Astronomy 100

Lectures Table of Contents Astro 100

Lecture 21
Spiral Arms and Galaxy Types

Outline

  1. Spiral Arms
  2. Galaxy Types

Terms to Know

spiral arm
spiral tracers
differential rotation
density waves
self-sustaining star formation
spiral galaxy
elliptical galaxy
irregular galaxy
barred spiral
Hubble tuning fork diagram

1. Disk galaxies have spiral arms.

One can trace out spiral arms in galaxies using spiral tracers:

Spiral arms look bright because they contain such bright stars. They are not as distinct as they appear.

Spiral arms cannot be fixed structures because differential rotation would cause them to "wind up". Strong spiral arms are formed by density waves that are excited by orbiting galaxies. It's like traffic slowing down behind a slow moving truck; new cars are always behind the truck but there is always a build up of cars behind it. In a galaxy its stars and gas that build up.

As clouds of gas "catch up" with the spiral arm they are compressed, which causes them to collapse and form stars. This forms stars of all masses but the most massive are the brightest and shine the bluest. As these stars die, lower mass stars live on but are dimmer and have already traveled past the spiral arm.

Energy from young stars and supernova explosions can trigger more star formation, called self-sustaining star formation. These areas of star formation can be stretched out by differential rotation making long linear structures. Several of these structures viewed together can make weak looking spiral arms.


The sky is literally covered with galaxies. Your fingernail held at arm's length covers a million galaxies , each one containing about 100 billion stars. Many of those galaxies are 10-15 billion light years away, close to the edge of the observable Universe, so we are peering back in time towards the Big Bang. What do they look like? How did they form? What clues do they hold about how the Milky Way came to be, and how Earth and the rest of the Solar System evolved?

2. Galaxy Types

Almost all galaxies belong to one of three major types:
  1. Spiral galaxies, or disk galaxies, like the Milky Way:
    • gas-rich
    • active star formation
    • generally blue due to hot young stars (halo may be red)
    • often dusty
    • disk stars rotate smoothly around center
    • sometimes show bars across their nuclei
  2. Elliptical galaxies
    • gas-poor
    • no active star formation
    • old red stars
    • stars move in haphazard orbits, not smooth flow
  3. Irregular galaxies
    • distorted shapes, sometimes due to two or more galaxies colliding
    • often gas-rich with active star formation
Hubble arranged ellipticals and spirals with and without bars into a diagram, the Hubble tuning-fork diagram, thinking it might help understand how galaxies were formed and how they came to appear the way they do. He thought perhaps ellipticals turned into spirals; now we think exactly the opposite!

Lectures Table of Contents Astro 100
Last updated: April 23, 2008 Neal Katz