University of Michigan - Department of Astronomy




Version: zoo

Characterizing the diversity of galaxies with Galaxy Zoo

Through binoculars the spiral nebula was
a smudged white thumbprint on the night sky.
Stories said it was a mark left by the hand
of Night, that old she, easily weaving
the universe out of milky strings of chaos.

- Minnie Bruce Pratt, The Blue Cup



The two basic types of galaxies: bulge and diskYou have seen in the lectures that galaxy structures come in two main flavors: galaxies with a dominant bulge (left), and galaxies with dominant disks (right).  The structures of these galaxies are a rich source of information about how the galaxy evolved: galaxies with dominant disks are thought to have evolved in relative isolation; galaxies with dominant bulges may have undergone interactions with other galaxies that randomized and disrupted the disks (or formed at earlier times, when interactions were more common).   In this lab, we will explore the structures of galaxies from a large sky survey, compare other properties with their structures, and contribute to an international research project.

The Sloan Digital Sky Survey (SDSS) has been using a dedicated telescope to perform an optical and spectroscopic survey of much of the sky.  During the course of the project, the SDSS has imaged over 930,000 galaxies.  Classification of these galaxies to learn about their structures (bulges, disks, bars and spiral arms, and interacting galaxies), is important, but impossible for small teams of astronomers to do alone. 

In 2007, an online project called the Galaxy Zoo was created to allow members of the public to help classify galaxies from the SDSS.  Users of the site are presented with an image of a galaxy and are asked a series of questions, designed to differentiate smooth round galaxies from structured disk galaxies, estimate the prominence of a bulge component, and characterize any features such as spiral arms or bars. 

One thing to note, Galaxy Zoo asks you to classify galaxies as either smooth or as having as disk or feature. In general, all galxies that are smooth will be bulge galaxies, however, you could also have a bulge type galaxy that appears to have a dust lane or other feature. These are somewhat rare, but you could come across one or two while doing your classifications, so be careful about taking notes as you go!

Before Class

It is stongly recommended that you register before class. You must register in order to save your information.
Please register before class at:
All that is needed is a username, e-mail address and password.

You may wish to go through the tutorial when you register to save some time in class.

Part 1: Background and Preparation

In general, galaxies with a dominant bulge have less cold gas and fewer blue stars than galaxies with a dominant disk.

  1. Based on your recollection of stellar populations from earlier in the semester, which color of stars will have a shorter main sequence lifetime – blue or red?

  2. In the light of your answer, does it make sense that one type of galaxy would have both cold gas and blue stars, but another type would have neither?  Explain your reasoning, and check your answer with your neighbor.


The Galaxy Zoo effort has involved over 250,000 volunteers so far. Having such a large number of users can be an advantage for several reasons.

  1. Why is it important to have more than one person classify the shape and features of each galaxy in Galaxy Zoo?

Go over the tutorial with everyone in your group to make sure everyone understands how Galaxy Zoo classifies things.

Part 2: Classify some galaxies

Have one person in your group log in and go to

For the first part of the lab exercise, you will classify 10 galaxies using the Galaxy Zoo website and record some of your observations in table 1. IMPORTANT: the information you record for the table is not exactly the same as the information Galaxy Zoo asks about.

  1. In the color column, record the color as R for red or B for blue. Color is sometimes hard to determine – many ‘red’ galaxies appear yellow or orange.  For the purposes of this exercise, any galaxy without an obvious blue portion is ‘red’, and galaxies with an obvious blue portion are ‘blue’. Note if you click the "invert galaxy image" button, it also inverts the colors, so be sure to do this step with the default, black background.
  2. In the type column, record an S for “smooth”, D for “features or disk” or A for “Star or artifact” (this is the first step n the Galaxy Zoo classification.)
  3. For galaxies you record as D (“features or disk”): In the Edge or Face on Disk column record E if it is an edge on disk, F if it appears to be a face on disk (such as a spiral), or N if it is not actually a disk (e.g. it is smooth except for a dust lane, or it is a lumpy blob).
  4. For all the galaxies where you recorded an N in the Edge or Face on Disk column, record whether it appears to be a bulge or disk galaxy, and explain why you think it is that type. For the other galaxies, record any notes about the galaxy, especially if there is “anything odd” like a dust lane.

Part 3: analyze your results

  1. Once you have finished filling in the table, add up the following:
    1. Number of bulge galaxies: _______

    2. Number of red bulge galaxies: ______

    3. Number of disk galaxies: ______

    4. Number of red disk galaxies:__________

    5. Number of red disk galaxies that are face on: ______
  2. Using a calculator or Excel, perform the following calculations:
    1. What percentage of the galaxies are bulge type? ___________

    2. What percentage of the galaxies are disks? ________

    3. What percentage of your bulge galaxies are red?  _____________

    4. What percentage of your disk galaxies are red? ____________

    5. What percentage of red disks are face-on?_______
Table 1: Galaxy Classification




Edge or face on Disk



Concluding questions

  1. Discuss if your results support each of the following statements:
    1. the majority of galaxies imaged by the SDSS are bulge type galaxies;
    2. the majority of bulge galaxies are red;
    3. disk galaxies generally contain younger stars (hint: consider Part 1 question 2)
    4. the color of disk galaxies depends on their inclination (edge-on vs. face-on.)

  2.  Do you believe members of the public can make a meaningful contribution to astronomy through projects like Galaxy Zoo?  Why or why not?

  3. Would you consider continuing your involvement with this project outside of this assignment?  Why or why not?

Updated: 11/3/2011 by SAM; Previous update: 11/2010 with input from EB

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