Classifying Galaxies

Pretend that you are an astronomer working shortly after Edwin Hubble made his startling discovery. Now that you know the “nebulae” are actually other galaxies like our own, you must come up with a way to classify the galaxies.

Exercise 1: Look at the following galaxies. Divide them into groups based on features they have in common. There is no set number of groups.

Use the Get Fields tool to look up each galaxy. SDSS images are sorted by regions of the sky, into runs, camera columns (camcols), and fields. The list below shows what run/camcol/field area each galaxy is in. Click on each galaxy’s field number to see a picture of the field. Because galaxies tend to cluster together, some of the frames have more than one large and interesting galaxy, but in all the other frames, the largest galaxy should be obvious.

Galaxy

A unique SDSS galaxy

Run

Camcol

Field

752

1


244

2662

4


243

752

1


331

1737

6


11

756

4


198

2738

2


196

752

1


432

3325

3


176

3325

3


319

3325

2


216

3325

2


215
(just left of center)

3325

3


230
(2 galaxies)

2738

3


122
(2 nice galaxies)

3325

3


352

3325

1


356

3325

1


359

Now, find another group and partner up with them to compare your results.

Question 1: Do you have the same number of galaxy classes as the other group? If not, what distinctions did one group draw that the other did not?
Question 2: What do your classification systems have in common? What makes them different?
Question 3: Try to combine the best features of each classification system. Repeat Exercise 1 with your new system.

The first person to classify galaxies was Edwin Hubble. He looked at pictures like those you saw, although his pictures were not nearly as clear as yours!

Hubble saw many of the same types of galaxies that you did. In the next few pages, you will learn a little about the types of galaxies that both Hubble and you saw. Then you’ll learn about the classification system that Hubble developed, which astronomers still use today.