass greater than 3 \(M_{Sun}\) become…

23.6: The Mystery of Gamma-Ray Bursts

  • Gamma rays – most energetic form of electromagnetic waves
  • Gamma rays are distributed isotropically.
  • Where do bursts come from?

The First Afterglows

  • Our instruemtns cannot pinpoint the exact place where gamma ray bursts are happening – not enouhgh resolution
  • Gamma ray bursts really are very energetic objects from far away in distant galaxies.

To Beam or Not to Beam

  • In order to produce as much energy so quickly so far away
  • Some radiation is concentrated in one or two directions

Long-Duration Gamma-Ray Bursts: Exploding Stars

  • Gamma ray bursts – short and long
  • Extra energy is the collapse of the star’s core from spinning
  • Sudden collapse is complex and produces a lot of radiation very quickly into a narrow beam.
  • Merger of two complact stellar corpos.
  • Kilonova

Short-Duration Gamma-Ray Bursts: COlliding Stellar Corpses

Chapter 24: Black Holes and Curved Spacetime

24.1: Introducing General Relativity

  • General theory of relativity – 1916, Einstein
  • Newton’s account of gravitation was good enough for everyday-life physics

The Principle of Equivalence

  • If you jump off a tall building and fall freely, you would not feel your own weight.
  • In a freely falling elevator, with no air friction, you lose your weight
  • There is no experiment someone can tell to see if they are in a freely falling elevator or an elevator floating in space

Gravity of Acceleration?

  • The orbiting ISS is falling freely around Earth
  • In free fall, astronauts do not have gravitational force; but everything else is falling too, so gravitational forces appear to be absent.
  • Local effects of gravity can be completely compensated by appropriate acceleration

The Paths of Light and Matter

  • Equivalence principle is a fundamental fact of nature
  • What happens when we use light in our experiments?
  • Beams of light seem to travel in straight lines. So if we shine a light at the front, we won’t travel along where our beam is
  • Maybe EP is wrong. Or — light falls with the ship in orbit around Earth.
  • Earth’s gravity bends the fabric of space and time, which keeps the behavior of light the smae in empty space and free fall

24.2: Spacetime and Gravity

  • How can light, with no mass, be affected by gravity?
  • Space and time iself are affected by large mass
  • Light always follows the shortest path, but it may not be straight

Linkages – Mass, Space, Time

  • Any event in the univerese can be pinpointed in the three dimensions of space and the one dimension of time
  • Newton considered space and time to be independent
  • Considering space and time together as spacetime gives a more accurate view of the physical world
  • Matter warps the fabric of spacetime – curving of spacetime is experienced as gravity. Things travel differently with and without nearby masses

Spacetime Examples

  • Because mass distorts spacetime, the shortest paths are curves, not lines

24.3: Tests of General Relativity

  • Einstein proposed a new theory of gravity – mass determines the curvature of spacetime, which controls how objects move.
  • Einstein’s theory only really makes itself apparent at large masses
  • When distorting mass is small, Newton’s universal gravitation works. But Einstein works more ‘universally’
  • Mercury orbits closest to the Sun and is most affected by spacetime produced by Sun’s mass.
  • Mercury has a highly elliptical orbit: Newtonian gravitation has errors – people thought there was some other body (just like with Neptune and Uranus).
  • Einstein – light near the Sun shoud follow a curved path under an eclipse: consistent with general relativity

24.4: Time in General Relativity

  • Generalizes space and time
  • The stronger the gravity, the lower the pace of time
  • Time is, it is thought, a democratic concept: all of us have it. But in fact it changes
  • 1959 atomic clock experiment – a clock closer to the ground should run slightly slower than a taller one
  • What does it mean for time to run slowly?
  • WHen light emerges from strong gravity where time slows down, light experiences a change in its frequency and wavelength
  • Light is a repeating phenomenon (a wave) – every light wave is a little clock
  • Stronger gravity makes the period of waves longer.
  • To maintain constant light speed, a lower frequency must be compensated with a longer wavelength – this is redshift. Gravitational redshift – produced by gravity
  • Why should I care about relativity? In fact it is everywhere. Satellites, planes, GPSs, etc.

24.5: Black Holes

  • If a core’s mass is bigger than three times the mass of the sun, nothing can stop the core from collapsing together.
  • Classical collapse
    • A rocket has to have high velocity to escape gravitational pull of Earth
    • Imagine we compress the Sun
    • Gravity depends on mass and distance from center of the mass
    • If the Sun is compressed, the mass remains the same but your distance gets smaller; gravity gets super strong
    • When the shrinking Sun reaches the diameter of a neutron star (20 km), escape velocity is half the speed of life
    • Escape velocity will soon exceed the speed of light, and nothing (including light) can escape; therefore it emits no light and nothing which falls into it can return
    • This is a black hole
  • Collapse with relativity
    • Gravity is a curvature of spacetime
    • Curvature gets larger and larger as gravity increases
    • Gravity is not pulling on the light; matter has curved spacetime; the concept of out has no geometrical meaning, the star is trapped in its pocket of spacetime
    • The star’s geometry ‘cuts off’ when escape velocity is equal to the speed of light; the size of the star is the event horizon
    • Events in the event horizon can never affect events outside of
    • The event horizon does not get smaller once a star has collapsed inside it. A horizon’s size depends only on the mass inside it
  • Myths
    • Black holes do not just suck things p with their gravity
    • Even if you orbit around a star which becomes a black hole, your orbit will not be significantly affected, even though there may be mass loss.
    • Only very near to the event horizon is gravitation so strong that Newton’s laws break down
  • It is possible in theory to construct a time machine with gravity to take you into the future.
  • A trip into a black hole
    • Black holes have no hair
    • What happens to a star core in a black hole? Material collapsed into a squozen point – zero volume and infinite density, singularity
    • Spacetime ceases to exist at singularity
    • The structure of a basic black hole can be described as asingularity surrounded by an event horizon
    • All matter falling into the black hole will look like it takes infinite time to fall through it
    • The event horizon is not a physical barrier
    • Theories of relativity: your measurement of the world is dependent on yrou frame of reference
    • Spaghettification

24.6: Evidence for Black Holes

  • Do black holes actually exist? HOw can we even find them?
  • Requiremetns for a black hole
    • Start by looking for a star which is part of a binary star system; we find systems where only one star is visibile
    • But not just invisible (can have confounders): there must be collapse. Using Kepler’s law, measure the mass of the invisible member: if the mass is bigger than 3 * mass of the sun, it is probably a black hole
    • Stars in binary systems can exchange mass: material falls towards hte black hole and spirals aruond it; this forms the accretion disk.
  • The discovery of stellar-mass black holes
    • X-rays are important tracers of black holes
    • Cygnus X-1
    • Flickering X rays – small collapsed object
  • Feeding a black hole
    • After an isoalted star becomes a black hole, it won’t grow much – other stars are too far to provide ‘food’
    • Black holes in the center of galaxies are more likely to find mass close enough to event horizons to pull in
    • Black holes in crowded regions can grow
    • There is a black hole in the cneter of our own galaxy
    • Feeding frenzy of supermassive black holes may be responsible for energetic phenomena in the universe
    • Many observations can only be explained if black holes do exist.

24.7: Gravitational Wave Astronomy

  • Any rearrangement of matter creates a disturbance in spacetime
  • Gravitational wave created by disturbance

Proof from a Pulsar

  • PSR1913+16 – orbiting another neutron star, moving 1/10 speed of light; the two should spiral closer together

Direct Observations

  • LIGO stations: if Einstein is correct, waves will affect local spacetime and affect the distance laser light travels between mirrors
  • New field of gravitational wave astronomy
  • Neutron star mergers are a significant source of heavy elements.

Chapter 25: The Milky Way Galaxy

25.1: The Architecture of the Galaxy

  • 1785, William Herschel – stars lay in a flattened structure encircling the sky – Herschel concluded that the Sun belongs to a disk or wheel
  • The galaxy is a 100k light year diameter disk
  • Youngest stars are found within 100 light years of the plane of the milky way galaxy
  • Stars, gas, and dust are not spread evenly – concentrated into a central bar and spiral arms
  • Central bar: mostly old yellow-red stars
  • Barred spirals – bar-shaped concentrations of stars in central galaxies
  • Stars form a central bulge close to the galactic center
  • Embedded in a spherical halo of old, faint stars
  • Mass in the Milk Way extends further – dark matter, does not emit any light. Detectable via gravitational effects on orbits of distant star clusters

25.2: Spiral Structure

  • The galaxy has two main spiral arms
  • Shorter arm – Orion Spur, 10k light years long
    • It’s hard to see farther b/c of galactic dust
  • The Galaxy does not rotate like a CD
  • Objects rotate like the solar system – differential galactic rotation
  • Differential rotation explains the spiral arms – differential rotation of the galaxy stretches any original distribution into spirals.
  • How do other galaxies evolve?
    • Begin as clumpy star forming regions
    • After a few billion years, galaxies settle down and become spirals with a central bulge
    • Multi-armed structures appear later

25.3: The Mass of the Galaxy

Kepler helps weigh the galaxy

  • The sun orbits the center of the milky way.
  • It takes us about 225 million years to go once around the center of the galaxy – this is a galactic year.
    • Earth is 20 galactic years old
  • We can calculate the sum of masses of the Galaxy and the Sun using Kepler’s third law.

A galaxy of mostly invisible matter

  • A lot of invisible matter exists far from the galactic center
  • The Galaxy needs to have a lot more gravity than is supplied only by luminous matter – the source of extra gravity has to extend much beyond the Sun’s orbit.
  • The total mass of the Galaxy is 2 times 10 to the 12 solar masses; dark matter extends 200k light years from the center of the galaxy
  • What is dark matter made of? Nothing that we know – exotic subatomic particles.
  • Which galaxies ‘orbit too fast for their own good’
  • Maybe as much as 95% of the mass int eh galaxy is invisible and we don’t know what it is about.

25.4: The Center of the Galaxy

  • Core of the galaxy contains a large concentration of mass
  • Supermassive black holes – mass they contain is much larger than that of a typical black hole
  • Other wavelengths flow unimpeded past dust particles without any dimming

A Journy toward the Center

  • Radio emissions from hot gas heated by clusters of hot stars or supernova blast waves
  • Sagttarius A – supermassive black hole. A lot of dust and molecular gas revolves around the black hole

Finding the Source

  • Where deos the galactic black hole come from
  • Maybe a large cloud of gas near the center of the milky way collapsed
  • A black hole can grow by devouring nearby stars and gas clouds, and merging with other black holes
  • The black hole is continuing to eat matter – gas and dust falls in at 1 solar mass every 1k years.

25.5: Stellar Populations in the Galaxy

  • Halo stars – Baade could examine other galaxies
  • Population I stars – found in the disk, follow circular orbits around the galactic center
  • Population II stars – no correlation with the location of the spiral arms, found throughout the galaxy;
  • Population II is older than population I, and has lower abundances of heavy elements
  • Stars are born over time with larger supplies of heavy elements

The Real World

  • There is no sharp boundary for what the disk counts as. Older stars define a 2k light year thick disk, but the new young stars have 200 light years thick.
  • Highest density of stars is found in the central bulge; star formation formed very rapidly after the formation of the milky Way.
  • Small megallanic cloud – very crowded, star formation happens slowly, lower heavy elements

25.6: The Formation of the Galaxy

The protogalactic Cloud and the monolithic collapse model

  • Oldest stars are distributed in a sphere around the galaxy – perhaps a protogalactic cloud was spherical; then, like in star formation, it collapsed and formed a rotating disk
  • Gravitational forces cause the gas to fragment into clouds, which fragment to form stars

Collision Victims and the multiple Merger Model

  • Sagittarius galaxy is much smaller than Milky Way
  • When a small galaxy comes too close, our galaxy tugs on the near side harder, which pulls on the orbit of the small galaxy
  • The Milky Way will get swallowed by the Andromeda galaxy also into a merged galaxy

Chapter 26: Galaxies

26.1: The Discovery of Galaxies

  • 1920s – astronomers believed the Milky Way encompassed all that exists in the universe
  • Kant – suggested that some nebulae might be distant systems of stars

Other Galaxies

  • Some nebulae have been correctly identified as star clusters; others incorrectly as gaseous nebulae
  • Edwin hubble – discovered variable stars
  • Measured the Andromeda galaxy as 900k light years away
  • Extragalactic astronomy.

26.2: Types of Galaxies

  • Hubble – observe other galaxies more closely – noting shapes, properties, contents
  • Describing new objects

Spiral Galaxies

  • Our Galaxy and the Andromeda galaxy – spiral galaxies with a central bulge, halo, disk, and spiral arms
  • ISM spread through disks
  • Most spiral galaxies form a bar at some point during evolution

Elliptical Galaxies

  • Consist basically all of old stars and have spheroid or ellipsoid shapes, with no trace of spiral arms
  • Mainly older reddish stars
  • Ellipticals don’t appear to rotate in a systematic way
  • Dwarf ellipticals

Irregular Galaxies

  • Irregular galaxies hav elower masses and luminosities than spiral galaxies
  • Look very disorganized
  • Large Magellanic Cloud, Small Magellanic Cloud

Galaxy Evolution

  • Astronomers hoped to find a scheme to understand the evolutionary stages of life of galaxies, like the H-R diagram
  • No simple scheme for galaxy evolving
  • Active field of research

26.3: Properties of Galaxies

Masses of Galaxies

  • Astronomers can measure the rotation speed in spiral galaxies by measuring wavelength changes
  • Elliptical galaxies do not rotate in a systematic way; we cannot determine rotational velocity
  • How much mass should a galaxy contain to hold stars?
  • Broadening – stars provide both redshifts and bleushifts, which broadens the range of speeds at which stars move w.r.t. the cneter of the galaxy

Mass to Light Ratio

  • Most stars are smaller and less luminous than the Sun
  • Most material in the universe cannot be observed directly via the electromagnetic spectrum

26.4: The Extragalatic Distance Scale

  • We need to know how far away a galaxy is before we compute other important properties
  • Cepheids – intrinsically luminous variable stars
  • Stars are not standard bulbs – it is not true that brighter stars must be farther away. Type 1a supernovas are easily visible and reach the same luminosity at maximum light.
  • The luminosity of a spiral galaxy is realted to its rotational velocity

Chapter 27: Active Galaxies, Quasars, and Supermassive Black Holes

27.1: Quasars

  • Quasar – quasi-stellar radio sources
  • Sky is full of radio waves

Redshifts – Key to Quasers

  • Spectra of blue radio stars have unknown emission lines
  • Star is receding from us at a speed of 45km per second – so there is a massive doppler shift
  • Radio stars are not stars in our own galaxy – these objects only look like stars because theya re compact and very far away
  • Quasi-stellar objects
  • All spectra show redshifts and no blueshifts

Quasars Obey the Hubble Law

  • Quasars found at the cores of spiral and elliptical galaxies
  • Quasar host galaxies are often involved in a collision with another galaxy

Size of the Energy Source

  • Quasars are very far away, and have to be very luminous to be visible to us
  • Quasars emit energy at X-ray adn ultraviolet wavelengths and some radio sources
  • Quasars have massive luminosities
  • Irregular variation in luminosity
  • Part of a quasar which varies must be smalelr than the distance light travels in the time it takes for this variation to occur.

Earlier Evidence

  • Heber Curtis – photographs a jet coming from the nucleus of the galaxy
  • Astronomers identified a zoo of active galaxies or active galactic nuclei

27.2: Supermassive Black Holes – What Quasars Really Are

  • Quasars are hugely powerful
  • Quasars are tiny
  • Quasars shoot out jets at close as the speed of light
  • Quasars can’t be powered by nuclear fusion
  • milky Way has a black hole in its center; energy emits from a small region
  • A black hole with a bunch of mass can produce a lot of energy

Observational Evidence for Black Holes

  • Energy emitted by quasars also produced by a hot accretion disk
  • X-ray band: quasars vary rapidly, light travel
  • Onlya black hole can explain the large amount of mass in a small amount of space
  • There is a mass of at least 3.5b solar masses in the center of M87

Energy Production around a Balck Hole

  • Central black hole attracts matter in dense nucelar regions – forms an accretion disk
  • Matter gets very hot and radiates a lot of energy as it falls into the black hole.
  • Dropping things from far away into strong gravity produces a lot of energy – releases a lot of electromagnetic radiation
  • Gas falls in streams into a black hole, reaches 10% c or more
  • Quasars are much mroe efficient than a hydrogen bomb
  • Many black holes don’t show quasar emission – quiescent.
  • But can be woken up with fresh gas. The gas has to take time to swirl around the star.

Radio Jets

  • Material escaping from the neighborhood of a black hole can do so easily perpednicular to the disk

27.3: Quasars as probes of Evolution in the Universe

  • Quasars tend to be far away, and so we are seeing them as they are long ago.

The Evolution of Quasars

  • Quasars – suggest that we live in an evolving universe.
  • Significant drop-off in the number of quasars over time.
  • Accretion disks around black holes peak early and then fade
  • Drop-off in quasars can be explained if there is more material avaialble to be accreted earlier on
  • Collisions were more common earlier than they are now

Codependence of Black Holes and Galaxies

  • It seems thatt he mass of a central black hole depends on the mass of the galaxy

How a Galaxy Can Influence a Black Hole in its Center

  • Without a lot of material to ingest, black holes glow only weakly, as there is not that much material swirling around to emit energy from
  • Galaxies begin with a lot of interstellar gas and dust
  • Both elliptical galaxies and nuclear bulges of spiral galaxies have little material left for black holes. So black holes are pretty quiet.
  • A star can be pulled apaert if it gets to close to a black hole
  • Collisions are less freuqent today; most galaxies the size of the Milky Way are merging iwth dwarf galaxies
  • Merging of supermassive black hole emits a burst of gravitational waves

How does the black hole influence the formation ofs tars in the galaxy?

  • Jets
  • Winds of particles streamed away from the accretion disk
  • Radiation from the accredition disk

The Brith of Black Holes and Galaxies

  • Puzzle – how did supermassive black holes in the galaxy get started?
  • Seed black holes must have been present near the formation of the solar system
  • Possible – stars form from angular momentum of gas; supernovae create black holes which merge into rich gas supplies.

Chapter 28: The Evolution and Distribution of Galaxies

28.1: Observations of Distant Galaxies

  • Distance from galaxy is derived from redshift

28.2: Galaxy Mergers and Active Galactic Nuclei

28.3: The Distribution of Galaxies Through Space

  • Hubble finds that the number of galaxies visible in each area of the sky is about the same
  • Hubble discovers that the universe is isotropic and homogenous – looks the same in all directions; any section looks the same as any other
  • Cosmological principle – the universe is the same everywhere
  • Inductive assumption essential to making claims about the universe

Local Group

  • We are discovering new galaxies all the time

Neighboring Groups and Clusters

  • Virgo Cluster – has the elliptical galaxy M87
  • Coma Cluster
  • Ellipticals are ‘highly social’, found in groups and hang out with other ellipticals
  • Spirals are ‘shy’; found in poor clusters or on the edges of rich clusters

Superclusters and Voids

  • Large superclusters – giant arcs of inkblots
  • Local Group is part of the Virgo Supercluster
  • Voids: separating the sheets of the supercluster. Empty bubbles within large galaxies
  • 90% of galaxies occupy 10% of space

28.4: The Challenge of Dark Matter

  • Galaxies contain large amounts of dark matter
  • The epistemic properties of dark matter are not foreign
  • Astronomers can routinely determine the location and amount of dark matter by measuring gravitional effects
  • No more than half of the mass in the region near the sun can be dark matter
  • About 90% of the mass of the galaxy is in the form of a halo of dark matter
  • Galaxies in clusters orbit a cluster’s center of mass; so how much mass is needed to maintain cluster consistency?
  • Gravitational lensing: general relativity tells us that mass bends spacetime, so dark matter can also reveal its presence by bending light
  • X-ray: galaxies have gas distributed between stars; gas heats up and shines; compare X-rays with visible matter
  • Mass-to-ligth ratio: amount of matter in galaxies
  • Dark matter apparently makes up most of the total mass of the universe
  • MACHOs: MAssive Compact Halo Objects. But MACHOs cannot account for all of dark matter
  • Hot dark matter – dark matter particles moved faster compared to ordinary matter. Cold dark matter – colder.
  • Dark energy – some mysterious energy which pushes spacetime apart, taking galaxies with it
  • Science always in progress

28.5: The Formation and Evolution of Galaxies and Structure in the Universe

  • How did the universe get this way? Dark matter, gravity, and time.
  • Galaxy mergers play an important role in the evolution of galaxies
  • The larger a galaxy, the larger its central black holes
  • Elliptical galaxies formed in a single collapse – top down.
  • Ellipticals are formed through mergers of smaller galaxies – bottom-up.
  • Spiral galaxies – bottom up.
  • Tiny dark matter seeds grew into larger structures as cosmic time passes. 5% normal attom, 27% cold dark matter, 68% dark energy
  • Big picture
    • Distribution fo matter is pretty much smooth and uniform
    • Each lump expands; regions of higher density acquire more mass
    • Universe built-itself fromt he bottom up, through mergers
    • Some galaxy collisions trigger massive star formation and black hole formation
    • Black holes found a lot of food and grew quickly
    • Massive black holes trigger quasars which shut off star formation

Chapter 30: Life in the Universe

30.1: The Cosmic Context for Life

  • The universe was born in the Big Bang ~ 14b years ago
  • Processes in stars created other elements besides hydrogen and helium – carbon oxygen, and nitrogen
  • Life on Earth is based on the presence of the organic molecule, which contains carbon molecules

What Made Earth Hospital to life?

  • 5 billion year ago, a cloud of gas and dust in this cosmic neighborhood began to collapse under its own weight
  • Atoms in your body are ‘in loan’ from the ‘library of atoms’
  • Cosmic evolution – our descent from the stars

The Copernican Principle

  • Formation of planets is a natrual consequence of the formation of stars.
  • Earth-like planets occur frequently, such that there are billions of exo-Earths in our Milky Way Galaxy.
  • The Copernican principle – the idea that threre is nothing special about
  • Are we lucky or are we unexceptional?

So Where Are They?

  • Intelligence seems like it might be very common
  • Fermi paradox

30.2: Astrobiology

The Building Blocks of Life

  • Amino acids
  • Proteins
  • Organic molecules are present in the places where interstellar dusdt is most abundant.
  • Miller-Urey experiments
  • Even the simplest molecules of genes contain millions of molecule units in a precise sequence.
  • Life uses DNA and RNA
  • Complex sequene of chemical reatctions in phoro
  • Stromalites – fossils of oxygen-producing photosynthetic bacteria in rocks.
  • Photosynthesis begins producing more ‘free oxygen’ into our atmosphere – interaction of sunlight with oxygen can produce ozone which protects us from ultraviolet radiation

Habital Environments

  • What conditions make for a habitable environemtn, one which is capable of hosting life?
  • Life requires a solvent – a liquid which chemicals can dissolve – to construct biomolecules and life.
  • Biochemistry is based on carbon, hydrogne, nitrogne, oxygen, phosphorus , and sulfur: CHNOPS
  • Maintaining biochemical machinery takes energy
  • One energy production method: combine hydrogne with carbon to make methane

Life in Extreme Conditions

  • Chemically – life is many types of molecules coming together to carry out the processes of life.
  • Your own biochemistry is highly tuned and contingent
  • Extremophile – organism which tolerates at minimum a ‘hostile’ world
    • Themophiles
    • Could slows down metabolism and cna cause physical damages

30.3: Searching for Life Beyond Earth

  • Two searchers for life
    • Direct searches
    • Looking for biomarkers

Life on Mars

  • Water might have flowed on Mars
  • No evidence of biological activity in the soil
  • Mars does not have a magnetic field and ozone layer
  • Habitable – liquid water + other enviornmental conditions like protection from solar radiation
  • Mudstone

Life in the Outer Solar System

  • So little sunilght
  • Europa – ocean for most of its history


  • More than 40% of stars have at least one exoplanet orbiting in a habital zone
  • Plants and microorganisms affect the color of light – we look greener
  • Biosignatures

30.4: The Search for Extraterrestrial Intelligence

Interstellar Travel

  • What is the cost of interstellar space travel? – to travel round trip to the nearest star at 70% speed of light is several hundred thousand years worht of total US electrical energy consumption
  • UFOs into NFOs and IFOs
  • People have difficulty accepting human accomplishments

Communicating with the Stars

  • Light travels through space the fastest – reaches the nearest stars in only 4 years
  • Radio waves are best for communciating – no interference with interstellar dust and gas

The Cosmic Haystack

  • SETI – search for extraterrestrial intelligence
  • Earth sents out a flood of adio signals
  • Ultra-short laser pulses can carry messages
  • Who speaks for planet earth?

Philosophical Relevance

Personal notes

  • Ptolemization (reactionary) vs Copernican (revolutionary, new coordinates)
  • Ptolemization requires an increasingly complex series of mecahnisms to accomodate reality
  • Eratosthenes: measure the world with an angle (particularity to understand universality)
  • Horoscopes: the arbitrariness of the scientific factors we choose to determine ourselves.
  • Astronomists adopt the fictitious celestial sphere to literally define the coordinates of the sky.
  • We need a star to measure and mark time
  • Calendars: the necessity of time and the flacidity of skipping time
  • Lunar eclipse: anyone who can see the moon can watch it be eclipsed by the ground they stand upon
  • In the discovery of Pluto, initially it was thought that a mysterious body must have been influencing Neptune because of reported anomalies in Neptune’s path. However later it was shown that Pluto could not have influenced Neptune and that these recording anomalies were likely false. This is a sort of negation of proposition which in turn renders a positive result.
  • Sublimation – from solid to gas.
  • Recursive composition, fusing, etc. in the growth of a star