Introductory Astronomy:Stellar Spectra

A star's spectrum is a graph of the amount of energy the star puts out at every wavelength. There are three different kinds of spectra: continuous, emission, and absorption. Stellar spectra are absorption spectra. They look like a continuous "rainbow" of colors interrupted by dark black lines. These lines are produced at certain wavelengths as atoms in the (relatively!) cool stellar atmosphere absorb energy coming up from the star's core. The absorbed wavelengths appear as dark lines against the continuous background.

By analyzing the star's spectrum, we can determine the surface temperature of the star by using either Wien's Law or the star's hydrogen lines (or some other aspect of its spectrum). The spectrum lets us determine the surface temperature (as opposed to, say, the core temperature) of the star because the radiation we measure as the star's spectrum comes from the surface layers of the star.

Once we have measured the star's spectrum, we can classify it. This means that we group the star with other stars that have similar spectra and give the group a name. Astronomers love to classify objects because it saves alot of time (and they have a whole universe to study!). Imagine that you have a cluster of stars and you want to measure their temperatures. To do this, you would need to measure the entire spectrum of each star and then use Wien's Law for each of them. This would take you several years, depending on how many stars were in the cluster. Now, suppose instead that you just compare the spectra of the stars; for example, you might compare the hydrogen lines of all the stars in the cluster. You would find that the spectra of many of the stars have hydrogen lines which look identical. You could then group together the stars with similar spectra, measure the temperature of only one member of each group (using Wien's Law), and Presto!you would have found the temperatures of all the stars in that group. (This is just one of the many clever tricks astronomers use!)

Today, astronomers classify (or group) stars, according to their spectra, into seven groups:

O B A F G K M

(This is easy to remember by reciting: "Oh, Be AFine Girl/Guy, Kiss Me.")

O stars are the hottest and M stars are the coolest. To be more precise, astronomers divide these groups into subclasses (for example: A0, A1, A2, ... , A9), which are based on surface temperature, and into luminosity classes (for example: I, II, ... ,VI), which are based on size. The sun, for example, is classified as a G2Vstar.