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Classifying Galaxies

          This is a statement about galaxies: "All ellipticals are galaxies, but not all galaxies are elliptical". After you learn how to classify galaxies, you will be able to explain what this statement means. When scientists see objects in nature that share some features, but not others, they find that grouping, or classifying them is a good idea. First they must decide what scheme or plan they will use to decide to which group something belongs.
        Your answers to questions below will come from the online lesson: Classifying Galaxies.

 

What is a galaxy?

1) billions of stars, dust and gas held together by gravity.  

 

In the 1920's, an astronomer named Edwin Hubble was able to gather pictures of many galaxies. He noticed that they were not all alike. He decided to group, or classify them. To group the galaxies in the photographs he studied, he could have used size, color, shape or any other feature that he noticed. Hubble decided to classify galaxies by their shape or form.

 Edwin Hubble looked at many galaxy pictures and decided to divide galaxies into three types according to the way they looked. He used letters to represent these three types of galaxies. 

 

2)The letter "E" meant  elliptical galaxies, 

 

3)"S" meant spiral galaxies, and 

 

4)"SB" meant barred spiral galaxies.

Galaxies had so many shapes that Hubble found that three types were not enough. He had to further divide these three according to a slight variation in shape between each type of galaxy in each category.  For the "E" or 

 

5) elliptical galaxies, he used the amount of flattening or 

 

6) roundness, elongation or stretching out, from E0( E zero) to E5, to put the galaxies into smaller groups.  For the "S" or 

 

7) spiral galaxies, he used how tightly the arms were wound around the bright 

 

8) center or nucleus to further classify these galaxies as Sa, Sb, or Sc.
For the "SB" or 

 

9) barred spiral galaxies, he used the increased openness of the 

 

10) arms or spiral arms to group these galaxies as SBa, SBb, or SBc.

 

Edwin Hubble took all these ideas and presented them all together in a simple diagram for classifying galaxies. This diagram is called the Hubble Tuning Fork Diagram. Most galaxies fall into one of his groups, according to how they look. Draw a shape for each of the types of galaxies below. Draw it above the letters which he used to identify the shapes. (#11-19)

                     

20) From the information you have seen today, explain the following statement: "All ellipticals are galaxies, but not all galaxies are elliptical". Put your answer below. 

 

Some ideas: there are several (three) main types of galaxies, classified according to their shape. Galaxies have many different shapes. Only one type of galaxy is elliptical; there are other types.