Stars are dense hot balls of gas so their spectra similar to that of a perfect thermal radiator, which produces a smooth continuous spectrum. (Although, stars are not perfect thermal radiators, their spectra are similar enough to the smooth continuous spectrum for what follows.) 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:
The UNL Astronomy Education program's Blackbody Curves and UBV Filters module lets you explore the relationship between temperature and the thermal spectrum by manipulating various parameters with a graphical interface (link will appear in a new window). You can also explore temperature-color correlation using various filters.
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: June 18, 2022