How Can Astronomers Tell What A Distant Object's Temperature Is?

Modern astronomical research has accumulated an astonishing wealth of knowledge about the universe despite extreme limitations on observation and data collection. Astronomers routinely report detailed information about objects that are trillions of miles away. One of the essential techniques of astronomical investigation involves measuring electromagnetic radiation and performing detailed calculations to determine the temperature of distant objects.

The color of light radiated by a star reveals its temperature, and the temperature of a star determines the temperature of nearby objects such as planets. Light is produced when charged atomic particles vibrate and release energy as light particles, known as photons. Because temperature corresponds to an object's internal energy, hotter objects will emit photons of higher energy. The energy of photons determines the wavelength, or color, of light; thus, the color of light emitted by an object is an indication of temperature. This phenomenon is not observable, however, until an object becomes extremely hot — about 3,000 degrees Celsius (5,432 degrees Fahrenheit) — because lower temperatures radiate in the infrared spectrum rather than the visible spectrum.

The concept of a blackbody is essential to measuring the temperature of astronomical objects. A blackbody is a theoretical object that perfectly absorbs energy from all wavelengths of light. In addition, the emission of light from a blackbody is not influenced by the object's composition. This means that a blackbody radiates light according a certain spectrum of colors that depends solely on the temperature of the object. Stars are not ideal blackbodies, but they are close enough to allow for an accurate approximation of temperature based on emission wavelengths.

A simple visual observation does not reveal the temperature of a star because temperature determines the peak emission wavelength, not the only emission wavelength. Stars generally appear whitish because their emission spectra cover a wide range of wavelengths, and the human eye interprets a mixture of all colors as white light. Consequently, astronomers use optical filters that isolate certain colors, then they compare the intensities of these isolated colors to determine the approximate peak of a star's emission spectrum.

Planetary temperatures are more difficult to determine because the absorption and emission characteristics of a planet may not be adequately similar to the absorption and emission characteristics of a blackbody. A planet's atmosphere and surface materials can reflect significant amounts of light, and some of the absorbed light energy is retained by the greenhouse effect. Consequently, astronomers estimate the temperature of a distant planet through complex calculations that account for such variables as the temperature of the nearest star, the planet's distance from the star, the percentage of light that is reflected, the composition of the atmosphere and the planet's rotational characteristics.