Color and Temperature

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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.

Color Index and Temperature

Hot stars appear bluer than cooler stars. Cooler stars are redder than hotter stars. The ``B-V color index'' is a way of quantifying this using two different filters; one a blue (B) filter that only lets a narrow range of colors or wavelengths through centered on the blue colors, and a ``visual'' (V) filter that only lets the wavelengths close to the green-yellow band through.

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:

  1. Measure the apparent brightness (flux) with two different filters (B, V).
  2. The flux of energy passing through the filter tells you the magnitude (brightness) at the wavelength of the filter.
  3. Compute the magnitude difference of the two filters, B - V.

B-V for a hot star is close to zero or negative

B-V for a cool star is greater than 2

Wien's Law and Temperature

wavelength of peak emission depends on the
temperature

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

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel