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 (see the Color project for a more detailed explanation). The temperature of such an object can be found by using the equation

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

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.