How Does Color Indicate the Temperature of a Star?

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A Rainbow of Stars

  • Stars gleam in the night sky in colors from red to orange to blue-white, providing stargazers with a veritable rainbow. Many don't realize that this show of colors is a doorway to understanding more about these "points of light." Through studying the color of a star, astronomers can determine its temperature. Hot stars glow a bright blue-white, indicating temperatures as high as 60,000 degrees Kelvin (K), while the coolest stars emanate a warm orange or red, revealing temperatures as low as 3,000 degrees K.

Wein's Law

  • Matter emits radiation, regardless of its size or composition. This principle was first proposed by Max Planck, who used the idea of a "black body" to describe something that could perfectly absorb and emit radiation, but it was German physicist Wilhelm Wein who studied thermal radiation more in-depth. Wein showed that all objects emit radiation, but as an object gets hotter, the wavelength of the radiation it emits becomes shorter and shorter. Very, very cold objects (somewhere just above absolute zero) emit radio waves. As temperatures increase, the radiation emitted moves up the electromagnetic spectrum into microwave, infrared and then visible light: red, orange, yellow, green and blue. If the object continues to grow hotter, it will glow white. Then it will leave the range of visible light and pass into ultraviolet, X-rays and gamma rays.

Types of Stars

  • Oh, Be A Fine Girl, Kiss Me! American schoolchildren have been taught that saying for decades as a way to remember the stellar classifications of stars. O stars are the hottest. The sun is considered a G star, with a surface temperature of 5,000 to 6,000 degrees K. Stars can be further classified by adding numerals from 0 to 9, with higher numbers denoting lower temperatures. So, O1 stars are hotter than O9 stars. Many astronomers also use Roman numerals to divide stars into luminosity classes, from white dwarfs (VII) to supergiants (I).

    New information continues to elaborate on these classifications. In 1998, astronomers added W (a very rare star with a surface temperature of up to 50,000 K) and two new letters for very cool stars, C and S.

The Practice of Measuring What We Cannot See

  • Determining the color of a star tells astronomers how hot a star is relative to other stars. But astronomers in search of a quantitative measurement cannot depend on relative measurements. One way astronomers measure the absolute temperature of a star is by measuring the star's magnitude with two or more color filters, usually yellow and blue. A hot star will be brighter through the blue filter than through the yellow. Astronomers use a formula to determine precisely where on the spectral temperature scale a star fits.

    Another way astronomers measure the temperature of a star is by studying the star's spectrum; they will then view the pattern of absorption lines created by the star, which vary depending on the star's temperature.

  • Photo Credit NASA/NSSDC
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