Conclusion

A carbon star

Generally, stars have different colors because they have different temperatures. However, as you learned in the last question, stars are not perfect thermal sources. For instance, a certain type of star called a carbon star has a very red color. Carbon in the star's atmosphere absorbs much of its blue light, making the cool star appear even redder than it should for its temperature.

A star's color can usually give you a good idea of its temperature, but it is always a good idea to double-check by measuring the star's spectrum with a spectrograph. In fact, whenever the SDSS sees an extremely red star, software automatically targets the star for follow-up measurements with the spectrograph.

The ultimate color-color diagram - for all stars in the SDSS's Early Data Release

You can see some of these unusual stars in the graph to the right, a color-color diagram for all the stars in the SDSS's Early Data Release. The black dots are normal "main sequence" stars. All the other stars were flagged by their unusual colors and selected for follow-up spectral measurements.

Most of the objects that lie outside the black main sequence are unusual stars, galaxies, or quasars. Although classifying objects by color is not a foolproof method, you can learn a lot about an object by looking only at its colors.

Astronomers use colors to classify many objects including stars, galaxies and quasars. They can use colors to estimate the spectral types of stars, the types of galaxies, to sort asteroids into different classes, and to estimate the distances to galaxies and quasars. In the Research Challenge below, you will use color to answer your own questions about the sky.