Classifying Stars

Pretend for a moment that you are an astronomer living in the early 20th century, before the OBAFGKM star classification system was developed. You are one of the first astronomers who ever looked at spectra of stars, and it is up to you to develop a classification system.

The table below shows a list of the first stars you are trying to classify. Look at their spectra and divide them into several groups. There is no set number of groups you should try to achieve, and the groups do not have to have equal numbers of stars (because not all types of stars are equally common). If you find a spectrum that has nothing in common with any of the others, a group of one is OK (but on the other hand, 16 groups of one is probably not useful either!)

Spectra in the SDSS are sorted by Plate and Fiber number. Click the fiber numbers below to go to the Object Explorer entry for each star. When you click the first number, the tool will open in a new window; when you click another number, the entry for the new star will appear in the same Object Explorer window.

Once you have a star loaded into the Object Explorer, scroll down in the left-hand frame and click “Spectrum.” You will see a full-sized picture of the star’s spectrum. If you click “Print Page,” you can print out the spectrum.

Plate

Fiber

Plate

Fiber

266/51630
21
266/51630
173
266/51630
275
266/51630
314
266/51630
365
266/51630
513
273/51597
2
498/51984
538
273/51597
157
273/51597
245
273/51597
589
281/51614
3
281/51614
4
281/51614
133
Exercise 1. Make a table as shown below. Classify the stars into groups, and record your groups in the table. Each star should fit in one of your groups. Make notes detailed enough that another group can duplicate your work. (You may have more than four groups or fewer than four groups…this is just a sample chart!)

Group

Star(s)

Characteristics

     
     
     
     

Now, find another group and partner up with them. Compare your spectral classification system to theirs.

Question 1. Do you have the same number of spectral types as the other group does? If not, what distinctions did one group draw that the other group 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 improved system.

You have just solved the same problem astronomers at the turn of the century faced when they developed the technology to take spectra of stars. If your classification system is different from the modern one, don’t worry. The original classification system underwent several revisions before astronomers arrived at a consensus. Originally, the classifications were the letters A through P. As time progressed, some classifications were added, some were dropped, and some were combined.