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Lecture Notes

ASTR 101


Table of contents
  1. Week 1 Monday – Introduction to Course
  2. Week 1 Wednesday – Patterns in the Sky
  3. Week 2 Monday – A Universe in Motion
  4. Week 2 Wednesday – Physics and Gravity
  5. Week 3 Monday – Terrestrial and Jovian Planets
  6. Week 3 Wednesday – Asteroids and Comets
  7. Week 4 Monday – Chaos and Contingency in Planetary Systems
  8. Week 4 Wednesday – Measuring and Interpreting Light
  9. Week 5 Monday – The Sun Amongst the Stars
  10. Week 5 Wednesday – Star Clusters and Stellar Ages
  11. Week 5 Thursday Section Notes
  12. Week 6 Monday – Dwarfs, Giants, and Gold
  13. Week 6 Wednesday – The Compact Corpses of Stars
  14. Week 7 Monday – Black Holes
  15. Week 7 Wednesday – Galaxy
  16. Week 8 Monday – Life in Our Galaxy
    1. Week 8 Wednesday: Galaxies
  17. Week 8 Monday – Dark Matter and Large-Scale Structure
  18. Week 9 Wednesday: An Expanding Universe
  19. Week 10 Wednesday: The Deep History of the Universe
  20. Week 10 Friday: Looking Back and Looking Forward

Week 1 Monday – Introduction to Course

  • Read the syllabus
  • Set up textbook
  • The universe and stuff

Week 1 Wednesday – Patterns in the Sky

  • Earth > solar system > milky way galaxy > local group > local supercluster (Laniakea) > the universe
  • Anasazi Sun Dagger petroglphy, New Mexico
  • Organizingt the Olympics – getting people from many different places to come together
    • Solution : use states of celestial bodies as markers
  • Astronomy is practiced in essentially every culture. You can’t hide the sun, the moon, etc….: celestial bodies are our universalities
  • Cultural history of astronomy occurs in constellations; stars stay together over time; the stars don’t appear to move w.r.t. each other.
  • Different interpretations of constellations: Orion re-interpreted as a hand by the Lakota
  • Stars are in fact moving at very fast speeds; but their movements are hard to detect and are in fact unnoticeable.
  • Various celestial bodies appear to rotate around the Earth in circular ways
  • Sun rises in the east and sets in the west
  • East – where things are rising
  • Longer patterns of motion and change – phases of the moon; month – moon; moon goes through noticeable changes in its resolution around the Earth
  • Gets brighter from the right hand side and then disappears towards the left side.
  • You can read by the light of the full moon; biological behavior adjusts
  • Fill in tables on rise and set time of different moon quarters
  • The motion of the sun changes location over time in clear ways.
  • Different constellations have different visibility at different times of the year due to the position of hte planet around the Sun
  • Eclipses are about shadows
  • Penumbral, partial, and total lunear eclipses
  • Red moon – the only light which hits it is light from the sun which is bent through the atmosphere and casts a red light on the moon.
  • Most of the tie, the moon is either above or under the penumbra. The moon’s shadow is small, the earth’s shadow is large: explains the difference between lunar and solar eclipse frequency.

Week 2 Monday – A Universe in Motion

  • Patterns we experience in the sky – clusters and patterns of motion which clearly define important time units
  • On most new moons, the moon’s shadow doesn’t hit the earth. You only get eclipses when everything is aligned well
  • Ancient civilizations often documented sky objects
  • Venus is unusual because it follows a different path htan other objects – it wanders around
  • Weird back-and-forth movement of Jupiter and Saturn
  • 7 things which move in the sky – 5 planets and the Sun and the Non
  • Babylonian cultures – noticed stars were wandering around ‘us’, centered around the Earth. The planets ‘care’ about us
  • Astrology – deep influence on culture and history. We have lots of astronomical data becuase people held astrological significance for these celestial bodies
  • Growing mythology in the Eastern Midterranan : a model of the universe in which the Earth sat inn the center with planets on giant wheels which spin around the Earth.
  • Aristotelian geocentric model of the solar system
  • Need to successively complicate a geocentrered theory: Ptolemization
  • Why doubt the world?
    • Parllax
  • The universe is spinning too – the entire solar system moves a 1m km per hour.
  • Our galaxy is being gravitationally pulled to other members of the local group, like M31.
  • All motions moving under gravity.
  • In every moment of your life, you are in a diffeent place – you can’t go home again
  • Relativity

Week 2 Wednesday – Physics and Gravity

  • Geocentric Ptolemaic model of the universe – the Earth sits in the center of nesting shells
  • Copernican model – a heliocentric revolution of the model (late 1500s, 1540s in Northern Europe)
  • The idea of the earth moving aroud the sun is in fact an ancient idea
  • If the blue dot is the earth, nearby stars compared to more distant stars should appear to move back and forth, taking one year to go around. This really does happen – parallactic motion – but it is too small to see this with the naked eye.
  • The closest star to the Earth beyond the sun – Alpha Centauri – really does move back and forth, but about the width of a human hair at an arm’s width.
  • Many people thought this was the issue which held back tot he heliocentric idea.
  • However most people thought that if the Earth was moving wouldn’t we feel a big wind blowing all the time – why would we not fall off of the Earth?
  • Ptolemy – the Earth must be the center of the universe – if the Earth is not at its center, then it must be in motion. It is therefore obvious that the Earth is not in motion.
  • Lots of old ideas intellectually assaulted in the 1500s – new fields of people in science
  • Galileo Galilei – refute basic tenets of Aristotelian physics.
    • Ambituous
    • Wanted to do big things
  • Aristotle – things want to be at rest. Galileo shows that objects do not come to rest because they want to but because there are frictional forces.
    • A pacification / de-agenting of objectss
    • Newton plagiarizes this observation in his first law
  • Most of Galileo’s contributions come with the users of the telescope
  • The moon looks like the Earth in a particular kind of way
  • In a few weeks Galileo was able to conclude that moons orbited Jupiter
  • When Jupiter was moving through the sky, the moons kept up – they weren’t left behind, they just kept on moving in this way
  • The view of Venus from Earth in the Ptolemaic system is insufficient. Venus at least orbit the sun.
  • Johannes Kepler – 1596, published a defense of Copernicus’ ideas. Worked to defend the heliocentric model.
  • Uses the planetary observations of Tycho Brahe to develop laws of planetary motion – uses ellipses instead of circles to describe how planets orbit the Sun.
    • Letting go of the circles
  • Kepler’s second law: planets sweep out equal areas in equal times – precise description of changing rate at which planets move in their orbits
  • Kepler’s 3rd law – extends the second law: if you’re always closer to the Sun, you’ll move faster than a farther planet – extended version
  • Kepler was able to produce the most accurate predictions of planetary motions ever – noticeably more than those of the Ptolemaic model or the original Copernican model.
  • Between 1550 and 1650, most astronomers shifted from believing the geocentric model to the heliocentric model.
  • There is no such thing as stillness – the death of staticity – there is only motion – all we do is move, move through time and space
  • There are no ‘objective’ coordinates
  • Why is this happening? The Earth is moving around the sun, but why?
  • Keppler tried: maybe the sun is a big magnet (magnetism recently discovered) pulling the Earth around the sun
  • Isaac Newton – formulated many of the ideas of motion and mass which woudl lead to gravity
  • Change in direction is an acceleration – you have to do something to change direction.
  • Newton’s three laws of motion
    • #2: force is mass times acceleration
    • #3: for any force there is alwyas an equal and opposite reaction force
  • The moon is accelerating because it’s changing direction all the time – this means that there must be some unseen forth operating on the Earth
    • This also means by law #3 that the moon and all objects act a force out
  • The smae force applies to apples on Earth and the moon around the Earth.
  • The moon is always falling around the Earth
  • Newton utterly breaks the old Aristotelian system, changes how we think about the universe. Before Newton, we could not study the moon – the idea that you can understand the moon by studying things carefully on the Earth you can make conclusions about things which you cannot directlye xperience: it opens up the universe to us for human understanding.
    • Interesting statements about universality.
  • Force between two stars depends on their mass: what is the ratio of the masses betweent them? This gives you the ratio of their strength, and that’s all you need to know.
  • Stars barely move because they’re so much smaller than the planet. Rotation around center of mass.
  • Mass / volume = density
  • Water has a density of 1 gm / cm\(^3\)
  • People have approximately a density of 1 gram / cube centimer
  • We can measure mass and radii of large bodies, and therefore their denities, from which we can guess what materials they are made of

Week 3 Monday – Terrestrial and Jovian Planets

  • Materializing the world – a new universality
  • Patterns in planets – ‘the big worlds’
  • Why is the universe so different? A large range of compositions
  • All the planets lie in the same plane
  • Same direction of rotation (almost)
    • Venus rotates in the other direction
    • Uranus rotates on its side; a day is longer than a year
  • Solar nebula model: like we see nebula in space, some of which are the sites of star formations
    • Nebula – a formation of dust and gas in place
    • Solar nebula is a good idea
  • Orion constellation
  • Forming stars are surrounded by disks of gas and dust
  • HL Tau: a disk formed around it with some gaps, where the mateiral is coalescing to form gaseous planets
  • You have a slowly spinning interstellar cloud (nebula) which is collapsing; it inevitably forms a dik.
  • Material falls in along the axis of rotation – rotation retards collapse in the plane perpendicular to the axis.
  • Major difference between the terrestrial planets and the jovial planets – their masses are much higher than the terrestrial ones
  • These worlds are not made of rock and metal – mainly hydrogen, helium, hydrogen compounds
  • We have visited all the planets at least once – 50 years to take a photo fo all the planets
  • Terrestrial planets are mainy rock and metal
  • The Earth is mostly rock – the atmospheres are very thin. Mercury and the moon basically don’t have forces which create interesting erosion on Earth and Mars
  • Moon and Mercury are littered with craters, and there is noe rosion or plate tectonics to renew the surface
  • Atmospheres are merely thin skins on solid, rocky surfaces
  • The Jovian planets are basically all atmosphere – no surface
  • It is believed that there is a ‘surface’, ‘core’ of highly pressurized gases
  • Very different compositions
  • Why are the planets like this?
  • In the conditions of the solar nebula, we have accretion – individual dust grains which begin to build up in size. Planetismals then something large enough to be round with gravity
  • Kep difference is where it forms. The Sun in the middle – it is very hot near to the sun, and very cold farther from it; so there ar edifferent choices for material. You have metal and rock which can be high solids near the sun. (Rocks and metals are actually quite rare galatically). Only choices are rocks and metal. But then you can have water ice; farther, you get methane ice, nitrogen, etc.
  • Protoplanets in the solar system can grow larger for gravity to begin attracting hydrogen and helim gas (most abundant materials in the solar nebular). Worlds go from super-sized terrestrial planets to gas giants
  • Gas capture phase – formed farther from the Sun; there is ice needed, which helps you to get large quickly and to collect large atmospheres. Terrestrial planets: can only form from rock and metal.
  • Jovian planets collected many moons – brojught a lot of solid material
  • Galileo discovered Jupiter’s Galilean moons – objects rotate around Jupiter
  • Io has fascinating geography: one of the oly bodies in the solar system which has active volcanoes. Most geologically active system; Io has a lot of sulfur; molten sulfur lakes on Io
  • Tidal heating
  • Europa – also has tidal heating, but not to the extent of sulfur volcanoes; enough to keep it warm. Europe also has a lot of ice. Europa is possibly covered in ice flotating on top of water.
  • Titan – it looks featureless, but bc it has a very thick atmosphere. However we landed on it. We see lakes of methane and rocks (made of ice).

Week 3 Wednesday – Asteroids and Comets

  • Addition of ices in the outer solar system means that worlds which start out as small amount of rock grow very quickly that they develop gravity in the solar nebula and begin to clear out an area of the nebula by sucking out gas
  • Titan – Saturn’s planet, very large moon
  • Every jovian planet has a ring system
  • Saturn has obvious rings which have been known for a long time; Galileo observed something was off; Christiaan Huygens observes Saturn through a Saturn-year and sees with the different profiles that it has a ring at a tilt; sometimes e see it at an angle or edge-on.
  • Saturn’s rings have small moons
    • F-ring: small shepherd moons in the rings help keep bands in check – to keep them in a flat structure, carving out gaps in them
  • Rings are very thin – less than a few hundred meters, which is very very narrow – partly because these moons keep things in line
  • Rings are made of many tiny ice particles which orbit Saturn
  • Ring particles can easily become charged and interact with Saturn’s magnetic field; therefore it begins to rain down on Saturn
  • Rings are not going to last more than a million years – either we’re very lucky or there’s new material going into the rings.
    • Rings are transient, which implies a short lifetime – what happened here to create these rings?
  • All Jovian planets have ring systems – it’s just that Saturn has very well defined rings.
  • Jovian planets have collected many moons; if rings are created by moons coliding and creating debris, then that makes sense
  • Saturn’s large rings may come from a particularly large impact, but the origin isn’t entirely clear
  • Jupiter gravitationally dominates a lot of the solar system
    • Comet Shoemaker-Levy 9 crashes into Jupiter, producing dozens of impact sites – massive explosions of energy
  • 40 meter object explodes in the atmosphere destroyed over 2000 sq km of forest.
  • Dinosaurs almost certainly went extinct because of an impact in the Gulf of Mexico –
  • Most impacts are small
  • Meteriorites hit the Earth ever few minutes and prroduce shooting stars
  • Every few years, we will eencounter a ad ‘
  • Any chunk of thing which falls into the atmosphere is a mteor; most likely degree
  • Most meteors are made of bits of astreorids and comets
  • Jupiter has its own colllection of asteoirds
    • Just approaching Jupiter forced Shoe-maker Levy 9 to break apart – shear forces getting to Jupiter
  • Comets are full of volaties (ices), frostline where ice sublimates – particles get pushed apart and form comets of their own. The tail isn’t streaking through space and stuff is falling behind – the tail is being carried away by the sun’s win; particles are now in a different orbit than the original comet
  • Churyumov-Gerasimenko – you can jets of material evaporating from its surface and steaming away into space
    • Rosetta mission – has a very weak and constructed shape
    • What do we do if one of these come by?
  • Asteroids are generally made more of rock and iron, but are loosely held together by gravity – well modeled as bubble piles. You’re throwing small stuff together; stuck for now but not for the future
  • Earth’s atmosphere is fairly thin – it won’t protect from large bodies
  • Few comets ever enter the inner solar system and are unlikely to strike Earth; most are found in the Kuiper belt beyond Neptune
  • Kuiper belt – area of many small rocky icy bodies which have been put out by the Jovian planets – leftover planet making material
  • Oort cloud – far away from the solar system is a lot of icy bodies thrown in a huge area with a spherical distribution; theorized to exist because comets have a lifetime; they get small everyt ime they come by the sun but we stilll see them; where are they coming from? Idea – very large cloud of material which holds material getting thrown into the solar system.
    • Things which are disc shaped are disc shaped because they come from the solar nebula
    • In close encounters getting thrown over th eplace with the jovian planets, we get spherical shapes; most of the solar system has been cleared out
  • Most asteroids are in the asteorid belt, but they are not that closely packed.
    • One of the few places in the solar system where you can throw in a rock and it can not get thrown around byt eh planets.
  • Near-Earth Objects – orbits cross with the orbit of Earth at some point.
  • Most 1km+ asteroids have been discovered.

Week 4 Monday – Chaos and Contingency in Planetary Systems

  • Rings are produced by asteroids and comets running into the moons of large Jovian planets – hits one of the moons and that kicks up enough dust to form a ring / contribute to a ring.
  • Comets reveal chaos in interactions with Jovian worlds.
  • Short-period comets tend to be prograde; long-period comets have no clear plane or preferred direction of rotation – long-period orbits show randomness clearly.
  • Comets are out there in a random spherical distribution because they are the comets which missed Jupiter and the moons – which almost ran into Saturn but different, and so their orbits got jacked off, now they get fucked far into the solar system due to contingent chaotic events.
  • Chaos and contingency – a comet which gets close to Jupiter – become a ring; if you miss, out into the Oort cloud.
  • The Oort cloud is pretty much it for the solar syestem
  • We need to look at other planetary systems to consider planets around other stars – can the model of solar system explainw ahtwe see in other planetary systems?
  • Stars like the sun are much brighter than planets.
  • Planets are a billion times dimmer than stars are – so planets are very hard to see bc they are so small and dim
  • We don’t find planets by finding planets: we find planets around stars by looking at the star – let the star reveal the presence of a planet – the center of mass of the sun changes a little bit – wobbles.
  • 1995, first exoplanet discovered – 51 Pegasi, routinely moves towards us, has a 4-day orbitla period around its parent star
  • Gravity – the planet is pulling on the star, pulling the Sun. You learn there is a planet and its gravitational pull, then its mass. We can calculate mass given the orbital speed.
  • Transit method – if a planet orbiting another star passes often between us and the star, then the star temporarily dim for us. This has to happen repeatedly before we can establish significance.
  • Not mass, but size (diameter) given by transit method
  • Transit methois imortat []
  • Large telescopes
  • Most plaenete do have solar systems – thea verage number of plaents per straris more than 1 – most stars
  • Planetary systems are very complex and basically form around all stars
  • Our model makes sense wutg terrerp’0
  • We can start with el= == a freckly.
  • Bizarre characteristic os fextrasolar planets
    • Semi-major axis by eccentricity: very low eccentricity in our solar system.
    • Jovian-type planets orbiting closer to their star than the Earth does – does not seem to line up with solar nebula theory
    • “Hot Jupiters” orbiting other stars – as close as 0.03 AU to their stars, orbits as short as 1.2 Earth days
    • Why? Not that planets formed close tot hose stars, but they formed far away from the stars by the solar nebula model and then they started moving around. Planets clear out gaps in the solar nebula; but there’s still material; if there is more material on one side, it begins moving inwards over time towards the gravitational pull of the side material as it clears out material
    • Gravitational forces can create all sorts of chaos, including migration, mergers, and highly eccentric mergers
  • If a body just misses a body, its orbit can change significantly
  • We believe that Jupiter and Saturn moved inwards and then things shift backwards again – this kind of moving around happens in our solar system in subtle and dramatic ways
  • Earth moon formation – formed that something hit us – if it missed us, no moon: the absolute contingency of the universe
  • Philosophically: but what is contingency, really?
  • Venus may have been hit so far it is spinning backwards; Uranus tilts over by 98 degrees
  • It takes studying other planets to understand the way our solar system is laid out is fairly arbitrary adn contingent – it’s almost random chance. There are simple rules – but once you start putting it together, what you get depends a lot on what happened.

Week 4 Wednesday – Measuring and Interpreting Light

  • Moving into the second act (first act – here on Earth), leaving our homes behind
  • We can undrstand distant objects,b ut we just need to know more about light
  • We know a lot more aobut how light works
  • Light – a way that energy propgates around the universe
  • Wavelength
  • Frequency: how many waves pass per unit of time
  • No matter its wavelength or frequency, all light moves through the universe at the smae speed.
  • However becuase red has a lower frequency than blue, more blue waves move in space than red waves in any period of time or space
  • Light shakes electric and magnetic fields – hsorter wavelengths pack a lot more punch
  • Your eyes are sensitive to a very small range of wavelengths – if the energy is too low or large your eye cells aren’t being activated.
  • X-rays (ultraviolet): higher frequency than visible spectrum; radio waves, microwave radiation (infrared): lower frequency than visible spectrum
    • Infrared waves can be very long – meters or km long.
    • Ultraviolet – slightly higher energies than visible spectrum
    • Gamma rays – even higher frequency
  • Light which I see is reflected light – dyes reflect certain colors and not others
  • But I am also making light – I have a certain temperature hotter than the room around me, and I am producing infrared light, which we think of as heat, but it is really radiation
  • Light interacts with different things in very different ways
  • Light is based on temperature: molecules banging into each other, producing radiation, which leaves my body. What sort of glow you make depends on how much activity there is, and therefore how much energy they produce
  • There is a form of radiation that all things which are not transparent make which depend on their temperature
  • Smooth and continuous without break: continuous radiation
  • Hotter objects produce more light per area unit at all wave lengths, so the peak of hotter objects move to bluer colors at higher temperature
  • THings look good becasue they;re produing more blue than not blue
  • Electrons in any atom respond in a predicable way to particular energies
  • Every elment prouced a ‘barcode’ when exposed to light
  • Bombining continuous spectrum and birght light p specitrum – e=we that deelat proccesssssnt
  • Inteferes with motion: Doppler Shifrt / redshit – sounds waves are pressure waves – shacking air in the room – shaky air shakes the ones in y
    • Higher patch with higher frequency and shorter wavelength
    • Lower frequency and wavelength
  • Reflecting telescope – uses curvmrirros to eing ight to Earth; 1960s. Curr position matter
  • We use reflecting instead of refracting telescopes by size – you can make mirrors much much larger than you can make lenses – we can build 30-meter telescopes or more – a mirror just just some shiny stuff sitting at the bottom, much easier to engineer.
  • Why does size matter when it comes to telescopes? Currently planning a 100-m telescope. TWo reasons you need a big teloscope:
    • A telescope is a bucket which collects light: the bigger your telescope mirror/lens, the more light passes through it every second, so you can see fainter things. Fainter - not farther, but fainter. Sometimes things are faint because they’re far, but they can be faint because they’re small and dark.
    • Size of the telescope sets the level of detail you can see in your focused image. A big telescope improves the angular resolution of images – more detail. also called angular separation – smallest possible angle you can detect with your telescope system
  • For most time we’ve used the human eye – which is great. But the human eye ‘erases’ every 0.03 seconds or so – if we want to see something faint, it has to be bright enough to make an impression in .03 seconds, which restricts you
  • Mid-1800s, invention of photography: astronomy becomes astrophysics. You can take a picture of what is happening.

Week 5 Monday – The Sun Amongst the Stars

  • Last week – basic physics of light, different wavelengths and their ordering, relationship between light and heat.
  • Electrons in particular in atoms and absorb and interact with light at particular wavelengths.
  • We have huge telescopes today – also in space, so they don’t have to deal with atmospheric disturbances
  • How does the sun produce so much light? What does it tell us about light?
  • A basic understanding of the Sun has eluded mankind for almost all of human history
  • 1795, William Herschel – maybe the Sun was only hot on the outside and it would be cool in its interior. People living on the sun?
  • We figured out how the sun works by letting go of the sun as our sun and thinking of it as a star; we figured out how the sun works by figuring out how other stars worked. A different historical story about how we came to understand the Sun by understanding other stars.
  • How do we study more distant stars?
  • Almost all stars are so far away that they appear as unresolved points of light. What can we measure about these stars which lets us know more about them?
  • We can measure several things: how bright stars look from Earth, what direction they are moving – side to side, and also forward/backward with Doppler; their color (different wavelengths of light, UV vs Visible vs Radio – spectral energy distribution); absopriotn/emission lines present in the spectra
  • We want to use these observations to know six interesting physical quantities: luminosity, distance, mass, radius, temperature, age
  • Luminosity: total amount of power radiated by a star into space as measured at some fixed distance from the star’s surface. Not how much do we see, but how much is it really putting out (what if I put a bag around the star?)
  • Apparent brightness – the amount of a star’s light which eventually reaches us per unit area here on Earth.
    • \[= L / 4\pi d^2\]
    • Formula which gives both luminosity and distance
  • Measuring the apparent birghtness of a star is ‘easy’ – very straightforward.
  • Given that apparent brightnes is trivially accessible, we can get either the luminosity or the distance and we get the other for free; they are stuck together by measurement definition.
  • The earth wobbling around the sun is how we can measure distance – parallax, as the Earth orbits the sun, nearby stars should look like they are moving back and forth compared to background stars. Angle that the star appears to move – the parallax angle. The star’s distance is given by \(\frac{1}{p}\) if \(p\) is measured in units of arcseconds
    • parcsecs: one parallax arcseconds, about 3.26 light years
  • We have lots of parallax and now we have luminosity.
  • Can we know the luminosity of something and then get the distance? Yes – the standard candle method (a candle out in the distance with certain regularities)
    • If I assume that a star has the same luminosioty as the sun from assumptions about mass, diameter, etc.
  • Mass of a star can be measured from here on Earth, only by watching it move by gravity – we have to see it move under gravitational forces and that gives away its mass.
  • We can also figure out the diameter given ecclipsing of a body over a star – if it’s a bigger star, it’ll take a longer amount of time to move across.
  • Most stars have another star in orbit around them – not rare in any way
  • However this method requires an eclipse, so we can’t measure their sizes. Being able to make the eclipsing inbary star measurement is very tricky.
  • We often estimate a star’s radius by estimating the radius using basic physics: if you understand how much light is being produced, \(L = 4 \pi R^2 \sigma T^4\) – depends on the size of the star and its temperature – bigger stars put out more light; hotter stars emit more light at all wavelengths
    • If you want to figrue out radius from temperature and luminosity – the temperature is very important, taken to the fourth power. Small erors compound. You need to know the temperature very carefully.
  • Temperature – blue stars are hotter than red stars. But it doesn’t put a number on it. But you can get a gauge for temperature, but it svery inaccurate – plus or minus several hundred degrees Kelvin. Instead, you need to turn to the details of absoruption lines seen in our sun. Absorption lines – tell us something about the cheimicals in the atmosphere of the star.
  • Comapring spectra across stars - not all star spectra look like.
  • All stars have basically the same composition. THey are all roughly 75% hydrogen, including M-type stars which don’t show Hydrogen absorptiospectrums.
  • Gaposchkin in fact believed that shew as mistaken that all stars have the same composition.
  • It depedns on the temperature – very hot stars can prevent ionization; iron at low temperatures, can’t make the elecrtrons move.
  • Some planets are so cold that titanium and warera == = can go together uneder cold
  • OBAFGIG – quite detailed, patterns of absorption line – better review temprare’s . Can be crazy precise if you measure these
  • Spectral type really tells you the temperature. O B A F G K M – oh bitch ass fucking gorilla king man

Week 5 Wednesday – Star Clusters and Stellar Ages

  • Apparent brightness relates luminosity and distance
  • You can measure distance via parallax, you get luminosity for free
  • Knowing luminosity and measuring apparent brightness via standard candle method. Standard candle becomes almost only method when parallax doesn’t work
  • Only way to measure mass of a star is by watching it orbit. Stars in orbit around each other
  • There can be very complicated systems – strs bound under gravity orbiting mutually. Using basic Newtonian gravity you can figure out masses
  • Radius can be measured directly with binaries that eclipse each other w.r.t. Earth
    • However for the most part we use laws of thermal radiation to estimate the star’s size.
  • Temperature – basic ideas of temperature from color (blue hotter than red)
    • More importantly, we can measure the spectral type – looking at the absorption lines we see in their spectrum. Seeing hydrogen asorption lines – very specific temperatures produce certain absorption lines.
  • Hertzsprung-Russell (HR) diagram: plot out temperature (spectral type or temperature) by brightness (luminosity)
  • Middle sequence – stripe running from the upper left to the bottom right.
    • Over 90% of all stars lie on the main sequence; most stars spend most of their lives generating their luminosity in a similar, temperature-dependent way.
    • Clear relationship between birghtness and temperature for almost all stars
  • Over half of all stars are M dwarfs; although G type stars appear common, M dwarfs
  • Dwarfs are hard to see – things are hard to see and we can’t understand/appreciate them
  • Proxima Centauri – spectral type M, we can’t see it with our naked eyes: it would have to be in the solar system for us to see it. We don’t see almost all of the spectral type M stars.
  • Why are there so many M dwarfs?
  • How do stars change over time? Is there a process by which they change which is differential for bright vs dim stars? How do you figure out the age of a star?
  • You have to look at a whole bunch of stars and try to figure out which ones are young and old, then try to piece together a story of stellar evolution. It becomes a demographic problem, studying large groups of things.
  • “Open cluster” (PLeaides), clusters of stars tend to contain hundreds of thousands of stars; there is a lot of open space in between the stars; there is also gas and dust in between stars. Open clustes are different from…
  • Globular clusters, have a pronounced spherical globe-like shape. Round shape rather than open cluster. Highly concentrated, a lot more stars. Very dense. Very little gas or dust between each star.
  • Both populations are important for two reasons, two simplifications for the study of a star
    • While clusters are big, about every star in the cluster is approximately the same distance from the Earth, so we can ignore the distance and measure apparent brightnesses, and treat them as if they were luminosities.
    • Ignoring distance specificity helps – you don’t have to worrk about distance; apparently brighter stars are really more luminous, not just closer to us. This lets us plot a reliable H-R diagram of the cluster.
    • All of the stars in a cluster formw ithin 100m years of each other, because of the effects of radiation from newly formed stars – the formation of stars stops more stars from forming – you have a burst of star formation which stops itself
    • All tars are basically the same age
  • H-R diagrams are very different between each other. There are no O and B type stars in globular clusters
  • Another trend in the main sequence: relationship between mass and brightness. More massive stars have higher luminosity
  • Mass is a form of energy, just like other forms of energy – bundled up in particles ith mass
  • If you break apart an atom into other atoms, you will every time change the mass. Breaking apart – fission. Adding together: fusion.
  • Did you make the mass more or less? If you make the material heavier, you need to put in energy; if you lose mass, then you generate energy.
  • Stars have sufficient temperatures and pressures to cause four hydrogen to fuse into 1 helium nucleus via proton-proton chain. The Helium nucleus has less mass than the 4h nuclei, and so the lost mass energy is converted into neutrinos that stream away from the sdtar’s core.
  • Stars don’t blow up because they are so massive – mitigates and resists the flow of energy in a meaningful way.
  • How much energy is putting out? This sets the energy of a star. A more massive star has to generate more energy, putting out more energy, and that means that they’re bigger – blow up larger because there’s so much energy coming out.
  • Burn trhough hydrogen much faster than smaller mass stars. Turning hydrogne fuel into helium. Bigger stars have more hydrogen
  • Mass-lifetime relationship between stars in the main sequence. A B dwarf goes through fuel only in 32m years, F dwarf lasts 1.8 billion years, G dwarf 10 billion years, M dwarf 56 billion years
  • Whya re there so many M dwarfs? They just have longer lifetimes. The universe is only about 13 billion years old; every M dwarf ever made is still here.
  • If you know how long stars of different spectral types can stay on the main sequence, you can look at different clusters of stars and figure out how old they are.
  • On the main sequence, stars on the lower right are older
  • 6k degrees – about the same temperature as the Sun. Stars which are a little bit bigger than the Sun are still here – so a cluster needs to be less than 10 billion years old – between 2 and 10 billion years old.
  • By comparing stars, we can tell which stars are younger vs older.

Week 5 Thursday Section Notes

  • \(B - V\): higher is redder (presumably ratio between B and V – V being read)
  • \(Vm\) magnitude: higher is dimmer
  • \(Vm\) correlates with \(B - V\)
  • We can only see as dim ass 6-magnitude
  • H-R diagrams are analogous to color-magnitude diagrams
  • H-R: spectral type against luminosity
    • Less massive to most massive: bottom right to top left
    • Smaller to larger: bottom left to top right

Week 6 Monday – Dwarfs, Giants, and Gold

  • How do we know how th sun works? By studying other stars, lookingat the main sequence – explain relationship between temperature and brightness
  • Internally, all stars are turning hydrogen into energy such that mass correlates with luminosity
  • Properties of the main sequence
    • Why are there bounds on the stars?
    • Light coming out of some stars are so intense that you can’t make them bigger: so bright that they are basically falling apart, they lose mass instead of gaining it because they’re too bright – they fall apart, self-limiting. Have more gravity but the excess of light overcomes this.
    • At the low mass end, collapsing protostars are not massive enough to generate core pressure needed for fusion.
    • 8% of the Sun’s mass or less – not enough gravity to make fusion happen.
    • Brown Dwarfs
    • Main sequence stars stay the size they have because fusion pushes back gravity. If brown dwarfs don’t have fusion, how do they resist gravity? Why don’t they collapse?
    • Degeneracy pressure
      • So far, talked about gravity, radiation pressure of light, thermal pressure of heat
      • Degeneray pressure only comes out in specific situations, tied to subatomic properties of particles. For many types of particles
      • Electrons need their own ‘chair’; you can’t stack them in the same space at the same time
      • Gravity crushes the brown dwarf until electrons start ‘running out of chairs’ and start moving faster, having higher energies than the temperature would suggest. Electrons are forced into higher energy states. When degeneracy pressure comes in, things begin moving beyond temperature. These particles insisting on having their unique space creates another source of energy
      • Pressure
      • Stars end up freezing to death in spaces and emptyiness
    • At a very high temperature, you can fuse Helium into carbon, which is slightly more massive and releases some energy – \(10^8\) k degreeses needed
  • Once the core starts fusing carbon, it stops collapisng because the core stablizies – there’s new information
  • Core stops shrinking
  • horizontal branch in the top left – turning helium into carbon in their cores. More decidedly
  • Helyium into carbon, hydroge ijnto helum*. This is where we reach a major inlecition point’. How massive was the staros tarts are less
  • Dont have anything to dow tih planets
  • While the star is losing its atmosphere out into space, the carbon core continues to shrink under the voice of gravity until finally electron degeneracy pressure – the same thing that holds up brown dwarfs comes into play – the carbon-rich core becomes held up by electron degeneracy ppressure.
  • What aboutt he O type stars whicha re bigger than 8 x star mass?
  • Gravitational paper on carbon-rich core allows for the fusion of higher eleemnts
  • Becuase of all this external emergy, outer atmospheres cool off, etc. – go through red giant phase, except they are so bright that we call the supergiant stars.
  • THe sun could lose a lot of its energy mass – stars look a 10t o 90 percent of their ttoal mass to avoid it fro the very hot inside’- Stability continuous. Fution inside the core is working with more complicated elements. Iron consumes energy, it doesn’t produce energy. Iron makes it harder for fusion to happen – it consumes energy.
  • The core begins to build up a very distinct onion shell where deep in its shell you have more and more complex astrmonomy – an iron core with nowhere to go
  • \[p^+ + e^- \to n + v\]
  • iron core begins to shrink on itself and reaches mind boggling conditions.
  • Stops being iron, turns into neutrons, which releases a lot of neutrinos (energy in the form of neutrinos)
  • Core stops shrinking when the neutrons get so tightly packed together that degeneracy applies to them. However it still drops a lot of heat; the collapse superheats the neutron-rich core . Blows apart in the process of a few hours – supernova
  • So energetic and violent that many bizarre fusion processes happen.
  • Produces a bunch of radioactive nickel which decays into gamma rays; produces a lot of radioactive light; produces large amounts of elements
  • We didn’t know where the elements came from – all of the elements come from the explosion

Week 6 Wednesday – The Compact Corpses of Stars

  • What happens when a star leaves the main sequence and hydrogen in the core runs out?
  • Low-mass stars end their lives with a carbon-rich core and blow their stuff out into space in a planetary nebula – it blows itself apart very gracefully over 1 million years
  • Higher-mass stars blow up in a day – a huge explosion, very violent, such that very heavy elements are fused during the explosion itself. Most of the periodic table is made during these very rare and violent explosive events.
  • What happens after the explosions, after the planetary nebula phase?
  • Cores of stars – what is left?
  • Brown Dwarfs: not main sequence stars. Supported from its inside by degeneracy pressure, the need for electrons to occupy their own space. This keeps gravity from crushing brown dwarfs into a black hole
  • What is the difference between a really small brown dwarf and a really large planet?
  • Degeneracy pressure doesn’t go away – an object held up by degeneracy pressure is basically always stable
  • The core which is left behind is held up by degeneracy pressure – it is what supports the cores b/c there’s no more fusion going on.
  • Low-mass stars: the core stops collapsing hwen degeneracy pressure takes over and the carbon core is stable; this is a white dwarf. It has a very hot temperature, basically the exposed core of a star – very hot
  • The escape velocity is the same as the free-fall velocity when you hit the surface
  • Sirius A and B – A is a type-A main sequence star but b is a white dwarf, much brighter than A in X-rays
  • Globular clusters are older then open-type clusters
  • Globular clusters have many white stars – the high-mass stars have died and some of the low-mass stars have died and turned into whie dwarfs
  • Whenever any object is held up by degeneacy pressure, it bgegins to obbey nonstandard physical laws. More massive such objects are, the smaller they are.
  • How small can you make a white dwarf be? Once you get to 1.4 solar masses (Chandrasehkar Limit), the electrons are so small that extra velocity associated with pressure move at the speed of light and begin to behave in bizarre ways, and the degeneracy pressure becomes unstable and breaks down.
  • In binary star systems a white dwarf can accrete matter from a red giant star. Material gets stripped away from the red giant
    • Hydrogen building up on the surface can get so hot that it causes fusion, except it’s happening on the surface of the white dwarf and not the input. It denotates across all of the white dwarf’s surface in a few moemnts
    • The white dwarf becomes temorarily much brighter – nova
    • Repeating nova occurrences – small amount of mass, doesn’t disupr the system itself, but gets very bright for a handful of days
    • What isf a white dwarf was close to the Chandrasekhar limit and absorbed enough matter from a binary star to pull it in?
    • At the Chandrasekhar limit, degeneracy pressure fails, it shrinks, the temperatue rises significantly
    • 600 miillion degrees: fuse iron into other places. Hydrogen fuses into more complex aras.
    • What dwarf detones, leaving tothing behind, because it sblows up – it shrinks, heats up, and goes off as a bomb
  • So bright that the hriewdraft look slike no=’- White dwarf -
  • Are type A1a supernovas we see simple or complex? Do they grow slowly or quickly
  • 1.4 solar masses worth of carbon fuses and beats itseif up – the smae thing repeats everyt ime.
  • There is a physical similarity between all the commonality vs black and white
  • Finding the distance to one supernova helps to know th
  • White dwarf supernova are as bright as th o-=overall type of pretty much at work
  • You can know theyir idd==distnace if you can ssee
  • Neuron stars
    • Type II supernova
    • Deep inside a set of store
    • Forms during a supernova explosions – a neutral star
    • Held up by neutron degenercy pesssure.
    • Very high in radiation output
    • Neutrons can be very fast because they emerge very really violent explosions – if they are slightly tiled, they get shot out to one side
    • One way for life to be destroyed – a neutron comes barrelling into the solar system
    • Neutrons are very small with a lot of mass piled on top of them – 7 miles and 2.9 solar masses
    • You can only hold up so much mass with neutron degeneracy pressure; the mass can only be 2.9 solar masses
    • We don’t see neutron stars at this limit.
    • What happens in a high mass star is that degeneracy pressure is not enough and gets collapsed into a ball of neutrons. Nature makes neutron stars at 1.4 sollar masses.
    • Very dense – 2/3 the speed of light to get away from a neutron star
    • We don’t know what the inside of a neutron star is
  • Pulsars

Week 7 Monday – Black Holes

  • The mass of a star determines what is left behind when the mains equence star leaves the main sequence and loses its atmosphere
  • Neutron stars and white dwarfs are held up by neutron and electron degeneracy pressure
  • If you allow gravity to overcome degenercy pressure, it explodes into a white dwarf supernova, which requires mass transfer from a binary star
  • Idea of binary systems transferring matter happens with neutron stars too – neutron stars can acquire material from binary companions and we see that an accretion disk forms around a neutron star.
  • A white dwarf orbiting a neutron star – ripping apart the white dwarf – a white dwarf’s escape velocity is 2% the speed of light, but the neutron star pulls it completely apart
  • If we start shrinking a white dwarf, the carbon inside starts fusing, so it blows up
  • What happens if a neutron star shrinks? The neutrons don’t fuse into anything.
  • The surface gravity increases and the escape velocity increases. What happens when the escape velocity goes past the speed of light?
  • A very dark / dense object is actually a very old idea – 1783, John Michell and Pierre Simon-Laplace – took Newtonian relationships to derive the relationship between mass and size
  • Radius of an object with a velocity equal to the speed of light is 3 km for every solar mass
  • 1780s: changing conception in history from light as a particle to a wave
  • Could gravity affect light anyway if it was just a wave, if light doesn’t have a mass?
  • Bike and light – if I ride a bike at an observer with a flashlight at speed \(v\), does the observer see light with velocity \(c\) or \(v + c\).
    • Einstein’s answer: all observers see light moving at the same speed regardless of relative position
    • Girl and the spacecraft – viewing from outside the laser travels farther
    • Measuring time and distance differently
    • Time and distance are measured differently for objects which are in motion wrt each other
    • Time passes slower for objects in motion
    • Time is not some uniform constant thing for everyone everywhere – it is different
  • Gravity introduced via general relativity
    • Equivalence principle: you would feel exactly as though you were standing on Earth if you were in a ship boosting at 9.8 meters per second-squared
    • There is no experiment you could do inside the spaceship which could tell you if you are sitting on Earth or in the spaceshift
    • The curved path of photons in the accelerating rockets must also be seen if the rocket is just sitting in a gravitational field on Earth – light itself is affected by gravity; gravity affects light even though it has no mass
    • Gravity isn’t some sort of force-beam – it is what happens to space and time itself in the presence of mass. Everything moves through space and time – gravity warps spacetime like in a trampoline
  • Stars produce very strong distortions in spacetime
  • We see light being bent by neutron stars, whit edwarfs
  • You can get light ‘orbiting’ and never leave, get trapped in a part of spacetime – from a distance, it looks dark because no light is leaving that area.
  • Black hole – any object which can warp space and time so much that light gets trapped
  • Mass compressed into an infinitely dense point – the singularity. Event horizon – the boundary around the singularity within which no light can possibly escape.
    • Schawrzschildradius – radius of event horizon.
  • We don’t know what’s happening in a black hole – we can’t get information from it.
  • You have to get really close to black holes before they get real; otherwise it’s just like any other body you can orbit
  • All our planets would continue to orbit the sun if it became a black hole with no change – black holes don’t suck unless you get within 3 times the size of the event horizon.
  • You see moving clocks as running slower than your own clocks
  • Clocks near the top of the tower run faster than the clocks on the bottom of the tower – all that matters is how close you are to the center of Earth, meaning how deep are you in this pit of stuff
  • GPS timing – correct for quantum computation
  • Time travel – you just have to get close to the event horizon
  • Hawking radiation – right near the event horizon, it is possible for quantum mechanical properties to prdocue radiation at the event horizon itself
  • We’re not ereally sure what’s going on – it’s possible that the event horizon is full fo searing ultraviolet radiation and youa re turned into pure energy which stays on the vent horizon

Week 7 Wednesday – Galaxy

  • If the black hole forms from something which is rotating, then the blak hole must rotate
  • The planets wouldn’t change their orbits if the sun turned into a black hole
  • How do we observe black holes? Black holes are very small even though they are massive; less than the size of the Earth’s moon – small objects. We need to look at things near the black hole
  • Material falling into an object heats up the black hole; particles spinning around the black hole reach the speed of light; there are many X-rays getting away from the black hole
  • We can look at a binary system and figure out the mass that an orbiter has to have. It must align with how much light we observe. If it has really high mass but low light then it is a black hole.
  • We look at X-ray binary systems: invisible companion in a system producing a lot of radioactive energy.
  • We have figured out how the milky ways in the last 100 years – a new thing
  • Three parts of the solar system
    • Disk – 1k light-years thick (more like a few thousand thick though). Sun is in the disk, 28k light years from the center. Disk is very thin. Few hundred billion stars int hed isk of the milky way. Very large – not a solar system, a totally different beast. Disk merges with a bulge in the middle
    • Central Bulge or Spheroidal Region – 7k light years in radius, roughlhy spherical but more oblong
    • Halo – Disk and Central Bulge are embedded in this. The Halo is very large, 1m or more light years away.
  • Differences between sub-regions
    • Distribution of Inter-Stellar Medium (ISM). Empty space between stars is not empty – thre is thin grey and black dust, found between the disk and the bulge region of the milky way galaxy.
  • Radiation put out by the ISM determiens what radiation it gives out – e.g., gas leaving a Red Giant star - On the inside of the supernovae are high-speed gasses and intense radiation, carvivng out bubbles of hot classs
  • The ISM is so thick that the bubbles stop expanding and the gas in
  • When hydrogen cools further to 10k - 30k, it forms molecular hydrogen, which can give rise t o H2O, CO, NH3, etc. Where Carbon and Hydrogen can interact with each otehr!
  • Not very much energy to resist gravity – coler regions are more susceptible to gravitational pull nad stars are formed
  • if you want to form stars, you need cold gas to start with so it will
  • “Pillars of creations” – denses cores are able to resist wave of radiation passing them. At the tip of each of the pillars is a new star.
  • Star formatino only happens significantly in the disk and bulge, because that’s where the ISM is concentrated. There is little cool ISM in the halo
    • Stars are forming in the disk in the bulge of the galaxy and not in the halo.
  • Another distinction between the disk and bulge and halo: different compositions.
    • Because stars are still formingi n the diska nd the bulge, there are old and young stars. High-mass stars cause some fraction of heavy elements to populate the area.
    • More iron in the disk and the bulge
  • How do the stars move in the diffeent regions? Stellar orbits in the galaxy
    • Bulge and halo are similar: the stars in the bulge and the halo are both randomly distributed in orientations, eccentricity, etc. When you take orbits from different angles, you end up with a largely round strucuture, roughly spherical.
    • Both are roundish because the objects that mov ein them have no preferred plane; prefer orbiting aorund the milk way
    • Stars in the disk have a prferred galatic plane
    • Pull of different stars – in general just move around itn he middle of the galaxy./
    • Sun has orbited about 19 galactic-years
    • Stars don’t run into each other, pass by
    • The Sun eventually gets up above the rest of the stars, at which point the gravity of the whole disk pulls the sun right back down; there is this sort of bobbing and weaving motion of the sun; this sort of bobbing is why the disk has a thickness - fluctuation up and down.
  • Stars orbit the center of mass of the entire milky way, not a central object in the middle of the milk way which is pushing everything around.
    • We are not orbitign some highly concentrated mass per se
  • While most of the Milky Way’s mass is not piled up in teh center, we can see towards Sagittarius and the galactic center. If we do not llook through visible lgiht, in the thickest part of the area, but rather look through infrared, we can see what is hapepning at infrared wavelengths, we can see more
    • We are orbiting something at the location of the center of the galaxy – mutliple stars are orbiting around it
    • We observe that some stars are traveling very very quickly
    • How much mass do we need?
    • Supermassive black hole lying in the center of the solar system
    • Totally different from regular black holes
    • Andromeda galaxy – also a supermassive black hole there, too, probably
    • We see these supermassive black holes in basically every galaxy.
    • Supermassive black holes are so big that we can take pictures of them. Event horizon is larger than our solar system
    • How do supermassive black holes form? We don’t really know what’s going on.
    • We can also measure the shaking of space and time – gravity waves. As objects move around each other on the trampoline of spacetime, we can measure the waves which carry away energy and make them merge over time – literally space and time shaking as objects merge. Intermediate-mass range black holes
    • Supermassive black hole is not closed yet.
  • “Habitabele zoon” – ‘goldilacokcszoom- We see the planets in histb=p on
  • Proama – nearest solar sysren near to aaaa

Week 8 Monday – Life in Our Galaxy

  • Functionally defining intelligence as astronomer – understanding the cosmos
  • Frank Drake – mathematical formalism for the number of laien species in the galaxy capable of communicating ieth Earth
    • Routinely criticized because differences in how we think about the terms gives wide variation in the predicted quantity
    • Not supposed to give you an answer, but rather to break a hard question into simpler questions
  • Four factors
    • How wmany habitable planets are there in our galaxy? – capable of having life on their surface
    • What fraction of those planets ever had life?
    • What fraction of those planets ever had intelligent life capable of communicating with us?
    • What fraction of all of those civilizations currently exist?
  • We know about how many stars there are and how many of htem are habitable. There are hundreds of billions of main sequence stars. But not all of them are suitable for life.
  • Most stars aren’t high mass – live a longer time than the sun does. Still plenty of mains equence stars out there.
  • Munes of Jupyter – oceladus and other – have oeans, maybe thes=on\
  • Pretty much all starss have planets – 90% or higher have planets aroound 10 billkon aticipated habitable workl in SEPPPPPPPPPPP
  • We only know of our life in all ofr-…
  • Arew we 10m
  • Life appears tos how up everywehre on the earth – simplel eaf seesm s very robust
  • We n find the NDA between ours and neothanderals – many atrophysicists think that life probably
  • Untula few uhnde=red yearoth that gives us ufuntinoalitu=88ee;
  • We hve a very popular e’ us8ccessufleeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
  • questinon is not when will intelligent lifeaooear but this is the wrgina;If it happens then it hapens
  • How long to ciivilizaion ls ast
  • Fermi paradox
  • SETI has searched most of our galaxy but we have found nothing
  • How dow e resolve Fermi’s paradox?
  • How muc intelligent life is there? Maybe intelligent life is exceedingly rare
  • How contingent is intelligent life? - Possibly no necesary need to explore the stars
  • However there just are too many stars – most astronomers don’t believe the Earth is th eonly intelligent speciess
  • Possible answers
    • Prime directive – aliens are keeping an eye on us
    • Alien civilizations are unable to traverse interstellar space so quickly – has a lot of engineering support
    • Optimistic and pessimistic – life is common, but intelligent life is much more rare. THere are aliens but many die off, and only a few can ponder space travel. Space travel is possible but very difficult. No one has figured out how to do it yet
  • Alcubierre drive – gravity warp in front, anti-gravity warp behind

Week 8 Wednesday: Galaxies

  • There are a lot of planets out there
  • Water is not rare in the universe
  • How common is simple life? We don’t know
  • 1920s, capturing things we know now are galaxies
  • Shapley: what we are seeing is a solar nebula, and there is an accretion disk forming inside the disk. Saw they saw stars forming
  • Ohters suggested that there was instead galaxies – island universes, like the milk way but outside
  • Star inside the milky way or a different thing to begin with?
  • It is a question of distance – stars must be closer to us if they are stars forming than if they are outside
  • Cannot measure well with parallax
  • All they knew was that none of the stuff was really close to the earth
  • Standard Candles – apparent brightness, luminosity, and distance are related; if you can know the luminosity of an object, you can easily find its distance.
  • However standard candles don’t work either. But there was a better approach: Cepheid variable.
    • Stars which have left the mian sequence, going up the red giant branch
    • While these stars are red-gianting, they go through a phase of pulsing: get bigger and smllaer by a
    • Complicated about why it pulses
    • henriettra Leavitt – pulsing.
    • How long does it take to pulse? The oners that take a long time are also golder
    • You can easily get distance via luminosity
    • You can see Cepheid variables
    • Based on tru the appaee os jpweve veua
    • Giq siwa rgw ayrwj ewaponsq hwn -Almostr eceryone we sat there i
    • in sppirold al areas, cold material is May have additionl structu
  • Elliptircal galxiez tend tobem j=much rderdddddddddd Indeed, ass=ming you couldgetinto
  • What do they have in common? Ireggularity areas are very blue aslo – er]wwwwwwwf
  • Large and small magellanic clouds – dwarf irregular galaxies and satellites of the Milky War get you easily visible to tehe naked eye
  • Galaxies are not typically isolated from each other – maps galaxies have loose groups with dozens of galaxies.
  • Local galactic group – 40 or so galaxies are all in the local galactic group of galaxies. The galaxies are gravitationally stuck together, orbiting the milky way and the andromeda galaxy, which are the two largest galaxies and about the same size
  • Other groups of galaxies – Sculptor Group, Canes I Group, M81, etc.
  • Virgo Galaxy Cluster – most galaxies congregate into very large collections which form a cluster – instead of100 or so galaxies, you have 1ks or 100ks of galaxies, stuck together and meaningfully together. Difference is size: groups are small, clusters are large and have a large number of galaxies in them
  • Big galaxies tend to be elliiptical, redder-type galaxies
  • Elliptical, appear redder
  • How do galaxies form and evolve? Why are there differences? How do they form and change over time?
  • Easy trick to find old galaxy vs young galaxy. When you look at any galaxy, you can see clearly that all galaxies have old stars which are 12-13.5b years old. No stars older than that. All galaxies in the universe started forming approximately at the same time
  • When you’re looking at a distant galaxy, you are looking back in time. They are 13 billion years old; if I am seeing light from 10 billion years ago, I am seeing a 3 billion year old thing.
  • To see young galaxies, look far; to see old galaxies, look close.
  • At some point in the universe’s history, galaxies formed from pure hydrogen and helium gas
  • Top-down approach to galaxy formation. Gas clouds collapse and form globular clusters as the gas collapses and lfattens into a disk (like in a solar nebula) which ends up with something like spiral galaxies
  • Elliptical galaxies: maybe formed by a protogalactic cloud with less angular momentum and just doesn’t collapse, or it is very high density and it makes all of its stars before it has time to flatten out into a disk.
  • How do supermassive black holes form? All of these galaxies have supermassive black holes
  • Important idea – collissions between galaxies. Galaxies are packed pretty close together relative to their sides. Galaxies run into each other in a way that stars in galaxies don’t, because stars are small relative to the space between them.
  • You get giant irregular galaxies – probably formed through multiple mergers with other galaxies.
  • The big galaxies tend to be elliptical – you can see separate subcores even in some of these elliiptical galaxies
  • Milky Way and Andromeda galaxy will collide one day
  • How does your position in the univers eaffect your scientific understanding?

Week 8 Monday – Dark Matter and Large-Scale Structure

  • By studying galaxies at varying differences, you can sample galaxies of different ages
  • Groups – smaller, less dense groups of galaxies; clusters – larger, denser groups of galaxies
  • Collisions – galaxies change over time by colliding and ‘interacting’/’encountering’ with each other in ways which planets just don’t
  • Collisions are driven by gravity; we need to know the masses of galaxies – how do you weigh a galaxy?
    • You need masses and distances per Kepler to understand gravity
  • Good way to figure out mass – watch how it moves under gravity
  • You can calculate a mass needed to Push the Mass around every so many years.
  • Rotation curve – distance from center of milky way against orbital speed
  • Orbital speeds stay very high after an initial climb
  • Why is this a little weird? – orbital velocity by semi-major axis:
  • The Su m is basically all o f mass i
  • Spiral galaxies – need to be tilted towareds you, so yo can b uild BEsl
  • All the stars and thre gas and matter are in the first fifty distance from the center – at’s wheere the matrttr and stuff is.
  • You can’t see the mass beyoboooooooooooooooddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddxb
  • How much mass is in the univereal in totla?
  • Quiz question: 3 different ways that one can measure the mass of glusters’
    • You watch thing love (?)
      • Clustrers form thousands of stars
      • A few undred datrapoints give youa 4un for their money
      • d
    • Look at X-ray imafges o f a nearby
      • hot gas is trapped there by gravity, some of it has to dow ith the temperature of the gas itself; wans to leave the clusrer, requires energy
      • Want to leave the cluste rbut can’t; are heavilyc onstrained
      • Escape velocity / temperature is correlate with mass
    • Einstein’s Theory of Relativity: some light finds itself bent by the presence of masses; we can measure how much the degree is juw53e If you look ata a cluste orf galaxies and you ust cound hot many things there are, you end up with 80 percent of thr e
      • MACHOs move between each other – looks like a very temporary brightening of one star which happens once
  • Dark matters are likely composed of new subatomic particles – they interact only very weakly and with normal matter and with radiation, also nonzero mass, and a measurable gravitaional influence
    • Weakly Interacting Massive Particles (WIMPs)
  • A lot of research in dark matter happens now in computer simulations? How do initial seeds lead towards different conditions?
  • So hwo do you make a map of the universe? How do you get distance?
  • You can’t use parallax or main-sequence fitting or Cepheid variables, becuase we can’t see things farther away enough
  • New method of distance measurement: there is a relationship between how far away a galaxy is and how high its velocity is. Hubble’s Law. Relies on linear relationship by Hubble’s constant.
    • Just measure velocity for galaxies to calculate distances
    • Measure Doppler velocities, use Hubble relationship
    • How do you measure the distance some other way to make sure the line really is a line?
    • Use a brighter candle beyond Cepheids – White Dwarf Supernovae. Very bright standard candles. Can be observed billions of light years away
    • Hubble’s Lw remains a straight line even for very large distances
    • Hubble’s line is real

Week 9 Wednesday: An Expanding Universe

  • Most of the mass of the galaxy lies far out in the halo, not tightly concentrated in the center in the same way that luminous matter – somehow distributed meaningfully throughout the halo
  • For some reason this matter isn’t settling down and settling in the middle of the galaxy, it’s staying out. Gravity wants to keep things together
  • What we are seeing in dark matter is that it doesn’t even interact with itself, don’t even bounce with each other and lose energy and settle down. Just stray in these halos
  • Amount of dark matter is very large, 80=90% of the gravitating material in the universe.
  • We see lots of independent lines of eivdence pointing towards black matter – we don’t know what most of the stuff in the universe is.
  • Appears to be some sort of a Weakly Interacting Massive Particle.
  • Maybe the issue is also that we don’t understand gravity on galactic scales and there isn’t dark matter.
  • Different simulations for how galaxies are believed to have formed
  • Large maps are made with Hubble’s law, which ties a galaxy’s redshift to its distnace.
  • We can confirm Hubble’s Law with white dwarf supernovae as standard candles
  • Hubble’s law really is a line, although there are interesting changes in distances
  • On very large scales, there doesn’t appear to be super-clusters – just gets very weirdly smooth. The early universe is less structured than it is now – it was more homogenous in the distant past.
  • Outside of the local group of galaxies, all galaxies are moving away from us. The farther away galaxies are from us, the faster they’re moving.
  • Not that surprising to people in the 20s, becuase of the development of general relativity – space and time are not stratic and uniform, but distortable.
  • Einstein was deeply concerned that all of the matter in the universe would eventually contract into a giant black hole and pull everything into it; gravity is always pulling on you and it will eventually pull everything into one spot.
  • Einstein sticks in an anti-gravity term to prevent things from contracting together over time “cosmological constant”
  • Greatly relieved by the expansion of the universe; the univerese is expanding, no need to protect it form itself
  • The galaxies aren’t really moving; it’s the space and time between the galaxies which is expanding; spacetime stretches apart the empty expanses between galaxies and affects the wavelenght and freuqency of light passing through it
  • Light looks redder when it gets stretched out through spacetime
  • Gravity can overcome the expansion of the universe in some local regions – gravity is strong enough to keep space from expanding.
  • Expansion of spacetime isn’t keeping Andromeda from running towards us. Expansion of the universe isn’t happening in our local galactic group.
  • Gravity is always working to bring things together; trying to pull things together now
  • The possibility that the entire universe might have enough stuff in it that gravity pulls on that in some distant future it might recollapse upon itself (big crunch) – the universe might collapse on itself
  • All about the tension between expansion and collapse from gravity
  • Four models for the future of the universe
    • Recollapsing universe, expansion will someday halt and reverse, the big curnch
    • Critical universe: universe doesn’t have enough to collapse, but will have enough to approach a stop in the infinite future; stops expanding, slows in the infinite future until no more expansion
    • Coasting universe: the universe will expand forever at the current rate or with little slowdown.
    • Accelerating universe: the expansion will not only continue, but actuallya ccelerate with time.
  • Will the past tell us how things will change over time?
  • These models predict not onlys omething about the future but also about the future
  • Recollapsing model seems like it’s not plausible because it predicts too young an age fro the universe; also the critical expansion method; so really it is the accelerating universe hypothesis which is true.
  • For galaxies to pick up speed, we need some sort of anti-gravitational force, but we don’t know about any such force.
  • Expansion of the universe used to be slow and is now faster than it used to be; it has gotten faster over time.
  • Why would the univere be accelerating?
  • Dark energy – shot-out to dark matter as things which are mysterious that we don’t understanda bout the universe.
  • What might be causing the expansion of the universe?
    • Einstein suggests that the universe has a constant force pushing against gravity – we have put the cosmological constant back. Also just a constant, pretty easy to keep up with.
    • May be a fifth force out there – an anti-gravitational force which is very weak and we don’t observe it, but it takes over in the empty spaces between mass, and it is what takes over: quintessence. Aristotle, the magical fifth element that things are made of – now a possibility for dark energy
    • You might be able to come up with a force that explains dark energy and dark matter
    • Maybe dark energy gets so strong that electrons are pulled off of neutrons and you don’t have atoms; nuclei themselves are pulled apart, the Big Rip – everything is so stretched out that you can’t have atomic structure anymore
    • Or maybe it gets negative and lumps in with gravity, collapses and cruches on itself
  • We can only look into the past to look at the future; where did you come from to know where you will be going.
  • The sky is dark at night – this is actually very profound. Kepler points out that this shows us that the universe cannot be infinite. There would be an infinite number of stars in the background, and everywhere you look you should see a star; and so everything should be lit up everywhere
    • Kepler: Olber’s paradox, the number of stars are small, and only located close to the Earth, like a small forest – the universe might just be the milky way
    • Edgar Allen Poe proposes a more accurate solution – if the universe were finite in age, it would be finite in its observable size. Light from very distnat stars simply would not have had time to reach us since the univrse began. So actually the universe could be infinitely large, but not infinitely old. The universe must be finite in its age.
  • From this simple observation, the universe must have had a beginning. There was a time before galaxies, stars, etc. – what was it like?

Week 10 Wednesday: The Deep History of the Universe

  • Hubble’s law really does hold
  • Expansion for like 13 billionyears; by running the film backwards, we can conclude that the rate of expansion is accelerating.
  • There does not appear to be enough gravitating matter to eventually halt or rverse expansion even on a universal scale
  • Many independent measurements suggest that expansion is accelerating.
  • We really don’t know – predictions into the far future are unclear
  • The future is not accessible to us; we cannoot see the future.
  • What was the time before galaxies and stars?
  • Georges Lemaitre – a primieval atom as the precursor of the universe; how long would this take? About 13.5 billion years. Lemaitre: if things get smaller, it gets hotter; so the universe must have been really hot.
  • Hadron collider: temporarily creating very high temperatures and densities, temperatures which will basically take us to a few hundred trillion degrees, simulating the universe in its first nanosecond.
  • Particle creation: gamma-ray photons form an electron and an antielectron
  • Particle annihilation: electron meets antielectron and forms gamma-ray photons.
  • You reach an equilibrium with equaal matte rand antimatter. But only under extreme condiiton
  • Asthee universe expans=ds, it cools off; when i reaches a millisecond of waves, there isn’t enough temperature for grmmma-rays to be there, not sufficient to make new products
  • Theoretically, particles and antiparticles should have annihilated each otehr completely
  • Someohow one parts per billion fo matter sticks around – we didn’t make equal amounts of anti-matter and matter. This tiny sliver went on to become everything, including dark matter. All of this is the one little piece which is left over.
  • Evidence for this having happened – why do we think the big bang?
  • Not observational support, but why is there slightly more matter than antimatter? WHere is not a lot of antimatter in the universe;antimate rna dmatter produce high radiation, but we can’t see that much of it whne we looka round
  • before,hydrogen is merfging and complexity.

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  • Radiation from the cloud surface detectable through many different mechanisms – a better way toe xperience dark microwave radiation
  • Noise from the radio is the cosmic background rdadiation, coming at you from all directions and coming to tickle your antannae from crossing the lnegth of the observable universe
  • Radiation touches every galaxy in the universe right now.
  • Cosmic microwave background is interestingly homogenous
  • The universe was red, but now it is microwave in color.
  • The universe has been losing energy as it has expanded, so it has been redshifted; originally red visible light but redshifted way past infrared towards microwaves.
  • Era of nuclei to era of atoms
  • Universe continues to expand but the temperature is still very high temperature – but you can’t make stars from gas which is that hot
  • We have to let the univers ecool off quite a bit before we get stars and galaxies – we have a few hundred million years as the gas cools for stars to form. But we see this – the ‘Dark Age’ before stars lighted things up in a big way is also observed.
  • Four major events which should have happened
    • Formation of light elements
    • Release of the cosmic microwave background
    • Dark ages before the formation of the first stars and galaxies
    • Recession of distant galaxies as the Universe continues to expand
  • Earlier nuclear formation produced very hot matter which was very hot and opaque until it was a few thousand degrees and became transparent, a cloud forms in all directions
  • This still hot gas cooling should make stars, but we have dark ages before then.
  • In the modern day, we should still see things expanding apart
  • The evidence is overwhelming.
  • Trying to make radiation look like how i tis observed is difficult unless everything was once in a much smaller volume.
  • No other cosmological model describes the availalbe evidence remotely as well as the big bang model does.

Week 10 Friday: Looking Back and Looking Forward

  • Current expansion of the universe suggests that the universe was very dense and hot
  • Big Bang model – what would the universe have been like if it was really hot and dense? What would have happened in a hot and dense expanding universe? And we see evidence corroborating this theory
  • If it were really hot and dense, fusion would have happened everywhere and produced heavier elements from the very simplest elements. As the universe expands, it naturally cools off and neutralizes; gone from plasma to a neutral gas through which light can pas; this time in which it goe’t see through it to can see through it should be visible, these glowing microwaves (CMB)
  • It is still expanding now and you can see this is happening
  • Dstribution of clouds, expansion of galaxies – alle vidence matches the model
  • But where did the initial energy input to start the universe’s expansion come from? What was ‘before time’?
  • Where does it all come from? Maybe it is a cycle: big bang followed by big crunch
  • A lot of ideas in modrn cosmology are strikingly similar to very cold ideas
  • There is no beginning of time, there is no moment before time
  • Big bounce model eliminates a moment before time, it has always been there
  • Universe starts at some singularity and then expands on itself, dark energy flattens, heat death dies, dark energy rips the universe apart, takes it down to nothing everywhere; but in this completely empty universe, quantum fluctuations continue to produce something; so an infinitely empty universe with an infinite amount of time will eventually get a quantum level fluctuation which is big enough to make a new universe
  • Multiverse / membrane theory: our brane coexists with other branes; our universe collides with another universe in this multidimensional structure
    • No proferred time – this collision could happen at any time
  • Nietzsche and the devil