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The Question

(Submitted May 14, 1998)

How do I relate the temperature of a main sequence star to its color?

The Answer

Stars produce energy primarily by nuclear reactions in their deep interiors. The nuclear reactions produce very high-energy particles and gamma-rays, but these can't escape easily through the outer layers of the star; they must scatter many times on their way out. This scattering process, plus the total amount of energy produced by the nuclear reactions, are what determines the spectrum of radiation escaping the star. This spectrum is pretty well described by a shape called a 'black body', which also applies to many terrestrial situations, such as an incandescent light bulb. For a black body the temperature and colors are related in a simple way. In order to figure it out, however, you have to decide what you mean by color. Astronomers have a couple of different ways of doing this, the most popular being the 'UBV' (ultraviolet, blue, visual) system. You can find a table showing how these colors are defined in terms of the wavelengths of light at:

http://ads.harvard.edu/cgi-bin/bbrowse?book=hsaa&page=100

Then, an approximate correspondence can be made between temperature and 'B-V', which is the difference between the B (blue) and V (visual) intensities:

T (degrees Kelvin)B-V
25,000-0.2
10,000+0.2
4,000+1.2

I hope this helps,

Tim Kallman
for "Ask an Astrophysicist"

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