Spectrum

A spectrum is a definition of the magnitude of the frequency components that constitute a quantity. The spectrum may be either a line spectrum or a continuous spectrum.

There are three kinds of spectra that interest astronomers.

Continuous Spectra

The surface of a star is heated to such an extent that it glows with a particular colour. Red for cool stars, bluish-white for very hot stars. Because the light emitted at the surface has been absorbed and transmitted by many atoms by the time it reaches the surface, the discrete colours of the emission spectra of the atoms have been evened out to form a continuous spectra.

Absorption Spectra

Formed when continuous spectra from a star shine through a gas that absorbs only certain colours of light. The absorption spectra, therefore, look like continuous spectra with dark bands (absorption lines) at discrete wavelengths. These lines characterise the chemical composition of the gas which surrounds the star.

Emission Spectra

These usually arise from gas that is in the outer regions of stars, where the light is not absorbed and emitted many times before being transmitted to space. An emission spectrum consists of sharp peaks in the spectrum corresponding to the wavelengths of the emitted light.

See also: Cross Spectrum, Spectrum Analyser, Spectrum Analysis.