An object with a high redshift which looks like a star, but is actually the very luminous active nucleus of a distant galaxy. The name is a contraction of quasi-stellar, from their star-like appearance. The first quasars discovered were strong radio sources (quasi-stellar radio sources, or QSSs), but many more are now known which are relatively radio-quiet (quasi-stellar objects, or QSOs). At the distance implied by the redshift, the nucleus must be up to 100 times brighter than the whole of a normal galaxy. Yet some quasars vary in brightness on a time-scale of weeks, indicating that this huge amount of radiation originates in a volume only a few light weeks across. The source may therefore be an accretion disk around a black hole with 107 or 108 solar masses. Some quasars show little change in their light output, while others are much more variable: for example, 3C 279 has varied by a factor of nearly 500 in four months.
The first quasar to be identified as such in 1963 was the radio source 3C 273 at a redshift of 0.158, and it remains the optically brightest quasar as observed from Earth, at 13th magnitude. Thousands of quasars have since been found, many with high redshifts which imply that we see them as they were when the Universe was only about a tenth of its present age. The redshifted spectra of quasars show strong, often very broad, emission lines as well as continuum radiation. A rich absorption-line spectrum is also seen in the ultraviolet region (shifted into the optical region in high-redshift quasars), caused by clouds of intergalactic gas or interstellar material in galaxies between the quasar and Earth. The numerous absorption lines due to hydrogen in the clouds are together known as the Lyman-α forest.
The large distances to quasars and the dominance of light from the central regions makes it difficult to observe the surrounding galaxy with Earth-based telescopes, but the Hubble Space Telescope has clearly resolved the host galaxies, both spiral and elliptical. In many cases the host galaxies are seen to be interacting or merging with one or more neighbours. Such interactions supply the massive black-hole nucleus with gas or stars, which ultimately fuels the quasar outburst. Some apparently normal galaxies may contain the remnants of quasar activity in their nuclei, and some Seyfert galaxies and Markarian galaxies have nuclei that are intrinsically as bright as some quasars. Quasars which vary greatly in their light output are termed optically violently variable (OVV) and are classified as blazars along with BL Lacertae objects. See also active galactic nucleus.