Quasars are extremely luminous Active Galactic Nuclei whose central engines are powered by accretion onto supermassive black holes. The name originated as a contraction of quasi stellar radio source, reflecting their early appearance as point like objects in optical images and their strong radio emission in many of the first discovered examples. Modern usage is broader because many objects historically called quasars are radio quiet as well as radio loud.
Their enormous luminosities arise from the conversion of gravitational energy in rapidly accreting matter rather than from a stellar population comparable to the visible brightness of the host galaxy. A quasar can emit strongly across the electromagnetic spectrum and can vary over timescales that constrain the physical size of the emitting region. Their compact central regions can therefore produce more energy than entire galaxies containing hundreds of billions of stars, making quasars among the most powerful persistent sources known in the observable Universe.
Quasar luminosity originates primarily in the accretion flow surrounding a supermassive black hole. Gas loses angular momentum as it moves inward, forming a rapidly rotating disc or related flow. Gravitational potential energy is converted into thermal and radiative energy, with the inner disc reaching temperatures capable of producing strong ultraviolet and X ray emission. Some of this radiation is absorbed and reprocessed by surrounding dust, producing infrared emission, while ionised gas farther from the central source produces characteristic emission lines.
If a relativistic jet is present, synchrotron and inverse Compton processes can add strong radio through gamma ray emission. The observed spectral energy distribution therefore reflects multiple physical regions rather than one uniform source. Quasar variability provides additional evidence that much of the luminosity originates from a compact region because an object cannot vary coherently on timescales shorter than the light crossing time of the relevant emitting region.
Quasars are especially important because many are observed at very large cosmological redshifts. Their emitted light can take billions of years to reach Earth, so observations of distant quasars provide views of the Universe when it was much younger. The redshift of spectral lines measures the cosmological stretching of their light and, within a cosmological model, provides information about distance and cosmic epoch.
Large surveys have discovered quasars from relatively recent cosmic times as well as from the first billion years after the Big Bang Theory. The existence of luminous quasars at early epochs demonstrates that massive black holes could grow rapidly in the young Universe. Explaining this rapid growth is an active research problem involving the masses of early seed black holes, the rate and efficiency of accretion, mergers and the availability of dense gas. Early quasars also illuminate the intervening Universe because their light passes through absorbing gas on its journey to Earth.
Quasar spectra contain broad emission lines produced by rapidly moving gas near the central engine. Lines such as those associated with hydrogen, carbon, magnesium and other elements can be measured even at high redshift because their rest wavelengths are known. The observed shift of these features enables redshift determination and therefore helps establish the quasar's cosmological context.
Broad line widths provide information about gas velocities, and when combined with estimates of the characteristic size of the broad line region they can be used to estimate the central black hole mass. Spectral absorption features reveal intervening clouds of gas, including neutral hydrogen along the line of sight. At sufficiently high redshift, absorption by the intergalactic medium produces the Lyman alpha forest and increasingly strong absorption blueward of the redshifted Lyman alpha emission line. Quasars therefore function simultaneously as astrophysical objects and as background light sources for investigating matter between galaxies.
Quasars played a major role in the development of modern extragalactic astronomy. Early radio surveys discovered compact radio sources that did not resemble familiar nearby radio galaxies. Optical counterparts appeared nearly stellar because their angular sizes were too small to resolve with contemporary instruments. The identification of strong spectral redshifts demonstrated that some of these apparently star like objects were at enormous cosmological distances, forcing astronomers to recognise that their luminosities must be extraordinarily high.
Subsequent discoveries showed that powerful nuclear activity was not restricted to radio loud objects and that quasars represent a high luminosity regime within the broader AGN population. Improvements in detectors, spectroscopy and space based observations revealed accretion discs, broad line regions, jets and host galaxies. The historical significance of quasars lies partly in their role as evidence that supermassive black hole accretion can produce immense luminosity on galactic and cosmological scales.
Quasars can influence their surroundings through radiation, winds and jets. Their intense ultraviolet and X ray output can ionise gas in the host galaxy and surrounding circumgalactic medium. Powerful outflows can carry energy and momentum away from the central region, potentially affecting star formation and the supply of gas available for further accretion. Jets, when present, can create enormous radio lobes and interact with intergalactic or intracluster plasma.
The feedback effects depend on luminosity, accretion state, jet power, host galaxy properties and environment. Quasar activity is generally episodic rather than necessarily permanent over the entire lifetime of a galaxy. A galaxy can therefore contain a massive black hole that is currently faint but can become highly luminous if an adequate supply of gas reaches the central region. The observed quasar population consequently represents a particular phase of black hole growth rather than a permanent state of all massive galaxies.
Quasars are valuable cosmological and astrophysical probes. Their extreme luminosity makes them visible over enormous distances, allowing surveys to map the distribution of matter and galaxies across large volumes. Quasar absorption spectra provide information about intergalactic hydrogen, metal enrichment, gas temperatures and the distribution of intervening structures. Their apparent luminosities can also be investigated statistically in relation to cosmological distance models, although quasars are not intrinsically standard candles because their luminosities vary widely and depend on accretion physics.
Large quasar catalogues reveal how the cosmic population of active black holes changed with redshift. The peak of luminous quasar activity occurred at a much earlier cosmic epoch than the present day, indicating that the most intense phases of supermassive black hole growth were concentrated in the past. Their distribution also provides information about how black hole activity relates to the evolving galaxy population.
Quasars should be distinguished from Blazars, although the categories overlap. A Blazar is an AGN whose relativistic jet is directed close to the observer, causing strong relativistic beaming, rapid variability and often extreme nonthermal emission. A quasar can have a jet without being viewed along it, in which case its observed properties differ significantly. Quasars should also not be interpreted as stars merely because they appear point like in ordinary images.
Their spectra, redshifts, variability and luminosities establish their extragalactic nature. Another misconception is that every quasar is currently visible as it exists today in the same physical state. Because light travels for billions of years, a distant quasar is observed at an earlier stage of its history. The term therefore carries an observational and historical meaning as well as a physical description of an exceptionally luminous accreting supermassive black hole.
