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 open a new window containing the Object Explorer entry for each star.
Once you have a star loaded into the Object Explorer, scroll down and expand the “Optical Spectra” section. thou will see a full-sized picture of the star’s spectrum. If you click “Print Page,” you can print out the spectrum. Just below the “View Optical Spectrum” link, you will see a miniature version of the spectrum. Click on the miniature spectrum to see a full-size version. You can use the print command in your web browser to print out a picture of the full-size spectrum.
| Plate | Fiber | Plate | Fiber |
| 266/51630 | 346 | 266/51630 | 173 |
| 266/51630 | 275 | 266/51630 | 314 |
| 266/51630 | 365 | 266/51630 | 513 |
| 273/51957 | 2 | 498/51984 | 538 |
| 273/51957 | 157 | 273/51597 | 245 |
| 273/51957 | 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.