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The Definition of Color in Astronomy

In the last section, you looked at the colors of some stars in the SDSS database. You may have classified them as red, blue, yellow, or white. But you may have had trouble figuring out exactly what color some of the stars were. Was it red or orange? Yellow or white? Color is a subjective judgment; what one person calls "blue" may be a different shade than another person's "blue."

If astronomers are going to learn anything from star color, they first need to have a definition of color that everyone can agree on; a measurement that everyone can make to compare the colors of different stars. The measurement they chose is the one you found in the last section: color is the difference in magnitude between two filters.


Magnitude is a number that measures the brightness of a star or galaxy. In magnitude, higher numbers correspond to fainter objects, lower numbers to brighter objects; the very brightest objects have negative magnitudes.

An increase of one number in magnitude corresponds to a decrease in brightness by a factor of about 2.51 - a magnitude five object is 2.51 times fainter than a magnitude four object. The sun has magnitude
-26. The brightest star in the Northern sky, Sirius, has magnitude
-1.5. The faintest object you can see with your eyes has a magnitude of about 6; the faintest object the SDSS telescope can see has a magnitude of about 23. If you're curious about the magnitudes of other famous stars, take a look at this table of the 314 brightest stars.

When you say that a star has a certain magnitude, you must specify the color that the magnitude refers to. The magnitudes given above are magnitudes for yellow light.

SDSS measures magnitudes in five different colors by taking images through five color filters. A filter is a kind of screen that blocks out all light except for light with a specific color. The SDSS telescope's filters are green (g), red (r), and three colors that correspond to light not visible to the human eye: ultraviolet (u), and two infrared wavelengths (i and z). On SkyServer, the five magnitudes (through the five filters) of a star are symbolized by u, g, r, i, and z. The astronomers who planned the SDSS chose these filters to view a wide range of colors, while focusing on the colors of interesting celestial objects.

Color is symbolized by subtracting the magnitudes: u-g, g-r, r-i, and so on. Remember that all these quantities involve magnitude, so they decrease with increasing light output. A star with a high g-r color is redder than a star with a low g-r color.