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Classifying Stars

Pretend that you are an astronomer living in the early 1900s, before the OBAFGKM spectral types were developed. You are one of the first astronomers who ever looked at spectra of stars, and it is up to you to invent a way to classify them.

The table below shows a list of the first stars you are trying to classify. 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," you can print out the spectrum.





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

Look at the spectra of these 14 stars, and divide the spectra into several groups. There is no set number of groups you should have, 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 very useful!).

Exercise 1. Make a table like the one below. Use it to record your groups. Give each group a name, then write the plate and fiber numbers of all the stars in that group. Each star should fit in one of the groups. Under "Description," write some of the characteristics that helped you decide make notes detailed enough that another group of the star spectra in this group. Make your notes detailed enough that other students can classify all 14 stars into your groups just by reading your notes. (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 of students and partner 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 features did one group look at 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 improved system.

You have just solved the problem astronomers at the turn of the century faced when they learned how to take spectra of stars. If your classification system is different than the modern one, don't worry. The OBAFGKM classification system used by astronomers went through several changes before its final version. Originally, the classifications were the letters A through P. Later, some classifications were added, some were dropped, and some were combined.