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Chapter 20

Chapter 20

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McGill University
PHYS 183
Tracy Webb

20: Galaxies and the Foundation of Modern Cosmology 20.1 Islands of Stars  galaxies come in many sizes, colours & shapes  most galaxies located relatively nearby are the around the same age as the Milky Way  to study young galaxies must look far away (farther we look, farther back in time it is)  cosmology: the study of the overall structure and evolution of the universe  spiral galaxies: look like flat white disks with yellowish bulges at centres; disks filled with cool gas & dust with interspersed hot ionized gas; Milky Way  elliptical galaxies: redder, more rounded, often elongated like a football; contain very little cool gas & dust but very hot, ionized gas  irregular galaxies: neither disk-like nor rounded  colours in galaxies differ from different stars in them  dwarf galaxies contain 100 million stars; giant galaxies contain more than 1 trillion stars  spiral galaxies o thin disk extends outward from central bulge o made up on disk population & spheroidal population o disk component: flat disk; stars follow orderly, nearly circular orbits around galactic centre; always contains interstellar medium but amounts/proportions of molecular, atomic & ionized gases differs from one spiral galaxy to the next; larger bulge = less interstellar medium o spheroidal component: bulge + halo; stars have orbits with many different inclinations; contains very little cool gas/dust o barred spiral galaxies: spiral galaxies with straight bar of stars cutting across the centre & arms curling away from ends of bar  lenticular galaxies: lack spiral arms; considered intermediate class between spirals & elliptical since tend to have less cool gas than normal spirals but more than elliptical  elliptical galaxies o lack significant disk component o majority are small o usually contain very little dust/cool gas  some have relatively small/cold gaseous disks o some large ones have substantial amounts of very hot gas o lack of cool gas means little-no ongoing star formation o tend to look red/yellow since no hot, young, blue stars  irregular galaxies o blobby star systems o usually white & dusty o colours show they contain young, massive stars o distant galaxies more likely to be irregular than nearby galaxies  more common when universe was younger  some galaxies travel solo but many bound together by gravity  spiral galaxies found in loose collections of up to a few dozen galaxies (groups)  elliptical galaxies common in clusters (hundreds-thousands of galaxies) 20.2 Measuring Galactic Distances  measurements tell us distances, size & age of observable universe  measuring happens in a chain: radar range  stellar parallax  standard candle  main sequence fitting  Cepheids  distant standard candles   standard candle: suggests a light source of a known, standard luminosity, measuring based on apparent brightness; ie/ a street lamp looks dimmer when you are farther away from it  astronomical object can only serve as standard candle if we have some way of knowing its true luminosity without first measuring apparent brightness & distance  must follow 2 steps to use bright main sequence star as standard candle: o identify star cluster close enough for parallax & plot it on HR diagram;
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