PES 106        Spring 2003
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General Astronomy II
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Lecture Notes:

Galaxies: Different Types

text: Chapter 16 Sections 1 - 3

The Space Telescope Institute has several on-line explorations related to galaxies: http://amazing-space.stsci.edu/resources/explorations/

Galaxy Types:

far away => faint => harder to study

need good telescopes

Here is a video that zooms in on a region of space to show galaxies: Click Here for video

many galaxies do not look like the spiral arms and disk that we described for our Milky Way galaxy

Classification of Galaxies:

spirals can be subdivided based on

  • size of bulge
  • tightness of winding of arms

ellipticals can be subdivided based on

  • amount of flattening of galaxy (spherical -> very flattened)

Here is an on-line lesson on classifying galaxies: http://amazing-space.stsci.edu/resources/explorations/hdf/

This link also discusses galaxy classification: http://www.windows.ucar.edu/tour/link=/the_universe/Galaxy.html&edu=high

Want to test your ability to classify galaxies? Here is an on-line test: http://www.smv.org/hastings/galaxmov.htm

Properties of galaxy types:

  • spiral galaxies
    • clouds of gas and dust (15% of mass)
    • mix of young (Population I) and old (Population II) stars
    • disks rotate quickly
  • elliptical galaxies
    • very little gas and dust
    • mainly old (Population II) stars
    • rotate slowly
  • irregular galaxies
    • lots of gas and dust clouds (50% of mass)
    • many young (Population I) stars

hard to tell fraction of each type since faint galaxies are very hard to detect - we only see the bright ones

Click on Galaxy Hunter at this site: http://amazing-space.stsci.edu/resources/explorations/ to learn more about detecting different types of galaxies

Development of Galaxies (slightly newer model than in 3rd edition of text)

collapse of large clouds

observe distant galaxies

  • light has taken billions of years to reach us => seeing galaxies as they were billions of years ago
  • more galaxies and smaller in size - also more spirals
  • Galaxies appear to collide and merge

most galaxies are created as spirals with small bulges

collisions with other galaxies

  • cause periods of rapid star formation (gas and dust clouds collide and compress)
  • EITHER
    • expand the bulge area
    • OR destroy the arms to form an elliptical galaxy

do not always need actual collisions - close encounters can disrupt a galaxy

NOTE: Galaxy collisions do not typically involve star collisions - lots of space between the stars

Measuring Galaxies

Distance measurements

much too far for parallax

standard candles works for very bright objects

  • use Cepheid variables for nearby galaxies
  • use supergiant stars or supernovas in distant galaxies (not very accurate)

Hubble's Law

observe almost all galaxies have strong red shifts of their spectra => moving away from us

the speed that a galaxy moves away from us gets bigger with increasing distance from us

speed = (H) x (distance from us)
H = Hubble constant - not so easy to measure accurately

Size measurements

Galaxies are big enough that they are not just points of light in our telescopes

Can measure the angular size.

If we know the distance to the galaxy, can measure the diameter of the galaxy.

Mass measurements

similar to measurements in Milky Way
  • pick a star (or group of stars) near the outer edge
  • measure the period of rotation (from speed and size of galaxy)
  • determine mass from Kepler's Third Law

mass is almost always bigger than we expect from observable matter => "dark matter"

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