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

    • halo

      • 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

    

• ellipticals

  

  • 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!

• irregulars

  

  • 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 weird shapes...

  • 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?

    some clues...

    • 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