Other Ways to Classify Galaxies
A very blue spiral
galaxy from the SDSS
When you look at millions of galaxies as the SDSS does, you can't
classify every one by looking at it and placing it on the
Hubble Tuning Fork. If it takes 30 seconds to find and classify a
galaxy on the Hubble Tuning Fork, it would take more than 75 years to
classify all the galaxies in the SDSS's Data Release 3!
To classify all the galaxies, astronomers need a faster method.
Fortunately, you can use other properties of galaxies to
classify them. Astronomers have known for a long time that galaxy type
and color are related. Spiral galaxies tend to have more star
forming regions, and younger, bluer stars. Elliptical galaxies tend to have
mostly old, red stars.
A team of SDSS astronomers looked at the colors, spectra, and
visual images of many galaxies to determine how colors are
related to galaxy types in SDSS data. The researchers found that galaxies
fell into the clearest groups when they looked at the difference
between the ultraviolet (u) and red (r) filters. Specifically, the
researchers found that most early galaxies (elliptical, SO, and Sa)
had a u-r value greater than 2.22, and that most late galaxies
(Sb, Sc and Irregular) had a u-r value less than 2.22.
The method of classifying galaxies by their colors is not
perfect. There are some unusually red
spiral galaxies and some unusually blue elliptical galaxies.
However, the method works well enough that it can be used to analyze the
properties of large numbers of galaxies fairly easily.
Exercise 4. Look up the
following galaxies in the Object Explorer by clicking on their
object IDs in the table below. Classify them as early (E, S0, or Sa) or late (Sb, Sc, or Irr)
galaxies from their u-r values (u and r are located to the right of the galaxy's image). Then, look at their images and classify them on
the Hubble tuning fork. (Note: you may need to click on the image and open the
Navigation tool to get a better view of the galaxy.)
How many of the galaxies' colors match their Hubble
Tuning Fork types? How many do not?
Do you think color classification is a good (even if it is not perfect) way of
classifying galaxies? Why or why not?