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A gaseous envelope surrounding a star, planet, or satellite, and bound to it by gravity.

Electromagnetic spectrum

The complete array of electromagnetic radiation (light). In order of increasing wavelength (decreasing frequency and energy), the spectrum ranges from gamma rays through X-rays, ultraviolet light, visible light, infrared radiation, microwaves to radio waves.

Infrared (IR)

Invisible electromagnetic radiation (light) with wavelengths longer than red light and shorter than microwaves. Infrared light occupies the spectral band extending from 0.75 to about 200 micrometers.


Relating to or using light in visible wavelengths.


The path of one celestial body moving around another under the force of gravity.

Protoplanetary disk

A disk of dust, gas, and perhaps developing planets orbiting a young star. A transitional stage between a solar nebula and a solar system.


A scientific instrument that identifies the chemical elements in a soil or rock sample by subjecting the sample to energy (infrared, xray, etc.) and then measuring the different energies, or wavelengths, of light given off by the minerals in response.

spectrum (pl. spectra)The range of electromagnetic radiation (light) expressed in terms of frequency or wavelength. A rainbow displays the spectrum of visible light.


A self-luminous body held together by its own gravity and with a central temperature and pressure sufficient to generate nuclear energy.


The physical system that encompasses all matter, energy, and space that exists.

Visible light

The portion of the electromagnetic spectrum corresponding to the visible colors, with wavelengths longer than ultraviolet light and shorter than infrared radiation. Visible light occupies the spectral band extending from 300 nanometers to about 750 nanometers.


The distance between successive wave crests, or troughs. Light of different wavelengths has different properties, such as color.