Identifying the Spectral Types of Stars

So, what type of star did you think it was? The star had all the hydrogen lines, so that narrows our choices down to B, A, and F. However, it had no helium lines, so that rules out a type B star. The star did have ionized calcium (the H and K lines), which are characteristic of a type F star. So the star is a type F star. The star whose spectrum you identified is shown to the right. (The picture shows two stars close together; the star you identified is the larger one on the bottom left.)

Another way to find the temperatures of stars is to find the peak wavelengths of their thermal radiation curves. Any non-reflective object, including a star, emits light at a certain peak wavelength depending on its temperature. The temperature of such an object can be found by using the equation

T = 2.897 x 10-3 m K / lpeak.

The star whose spectrum was shown on the last page has a peak wavelength of about 4200 Angstroms (1 Angstrom = 10-10 m). This yields a temperature of about

T = 2.897 x 10-3 m K / (4.2 x 10-7 m) = 6900K,

close to the middle of the temperature range for a type F star.

The peak wavelength is not always in the portion of the spectrum we observe, however. The spectra taken by the SDSS do not cover the entire electromagnetic spectrum. If a star has a peak wavelength in the ultraviolet (beyond 4,000 Angstroms) or in the infrared (beyond 10,000 Angstroms), you cannot see the peak. However, you will still be able to see spectral lines for the star, so you can still use the OBAFGKM spectral type classification to find the star’s approximate temperature.

Now, it’s your turn to start classifying stars on your own.

Exercise 5. Use the Object Explorer tool to look at spectra of the stars in the table below. When you click on a link, the tool will open in a new window, displaying complete data on the star you have selected. Click “Spectrum” in the left-hand column to view the spectrum of the star. You may wish to print out each spectrum so you can compare them side-by-side.

Classify the stars according to their spectra (note: one or two of the objects are labeled as “huh” instead of a “star.” These objects have since been identified as stars, and their spectral types have been determined). Some spectral types may appear more than once. There is not necessarily one star of each spectral type. Be careful…some of the spectral types are difficult to tell apart!


Exercise 6. Use the Get Plates tool to choose about a dozen stars from the SkyServer database. Choose any plate you

Determine the spectral type of each star. Your sample is probably too small to ensure you will find a star of each type, but you may always look at more stars. like, then click on one of the “star” links to see the star’s spectrum.

Exercise 7. On the board, make a histogram showing the data from Exercise 6 for everyone in the class. Let the bins represent the spectral types (OBAFGKM). When you make the class histogram, label each square with the star’s plate number and fiber number. There are a couple of reasons for this. You don’t want to count the same star twice if two groups analyzed the same star. There is also the possibility that two groups analyzed the same star and disagree on its type. In this case, call in your “peer reviewers” to check the findings! Which types of stars are most common? Which types of stars are the least common?

Question 7. You may have noticed that type O stars are rare, especially in the SDSS database. Why do you think we see so few of them?

Question 8. Did you find any stars that did not fit into any of the spectral types? If so, what were their characteristics?