A quasar is the ultra-luminous core of a galaxy whose central supermassive black hole is actively devouring matter. Visible across the observable Universe, they outshine their own host galaxies.
Imagine an ordinary galaxy — say, ours, with its few hundred billion stars. Then imagine, at the center, a single lamp that shines as bright as the entire galaxy, sometimes a hundred times brighter. That lamp is a quasar: an active galactic nucleus whose luminosity exceeds 10³⁸ W, or 10¹² times that of the Sun.
The energy source is not nuclear, unlike stars. It is gravity. A supermassive black hole (10⁶ to 10¹⁰ M☉) at the core of the galaxy actively swallows gas, dust and sometimes entire stars. Before crossing the event horizon, this material forms an accretion disk heated by friction to millions of kelvin, radiating massively across the spectrum — UV, optical, X-ray, radio. The mass-energy conversion efficiency reaches ~10 % (versus 0.7 % for nuclear fusion), which explains the extraordinary efficiency of the mechanism.
Many quasars also launch relativistic jets perpendicular to the disk, detectable in radio across thousands of kiloparsecs. When a jet points almost at us, we call the object a blazar: extreme variability, strong polarization.
The historical lineage is well-marked. In 1959, Cambridge published the 3C radio catalog. Allan Sandage optically identified 3C 48 in 1960 as a 'blue star' — an anomaly. In 1963, Maarten Schmidt (Caltech) recognized in the spectrum of 3C 273 hydrogen lines redshifted by z = 0.158: not a star, but an object at 750 Mpc, thus intrinsically monstrous. Quasars were born as a category.
Today we understand them as an active phase in the life of galaxies: all massive galaxies probably went through a quasar stage, billions of years ago.
Luminosity. 10³⁸ to 10⁴¹ W, that is 10¹² to 10¹⁴ L☉. The brightest quasars outshine their host galaxy by a factor of 100.
Size. The main emitting region is less than a light-day across — we know this because some quasars vary in brightness on timescales of days, which imposes a causal maximum size. That is smaller than the Solar System, and yet it radiates more than a galaxy.
Central black hole mass. From 10⁶ to 10¹⁰ M☉. The record approaches 4 × 10¹⁰ M☉ for TON 618. Accretion rates reach ~1 to 10 M☉ per year (~2-20 Earth masses per second).
Redshift (z). Known quasars range from z ≈ 0.05 (nearby quasars) to z = 7.64 for J0313-1806, discovered in 2021. That one shows a black hole of 1.6 × 10⁹ M☉ already in place ~670 million years after the Big Bang — a headache for models of early black hole formation.
Distances. 3C 273, the first quasar identified, sits at 750 Mpc (z = 0.158). Visual magnitude 12.9 — reachable with amateur 200 mm telescopes. The most distant known quasars are beyond 13 billion light-years away.
All quasars are active galactic nuclei (AGN), but not all AGN are quasars. Naming depends on luminosity and on the angle at which we see the system.
Seyferts (types 1 and 2). Moderately luminous AGN, common in nearby spirals. NGC 4151 is the archetype.
Radio-loud quasars. About 10 % of quasars emit strongly in radio, with jets. 3C 273 is the prototype.
Radio-quiet quasars. The majority (~90 %), with modest radio emission.
Blazars (BL Lac + FSRQ). Quasars seen down the jet axis. Extreme variability, very-high-energy emission. Markarian 421 and Markarian 501 are observed by gamma-ray telescopes (HESS, MAGIC, CTA).
Notable examples. • 3C 273 (Virgo) — first quasar identified, 1963, magnitude 12.9, optically brightest quasar in the sky. • 3C 48 (Triangulum) — first identified as an 'anomalous blue star' as early as 1960. • TON 618 — record mass of ~4 × 10¹⁰ M☉. • SDSS J0100+2802 (z = 6.3) — very bright at high redshift. • J0313-1806 (z = 7.64) — most distant known (2021). • The Einstein Cross (Q2237+0305) — a single quasar split into four images by gravitational lensing. • J1120+0641, J1342+0928 — quasars at z > 7 studied by Hubble and JWST.
Optical spectroscopy. This is what first revealed quasars. The spectrum shows broad emission lines (hydrogen, MgII, CIV…) shifted to the red. Measuring z gives the distance and, combined with the observed flux, the intrinsic luminosity. The SDSS catalogue now contains several hundred thousand.
Radio astronomy. Radio-loud quasars emit jets detected by VLA, LOFAR and soon SKA. VLBI (Very Long Baseline Interferometry) resolves jets down to parsec scales.
X-rays and gamma rays. Space observatories Chandra, XMM-Newton, Swift and eROSITA follow quasars at high energies. Fermi and ground-based Cherenkov telescopes capture gamma-ray photons from blazars.
Gravitational lenses. Some distant quasars are multi-imaged by intervening galaxies. The Einstein Cross (Q2237+0305) is the finest example. These systems test general relativity and measure the Hubble constant H₀.
Amateur observation. 3C 273 is reachable with a 200 mm telescope under dark skies, in spring, in Virgo, at magnitude 12.9. It appears as an ordinary star — no detail distinguishes that point of light. The vertigo comes from realizing: the photons entering your eye come from a hot accretion disk at millions of kelvin, formed before complex life appeared on Earth, 2.4 billion light-years away. Our sky map tool lets you locate it.
Black hole. A quasar IS NOT a black hole — it is the LUMINOUS PHENOMENON powered by accretion onto a supermassive black hole. The black hole itself emits nothing; what we see is the accretion disk just above its horizon. Some supermassive black holes are in a quasar phase, others are quiet (like Sagittarius A* at the heart of the Milky Way today).
Active galaxy (AGN) — general case. 'AGN' is the umbrella term: it covers all galactic nuclei powered by accretion. Seyferts (less luminous), radio galaxies, blazars and quasars are subcategories. A quasar = an AGN whose luminosity reaches ~10³⁸ W or more, and whose nucleus often outshines the host galaxy.
Gamma-ray burst (GRB). An extremely brief transient phenomenon (milliseconds to minutes) from a supernova or neutron-star merger. Quasars, by contrast, shine continuously for millions of years.
Extreme variable star. No, quasars are not stars, despite their 'quasi-stellar' appearance on photographic plates. That confusion gave them their name, but spectra settled the matter in 1963.
Blazar. A blazar is a quasar viewed along its relativistic jet axis. Same physical engine, different geometry.
Because their luminous phase is an episode in the life of a galaxy, not a permanent state. The quasar golden age lies between z = 2 and z = 3, about 10 to 11 billion years ago, when galaxies were young and gas-rich. Since then, most supermassive black holes have exhausted their local fuel and quieted down. So we mostly see DISTANT quasars because this phase occurred EARLY in cosmic history. 3C 273, at 750 Mpc, is considered 'nearby' for a quasar.
Very probably yes, but a long time ago. Sagittarius A* is remarkably quiet today: its accretion rate is ~10⁻⁸ that of an active quasar. But clues (X-ray echoes on central molecular clouds, Fermi bubbles detected in gamma rays) suggest it has undergone activity episodes in the last few million years, and more strongly 6-10 billion years ago. All massive galaxies likely went through a quasar stage at least once.
Yes, 3C 273 is within reach of a 200 mm telescope under good skies. It lies in Virgo, a few degrees north of the M104 galaxy (Sombrero), at magnitude 12.9. It appears as an ordinary star — no detail will distinguish the bright dot. What is dizzying is the realization: the photons entering your eye come from a hot accretion disk at millions of kelvin, formed when complex life did not yet exist on Earth, 2.4 billion light-years away.
That is the puzzle of very-high-redshift quasars like J0313-1806 (z = 7.64). The standard growth-by-accretion model (Eddington limit) caps the rate, and a ~100 M☉ stellar seed takes too long to reach a billion M☉ in 700 million years. Two leading hypotheses: direct collapse of primordial gas clouds into massive 'seeds' (10⁴-10⁵ M☉) that then grow rapidly, or episodes of super-Eddington accretion. Ongoing JWST observations of early quasars should settle the question this decade.