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# 4.17 Radiation from Planets and Stars

Wien's law is a powerful physical principle that applies across the universe. Every object in space — every star, every planet — has a temperature that defines the peak wavelength of its thermal radiation. For astronomers, the most exciting application of Wien's law is that it gives us the ability to measure the temperatures of remote objects without visiting them. This technique is called remote sensing and it is the way we have learned the basic properties of stars and planets in the Solar System.

Graphic representation of Wein's Law. Click here for original source URL.

Using a telescope, astronomers can measure the exact colors of the radiation emitted by a star to figure out its surface temperature. Wien's law describes the mathematical relationship precisely. Each factor of two increase in wavelength is a factor of two decrease in temperature. You can even make a rough estimate of a star's temperature without a telescope. Stars that appear reddish have temperatures around 3000 K, white stars are in the range 5000 to 6000 K, and bluish stars are closer to 10,000 or 20,000 K. The colors of stars in the night sky are not only pretty to look at, they are also telling us about the temperature of enormous balls of gas trillions of miles away!

Artist impression of the Spitzer Space Telescope rendered against an infrared sky. Click here for original source URL.

Astronomers also use telescopes equipped with infrared detectors to measure the measure infrared thermal radiation emitted by planets. Cold planets radiate mostly at long infrared wavelengths. Warm planets have more thermal radiation at short infrared wavelengths. In a similar way, an instrument on a spacecraft can scan a planet for geothermal "hot spots," such as an active volcano. Reconnaissance satellites use the same principle to detect the heat pulse from nuclear tests or rocket launches.