University of Michigan - Department of Astronomy

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Discussion version

Classification of Galaxies

Our galaxy is one of billions of galaxies populating the universe. It would be the height of presumption to think that we are the only living things in that enormous immensity

-- Werner Von Braun

Overview

Introduction

In the early part of the 20th century, Edwin Hubble began studying “spiral nebulae.”  Distance measurements using the period-luminosity relationship for Cepheid variables would eventually show these “nebulae” were in fact other galaxies outside our own. Hubble came to recognize that there are several types of galaxies based on their visual appearance (morphology).  He expanded his studies to include all types of galaxies, and was eventually able to show that the universe is expanding (Hubble’s law).  To help his studies, he also developed a morphological classification system for galaxies.  Despite modifications and refinements over the years reflecting our increasing understanding of these beautiful stellar systems, the basic Hubble classification still remains in widespread use today.

In Hubble’s classification, galaxies are divided into 5 basic groups: elliptical, lenticular or S0, spiral, barred spiral, and irregular. Elliptical galaxies generlly lack any overal structure except for their general shape. Spirals and barred spirals have a distinct disk and bulge, and get their name from the fact that there appear to be spiral arms in the disk. Lenticulars appear very similar to the ellipticals but actually have a distinct bulge and disk. In most cases, the disk is very hard to make out in optical images. Irregular is the catch-all for everything else.

Tuning Fork Diagram

Each group is further divided based on characteristics within the group:

In addition, there are peculiar galaxies.  These are classifiable as a particular type but have something strange about them.  For example, it may be classifiable as an edge-on type Sc but the spiral disk may be warped, or as a E4 but have a distinct blob of gas near one edge. The first atlas of peculiar galaxies was compiled by Halton C. Arp in 1966. In many cases he classified galaxies as peculiar because there was an unresolved companion, so it looked like there was a blob on the side of the galaxy.

Overal there are 16 different classes for galaxies in this system. It works fairly well for the nearby bright galaxies Hubble was studying. However, as astronomy developed, especially at non-optical wavelengths, the need for more catagories arose. Several other classification systems were developed, but the most popualr was developed by Gerard de Vaucouleurs in 1959. He basically expanded the Hubble classification system allowing for inermediate classes such as Sab, and adding extra notations such as + and - signs or extra letters like m to indicate things like the the strength of the bar, disk structure in lenticulars, rings in sprirals, and different structures in irregulars. Using this classification system, there are hudreds of different types of galaxies possible. This is the system generally used if the Hubble classification system in insufficient.

In this activity, you use the Hubble classification system to develop a method for classifying galaxies from a set of pre-classified galaxies.  Then you'll classify several mystery galaxies.  After that, you will draw some conclusions about the history of several types of galaxies based on the color images. If you want more information on the objects or on any of the catalogues and classification systems, http://www.seds.org/messier/ is an excellent place to start.


Part 1:The Classification System

You need the checklist and 9 images with known classification.

Spread the images out in front of you in the same order as the checklist.  Based on your overall view of these images, answer the questions below.

  1. How do you tell the difference between the dense bulge at the center of a lenticular and a photographic/ printing effect making the center of an elliptical galaxy look like a dense center?







  2. How do you determine where the edge of the galaxy is in order to measure its diameter (consider especially the E0 and SBb galaxies)?







  3. How do you tell if a galaxy has a disk if it is (a) face on and (b) edge on?







Now you are going to develop a checklist to help you classify galaxies.

Answer the following questions for each of the pictures.  If the answer is yes, place a check in the appropriate space on the checklist and leave it blank if the answer is no.  If it asks for a number, fill in the number in the appropriate space.

This section will determine the basic type: elliptical, lenticular, spiral, barred spiral or irregular.

  1. Is there a distinct bulge at the center?
  2. Is the bulge round?
  3. Is there a distinct disk?
  4. Are there spiral arms visible?
  5. Do the spiral arms attach directly to the bulge? (the other option would be attaching to a bar that cuts across the bulge)
  6. If there are spiral arms, rank the wrap of the arms on a scale of 1 to 10 where 1 is very tightly wrapped (circles back around the galaxy) and 10 is very loose (curves away in an arc)
  7. Is there any structure other than a bulge visible or anything unusual looking?

This section will determine the number or letter for the elliptical or spiral.  You can skip these questions if it is an irregular or lenticular galaxy.

  1. Measure the major axis of the galaxy.  If it appears to be a round elliptical, take the longest of several measurements.  If it is a spiral-type and appears round, take the average of several measurements.  Enter this as x in the checklist. 
  2. Measure the minor axis if it is an elliptical (perpendicular to the direction you measured in step 8), or the diameter of the bulge if it is a spiral or barred spiral (parallel to the direction you measured in step 8).  Enter this under the y column in the checklist.
  3. Calculate the ellipticity e if it is an elliptical, or the bulge – to – disk b/d ratio for a spiral.  Enter this in the last column.

Once you have completed the table, you will have a check list to determine the galaxy type based on the known galaxies.  Answer the following questions before going onto part 2.

  1. What would the checklist look like for an E6 galaxy?







  2. What would the checklist look like for an SBa peculiar galaxy?







  3. How would you classify a galaxy with loosely wrapped arms but a large round bulge?  Why?








Part 2: Unknown galaxies

Your GSI will give you a list for the unknown galaxies. You will need to find the images of the galaxies on your list, then determine their classification as outlined below.

Begin by recording their names in table 1.

In the next column, record which checklist items from numbers 1 - 5 and 7 you answered yes to for each picture, and the rank for 6 if it is a spiral galaxy.

If it is an elliptical or spiral galaxy, measure x and y and calculate e or b/d (checklist items 8 - 10), and record the values in the next 3 columns.

Any additional comments should go in the second to last column. This is a good place to make notes about how you made your final determination of the galaxy type, especially for peculiars and boarder-line galaxies (e.g. an elliptical with e=4.5 or a spiral with a large bulge and loose arms)

Finally, determine the classification and record it in the last column.

Questions

  1. What problems did you run into classifying the galaxies?







  2. How confident are you in your classification: do you think your classification would match if you look up your galaxies in a catalogue? Explain.








Part 3: The Hubble UDF

Look at the Hubble Ultra Deep Field (UDF) image segment (available here if you want to see the original) Note there are no stars, only galaxies in this image. Light from most of the galaxies in this image left when the universe was about 1 billion years old, so they provide the earliest snapshot yet of what galaxies were like when the universe was young.

  1. Estimate the number of galaxies in the image. Describe how you estimated the number.







  2. Estimate the number of galaxies you can classify as spiral, elliptical or lenticular. What fraction of the total number of galaxies is this?







  3. Estimate the number of irregular galaxies. What fraction of the total number of galaxies is this?







  4. Is the Hubble classification system useful for the UDF?







  5. How were galaxies in the early universe different from galaxies in the modern universe?








Last modified: 9/1/05