Quasars
Brilliant active galactic nuclei in the distant universe.
Overview
A quasar is an extremely luminous active galactic nucleus, powered by gas falling toward a supermassive black hole at a galaxy's centre. They are among the most distant and energetic objects observable, and they served as the first evidence that black holes shape galaxies.
- The name is a contraction of 'quasi-stellar radio source' — they appeared starlike but were not stars.
- A quasar can outshine its entire host galaxy, which is why the host is often invisible in early images.
- Their extreme redshifts made them probes of the early universe before anything else could reach that distance.
- Quasars, Seyferts, blazars and radio galaxies are largely the same phenomenon seen from different angles.
The identification problem
Quasars were first noticed as radio sources with optical counterparts that looked like stars but had spectra nobody could identify. The breakthrough came in 1963 when Maarten Schmidt recognised that the spectrum of 3C 273 showed ordinary hydrogen lines shifted by an enormous amount — the object was not a nearby star but something extraordinarily distant.
That resolution created a new problem. At such distances, the observed brightness implied a luminosity far exceeding an entire galaxy, produced from a region small enough to vary noticeably over months. Only accretion onto a supermassive black hole could plausibly release that much energy from that small a volume.
The engine
Gas spiralling toward a supermassive black hole forms an accretion disc heated to enormous temperatures by friction and magnetic stress. The conversion of gravitational potential energy to radiation is far more efficient than nuclear fusion, which is what allows a region comparable in size to the Solar System to outshine a galaxy of hundreds of billions of stars.
Surrounding the disc are fast-moving clouds producing broad emission lines, slower clouds producing narrow lines, and an obscuring torus of dust further out. Many systems launch relativistic jets along the spin axis, which are prominent in radio observations and can extend far beyond the host galaxy.
One phenomenon, many labels
Active galactic nuclei were historically catalogued under many names — Seyfert 1 and 2 galaxies, radio-loud and radio-quiet quasars, blazars, BL Lac objects — because they look different. The unified model attributes much of that variety to geometry: whether the line of sight passes through the obscuring torus, and whether a relativistic jet points toward the observer.
A blazar, on this account, is an AGN viewed nearly down the jet axis, which is why blazars show extreme and rapid variability. The unified picture is well supported but not complete — intrinsic differences in accretion rate and black hole spin also matter, and some sources change classification over time.
Quasars as cosmological tools
Because quasars are luminous enough to be seen across most of cosmic history, they function as beacons. Absorption lines imprinted on quasar light by intervening gas clouds map the distribution and composition of the intergalactic medium along the line of sight — the Lyman-alpha forest is built entirely from this effect.
Quasar counts also peak at earlier cosmic times and decline toward the present, indicating that supermassive black hole accretion was far more active in the past. That evolution is one of the strongest constraints on models of how galaxies and their central black holes grew together.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- What powers a quasar?
- Accretion onto a supermassive black hole. Gas spiralling inward through a disc converts gravitational potential energy into radiation with efficiency far exceeding nuclear fusion, which allows a region roughly the size of the Solar System to outshine a galaxy containing hundreds of billions of stars.
- Are there quasars near us?
- Not in the present-day universe at full quasar luminosity. Quasar activity peaked billions of years ago and has declined since, as central black holes exhausted their fuel supply. Nearby galaxies host lower-luminosity active nuclei, and many — including the Milky Way — host black holes that are currently quiescent.
- Why do quasars look like stars?
- Because they are so distant and so compact that they are unresolved points of light, and so luminous that they overwhelm the host galaxy around them. That is the origin of the name — 'quasi-stellar'. Deep imaging with modern telescopes can often reveal the host galaxy once the nuclear light is carefully subtracted.