How do scientists determine the temperature of planets or stars?

Astronomical temperatures are usually estimated from spectroscopic measurements. The spectroscope was invented in 1859 by two German scientists, Robert Bunsen and Gustav Kirchhoff. It was first used to analyse the elements in a substance heated to incandescence. Each element gave off characteristic wavelengths of visible light. Bunsen used the device to identify two new elements, caesium and rubidium.

It was later found that the presence of certain elements in distant heavenly bodies, and their corresponding temperatures, could be analysed by the same colour yardstick, and by spectral lines, the patterns created by the emission and absorption spectra of the elements in stars and other heavenly bodies.

Over the years, scientists refined their classification of stars. Astronomers today take the entire electromagnetic spectrum into account, not just visible light. In general, cooler objects give off radiation of longer wavelengths while hotter objects give off shorter wavelengths. Infrared telescopes sent into space measure the wavelengths shorter than those of visible light and X-ray and gamma ray telescopes are trained on longer and hotter astronomical sources.

First published in April 2008