Lecture 20: Our Galaxy and Other Galaxies
Other galaxies come in a great diversity of sizes, luminosities, shapes...
- spiral galaxies
- like our very own Milky Way
- have 3 basic components
- thin disk of gas, dust, young and old stars
- for large galaxy, typically 100,000 lyr across, 3000 lyr wide
- spiral arms are where new stars are forming
- arms are bluer than halo or bulge because young, massive, blue stars live there
- are not entirely understood,
how do gas clouds start to collapse to form stars?
- material fills disk -- arms are not necessarily
where the stars are, but where stars are being formed
- central bulge
- rounder than disk
- thicker than disk (~15,000 lyr for Milky Way)
- no blue stars, no gas ---> no star formation
- spherical region around galaxy
- few stars visible, but traced out by the distribution
of globular clusters
- globular clusters:
very old structures ---> tracing out early shape of galaxy?
- some other disk galaxies
- S0 galaxies: similar to spirals in shape, but no spiral arms
- barred spirals: similar to spirals, but with bar-like structure across center
- resemble bulges of spiral galaxies:
no blue (young) stars, very little gas, mostly old, reddish stars
- most massive and luminous galaxies are ellipticals (but also many dwarf elliptical galaxies)
- no rotation: stars orbit center, but not all in the same direction,
orbits are randomly oriented
- no significant disk, but halo
- giant ellipticals may have thousands of globular clusters!
- young stars, gas, often considerable star formation
- no obvious structure -- what a mess!
- many dwarf galaxies are irregulars
- but there are also large galaxies with
- can result from close passages or even collisions
of galaxies (simulation)
- what causes different types of galaxies?
- not necessarily an evolutionary sequence
- did spirals and ellipticals form that way?
- or, did events in their past determine the way they look?
- spirals, irregulars are gas rich and are forming stars
- ellipticals are gas poor and are not forming stars
- disks of spirals have stars orbiting mostly in one direction (rotation)
- ellipticals have stars going in all directions, and bright ellipticals have no rotation
- ellipticals generally found in dense environments
- spirals generally found outside of dense environments
- ellipticals are the brightest galaxies
The Local Group
- Milky Way and Andromeda Galaxy (over 2 million light years away)
- Milky Way has dwarf satellite galaxies
- Andromeda has satellites also
- in total, 2 large spirals, one small spiral, many dwarf irregulars and spheroidals,
- new small galaxies are still being discovered!
- Andromeda-Milky Way will merge eventually (movie)