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Chapter 15

Chapter 15 Galaxies and the Foundation of Modern Cosmology.docx

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Department
Natural Science
Course Code
NATS 1740
Professor
Randy Hoffman

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Chapter 15 Galaxies and the Foundation of Modern Cosmology 15.1 Islands of stars  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 x 0  V is velocity (recession velocity), d is distance and H is called hubbles constant or 0 (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
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