15.1 Islands of stars Chapter 15 Galaxies and the Foundation of Modern Cosmology Cosmology: study of galaxies What are they 3 major types of galaxies? Spiral galaxies: like the milky way, look like flat, white disks with yellowish blues at their centres The disks are filled with cool gas and dust interspersed with hotter ionized gas and usually display spiral arms Elliptical galaxies: redder, rounder and often longer in one direction that then other like a football Compared with spiral galaxies, elliptical galaxies contain very little cool gas and dust thought they contain very hot ionized gas Irregular galaxies: appear neither disklike nor rounded Spiral galaxies - Spiral galaxies have a disk, bulge and halo just like the milky way Disk component (population I): flat disk in which stars follow orderly and nearly circular orbits around the galactic centre Always contains an interstellar medium of gas and dust but the amount may differ from one spiral galaxy to the next Spheroidal component (population II): contains little cool gas and dust and stars have orbits with many different inclination s Barred spiral galaxies: have a straight bar of stars cutting across the centre with spiral arms curling away from the ends of the bar Leticular galaxies (lens shaped): have disk and spheroidal components but lack spiral arms Intermediate class between spiral and elliptical Elliptical galaxies - Elliptical galaxies differ from spiral galaxies in that they don t have significant disks Sometimes called spheroidal galaxies Some are called massive elliptical galaxies or dwarf elliptical galaxies Irregular galaxies - Irregular galaxies appear to be in disarray Hubbles galaxy classes - Created a system to organize galaxies into a diagram shaped like a tuning fork (page 489) How are galaxies grouped together?
- Spiral galaxies tend to congregate in small groups while elliptical galaxies are primarily found in large clusters 15.2 Distances of galaxies How we do measure the distances of galaxies? Radar ranging: technique to measure AU in which radio waves are transmitted from earth and bounced off Venus Standard candles - We can determine distance by measuring the apparent brightness of an object whose luminosity we already know and applying the inverse square law for light Standard candle: light source of a known standard luminosity - The inverse square law for light tells us how an objects apparent brightness depends on its luminosity and distance Distance = square root of luminosity/ 4π x (apparent brightness) Main sequence fitting - Need to follow 2 steps to use bright main-sequence stars as standard candles 1.) Identify a star cluster that is close enough for us to determine its distance by parallax and plot its H-R diagram Because we know the distances to the cluster stars, we can use the inverse square law for light to establish their true luminosities from their apparent brightness 2.) We can look at stars in other clusters that are too far away for parallax measurements and measure their apparent brightness s If we assume that main-sequence stars in other clusters have the same luminosities as their counter parts in the nearby cluster, we can use the inverse square law for light to calculate their distances Main sequence fitting: technique for determining distances by comparing main sequences in different star clusters Cepheid variables - Cepheid variable stars are useful for measuring distances because we can determine a Cepheids luminosity from the period between its peaks of brightness Cepheid variable stars (Cepheids): alternately become dimmer and lighter and are used to measure distances between galaxies Been used for almost a century Distant standard candles - White dwarf supernovae are useful for measuring large distance because they are bright and all have about the same peak luminosity
What is hubbles law? Edwin hubble and the Andromeda galaxy - Edwin Hubble used Cepheids to prove that the Andromeda galaxy lies beyond the milky way in the 1920s Distance and redshift - A galaxies redshift tells us how fast it is moving away from us and the relationship between redshift and distance shows that the universe is expanding Hubbles law - More distant galaxies move away from us faster V = H 0 x d V is velocity (recession velocity), d is distance and H 0 is called hubbles constant or (H naught) - Hubbles law expresses a relationship between galaxy speeds and distances and hence allows us to determine a galaxys distance from its speed - We encounter 2 difficulties when we try to use hubbles law to measure galactic distances 1.) Galaxies do not obey hubbles law perfectly Hubbles law gives an exact distance for a galaxy whose speed is determined solely by the expansion of the universe In reality, nearly all galaxies experience gravitational tugs from other galaxies and these tugs alter their speeds 2.) Even when galaxies obey hubbles law well,the distances we find with it are only as accurate as our best measurement of hubbles constant - The quest to measure hubbles constant was one of the main missions of the hubble space telescope Parallax: we measure the distances to nearby stars by observing how their positions appear to change as earth orbits the sun These distances rely on our knowledge of the earth-sun distance determined with radar ranging How do distance measurements tell us the age of the universe? Universal expansion - The expansion of the universe implies that the universe came into being at a single moment in time Cosmological principle: matter in the universe is evenly distributed without a center or an edge The age of the universe - The rate at which the universe expands tells us how old it is about 14 billion years old
Lookback time - An objects lookback time is the time it took for the objects light to reach us Its the difference between the current age of the universe and the age of the universe when the light left the object Cosmological redshift: expansion of the universe stretches out all the photons within it, shifting them to longer, redder wavelengths The horizon of the universe - The size of the observable universe is determined by the age of the universe Cosmological horizon: marks the limits of the observable universe (boundary in time NOT space) 15.3 Galaxy evolution Galaxy evolution: development of galaxies How do we observe the life histories of galaxies? - Images of the deep universe allow us to study galaxies at many different distances and therefore many different ages How did galaxies form? - Theoretical models suggest this Hydrogen and helium gas filled all of space more or less uniformly when t he universe was very young (first million years) The distribution of matter in the universe was not perfectly uniform certain regions of the universe started out slightly more dense than others - Our most successful models of galaxy formation suggest that protogalactic clouds formed in regions of slightly enhanced density in the early universe Why do galaxies differ? 1.) Galaxies may have ended up looking different because they began with slightly different conditions in their protogalactic clouds 2.) Galaxies may have begun their lives similarly but later changed due to interactions with out galaxies Conditions in the protogalactic cloud Protogalactic spin: a galaxys type might be determined by the sping of the protogalactic cloud from which it formed If the original cloud had a significant amount of angular momentum, it would have rotated fast and collapsed The galaxy it produced would therefore have tended to form a disk and the result galaxy would be a spiral If the protogalactic cloud had little or no angular momentum, its gas might not have formed a disk at all and the resulting galaxy would be elliptical
Protogalactic density: a galaxys type might be determined by the density of the protogalactic cloud from which it formed A protogalactic cloud with high gas density would have radiated energy more effectively and cooled more quickly allowing more rapid star formation If the star formation proceeded fast enough, all the gas could have been turned into stars before any of it had time to settle into a disk making it an elliptical galaxy Lower density cloud would have formed stars more slowly leaving plenty of gas to form the disk of the spiral galaxies - Elliptical galaxies may have formed from protogalactic clouds that were spinning more slowly or were denser than those that formed spiral galaxies Galactic collisions - Computer models show that a collision between 2 spiral galaxies can form an elliptical galaxy - Observations show that at least some elliptical galaxies have experienced past collisions Central dominant galaxies; found at the centre of many dense clusters (they are giant elliptical galaxies that grew in size by colliding with and consuming other galaxies) Starbursts - There are 2 ways from which an elliptical galaxy may have formed 1.) Result of birth in an unusually slow-rotation or high density protogalactic cloud 2.) Result of later collisions and mergers of spiral galaxies - Starburst galaxies are forming stars so quickly that they will run out of star-forming clouds in just a few 100 million years Starburst galaxies; small number of galaxies that appear to be in the midst of such bursts of star formation Galactic wind: hot gas that erupts outward into space 15.4 Quasars and other active galactic nuclei Active Galactic nuclei: bright galactic centres Quasars: most luminous active galactic nuclei What are quasars? Supermassive black holes: mass millions of times that of our sun Discovery of quasars - Quasars can be more than a trillion times as luminous as the sun - They were first named quasi-stellar radio sources Evidence from nearby active galactic nuclei - The immense luminosities of quasars and other active galactic nuclei come from regions no larger than our solar system
Seyfert galaxies: galaxies that look like quasars but are less powerful - The incredible luminosities of active galactic nuclei and quasars are apparently being generated in a volume of space not much bigger than our solar system Radio galaxies and jets Radio galaxies: emit unusually strong radio waves (comes from pair of radio lobes) - Active galactic nuclei in radio galaxies can propel huge jets of particles moving at nearly the speed of light What is the power source for quasars and other active galactic nuclei? - Quasars are probably powered by matter falling into supermassive black holes Do supermassive black holes really exist? - Observations of rapidly orbiting material at the centers of galaxies indicant that at least some galaxies and perhaps all of them harbor supermassive black holes