Classifying Galaxies
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What is a galaxy?

A galaxy is made of billions of stars, dust, and gas all held together by gravity. Galaxies are scattered throughout the Universe. They vary greatly in size and shape. Not all galaxies look alike.

This is a galaxy.

This is a galaxy.

This is also a galaxy

Did you notice that these galaxy pictures don't look alike?

When telescopes led to the discovery of galaxies, astronomers observed many differences. In 1926, an astronomer named Edwin Hubble decided to classify the galaxies, grouping them according to some logical scheme.

He could have classified them according to color, because galaxies are different colors. You can see many different colored galaxies in the Hubble Deep Field image.

He could have classified them according to size, calling small galaxies "dwarf galaxies" and calling large galaxies "massive galaxies".

After considering different schemes, he decided to arrange or group them by shapes. He would classify them according to the way they looked. In science, the study of something according to its form or structure is called "morphology".g

Let's consider the three galaxies we saw earlier. They are typical of the three main types of galaxies that Hubble classified.

 The first type of galaxy you saw is an elliptical galaxy. The word elliptical refers to its degree of "roundness". Hubble used the letter "E" to stand for elliptical galaxies. To see elliptical galaxies, click on the Galaxy Guide. Galaxy Guide by Josh Kennedy

Hubble called the second type of galaxy you saw a  spiral galaxy. It reminded him of a pinwheel or whirlpool. He used the letter "S" to stand for spiral galaxies.

To see spiral galaxies, click the Galaxy Guide.

The third type of galaxy reminded Hubble of a spiral with a solid bar across the center. He called it a barred spiral galaxy. He used the letters "SB" to stand for barred spirals.

 To see barred spiral galaxies, click on the Galaxy Guide

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Many galaxies have strange, irregualr shapes, and do not fit neatly into any of these three categories.

To see an example of a nearby irregular galaxy, click on the Galaxy Guide.

Can you name the three main types of galaxies? Did you discover that each main type is divided into smaller groups?   Click here to review.

To show the relationship of the galactic shapes, Edwin Hubble arranged the main types of galaxies and the sub-types into a chart that has come to be called "The Tuning Fork Diagram".

The Hubble "Tuning Fork Diagram" is the simplest way to classify galaxies. When you look at a picture of a galaxy, and try to classify it, you are trying to place it on the diagram where it belongs.

You have seen some pictures of galaxies in this lesson and have looked at how they were classified. Now it's time for you to classify some galaxy pictures. Are you up to the challenge? Click here.

NOTE: You must have a java-enabled browser in order for the next page to work. If your browser does not work with java applets, Click here.

The Hubble Telescope has looked far into space and discovered many galaxies. Travel to the Hubble Space Telescope Institute to look at an image of a newly discovered galaxy. See if you can classify it by its shape.

Read the summary written below the picture to see if you were correct! Use the Back button at the top of your Web Browser to return here when you have finished your visit to Space Telescope Science Institute.

Space Telescope Science Institute

The Hubble Space Telescope is making frequent new discoveries. If you want to do more galaxy exploring at the Hubble Space Telescope Institute, Click Here

Now that you have had practice at classifying galaxies according to shape, you are ready to take the challenge of becoming a member of the Hubble Deep Field Academy.

Remember this picture?
This image was taken by the Hubble Space Telescope in December of 1995. It shows lots of previously unknown galaxies of different colors, shapes, and sizes.

Use this link to the "Cosmic Classifier" to test your new skill at classifying galaxies in the Hubble Deep Field image.

Want to find out more about Edwin Hubble's Classification system? Click on the obsrvatory dome!