Stars are dense hot balls of gas so their spectrum is close to that of a perfect thermal radiator, which produces a smooth continuous spectrum. Therefore, the color of stars depends on their temperature---hotter stars are bluer and cooler stars are redder. You can observe the star through different filters to get an approximate temperature. A filter allows only a narrow range of wavelengths (colors) through. By sampling the star's spectrum at two different wavelength ranges (``bands''), you can determine if the spectrum is that for a hot, warm, cool, or cold star. Hot stars have temperatures around 60,000 K while cold stars have temperatures around 3,000 K. The filter diagrams are shown below.
A hot star has a B-V color index close to 0 or negative, while a cool star has a B-V color index close to 2.0. Other stars are somewhere in between. Here are the steps to determine the B-V color index:
Another way to measure a star's temperature is to use Wien's law described in the electromagnetic radiation chapter. Cool stars will have the peak of their continuous spectrum at long (redder) wavelengths. As the temperature of a star increases, the peak of its continuous spectrum shifts to shorter (bluer) wavelengths. The final way to measure a star's temperature is more accurate than the previous two methods. It uses the strength of different absorption lines in a star's spectrum. It is described in full a little later in the chapter. The temperatures of different types of stars are summarized in the Main Sequence Star Properties table.
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last updated: November 12, 2002