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Stars seen by
the SDSS camera

Types of Stars

When astronomers look through their telescopes, they see billions of stars. How do they make sense of all these stars? How do they classify stars into types, and how do they tell which types are common and which are rare? Most importantly, how do they use the star types they see to learn about stars?

The only information we have about a star is the light it shines on Earth. No two stars are exactly alike, so you might expect that we could tell them apart by closely examining their light as it reaches us.

The "Fingerprints" of Stars

The best tool we have for studying a star's light is the star's spectrum. A spectrum (the plural is "spectra") of a star is like the star's fingerprint. Just like each person has unique fingerprints, each star has a unique spectrum. Spectra can be used to tell two stars apart, but spectra can also show what two stars have in common.

The spectrum of a star is similar to the spectrum of colors you see in rainbows. Stars give off light in a range of different colors. The spectrum of visible light - the spectrum you see in a rainbow - is shown below.

The wavelength of light determines its color. The wavelength on the spectrum above is measured in units called Angstroms; 1 Angstrom = or 0.0000000001, or 1 x 10-10, meters.

Stars do not give off the same amount of light at every wavelength. If you made a rainbow graph like the one above for a star, some parts of the graph would be much brighter than others. Scientists used rainbow graphs for many years; but in the past 20 years, they have begun to use an x-y graph to show a star's spectrum. The x-axis shows wavelength of light. The y-axis shows how bright the light is at that wavelength. Today, when scientists say "spectrum," they usually mean this x-y graph.

A typical spectrum for a star has a lot of peaks and valleys. You can see a typical star's spectrum below.


Click on the image to see it full size

Many of these peaks and valleys have labels on them. You may recognize some of these labels as symbols of chemical elements. Each star has a different set of peaks and valleys that can be used to divide the stars into different "spectral types."

The spectral types that astronomers use are given by the letters O,B,A,F,G,K,M (and there are some new spectral types that have been added in the last couple of years...more on those later!) For example, our sun is a type G star.

Before you find out what these letters mean, take a shot at developing your own system for classifying stars based on their spectra.