Quasars: The Brightest and Most Powerful Objects in the Universe

Quasars are the most luminous persistent objects in the universe, powered by supermassive black holes that can outshine an entire galaxy of hundreds of billions of stars. A quasar is the blazing core of a distant galaxy, where matter spiralling into a central black hole releases staggering amounts of energy as light and radiation. The brightest quasar ever measured, J0529-4351, shines with the energy of roughly 500 trillion Suns, making quasars the leading answer to the question of the brightest thing in the universe.

Are Quasars the Brightest Thing in the Universe?

Yes — quasars are the brightest continuously shining objects known in the universe, far outshining ordinary galaxies and individual stars. A single quasar can emit thousands of times more light than the entire Milky Way, despite being concentrated into a region not much larger than our solar system. The only events that briefly rival or exceed quasar luminosity are transient explosions such as gamma-ray bursts and supernovae, which fade within hours, days, or weeks, whereas a quasar can blaze steadily for millions of years.

The reigning record holder is the quasar J0529-4351, identified in 2024 as the most luminous object yet discovered. Its host black hole consumes the equivalent of one Sun's worth of matter every day, making it both the brightest quasar and the fastest-growing black hole ever observed. This combination of extreme brightness and rapid growth is exactly what places quasars at the top of the cosmic luminosity scale.

What Is a Quasar?

A quasar is an extraordinarily bright, compact region at the centre of a distant galaxy, powered by a supermassive black hole drawing in surrounding gas and dust. Quasars are a type of active galactic nucleus, and they appear star-like through a telescope despite being entire galactic cores billions of light-years away. The energy they release comes not from nuclear fusion like ordinary stars, but from gravitational and frictional heating of matter as it falls toward the black hole.

Quasars share several defining characteristics that distinguish them from ordinary galaxies and stars:

• Point-like appearance — they look like single stars in optical images, which is why early astronomers misclassified them.
• Enormous distance — most quasars lie billions of light-years away, so their light shows us the early universe.
• Extreme luminosity — a quasar can outshine its entire host galaxy combined.
• Strong redshift — their light is heavily shifted toward longer wavelengths, revealing how fast they are receding with cosmic expansion.
• Powerful radiation across the spectrum — quasars emit radio waves, infrared, visible light, ultraviolet, and X-rays.

Modern radio and optical telescopes have established that quasars are not stars or galaxies in the conventional sense, but star-shaped objects radiating an immense amount of energy from a tiny central engine.

Etymology and Naming of Quasars

The word "quasar" comes from "quasi-stellar radio source," the term astronomers coined in the 1960s because these objects looked like stars yet emitted powerful radio waves no ordinary star could produce. The "quasi-stellar" part captures the central puzzle of their discovery: they appeared star-like in images but behaved nothing like stars when their spectra were examined. As astronomers later found many such objects that were radio-quiet, the broader term "quasi-stellar object" (QSO) came into use, though "quasar" remains the popular name.

How Bright Are Quasars Really?

Quasars routinely shine with the luminosity of trillions of Suns, releasing more energy from a region the size of our solar system than billions of stars spread across a whole galaxy. Their brightness is what first made them detectable across the vast distances of the universe, and it is the single property that makes quasars stand out as cosmic landmarks. The most extreme examples push the limits of how much light a single object can physically produce.

Quasar Luminosity Compared to Galaxies and Stars

A luminous quasar can outshine the combined light of the hundreds of billions of stars in its host galaxy by a factor of a thousand or more. For comparison, the Milky Way is a hundred thousand light-years across and contains on the order of a hundred to two hundred billion stars, yet a single quasar packed into a far smaller volume can radiate vastly more energy than all of them together. Measured against our own Sun, the brightest quasars emit hundreds of trillions of times more light, which is why they remain visible even when their light has travelled for more than ten billion years to reach Earth.

How Quasar Brightness Is Measured

Astronomers measure quasar brightness by combining the object's apparent brightness in the sky with its distance, derived from the redshift of its spectrum, to calculate its true luminosity. Because quasars are so far away, instruments such as the Hubble Space Telescope, the Very Large Telescope operated by the European Southern Observatory, and the James Webb Space Telescope are used to capture their faint light and resolve detail. Spectroscopy then reveals how much the light has been stretched by cosmic expansion, which fixes the distance and, in turn, the absolute energy output. In some cases, gravitational lensing magnifies a distant quasar, requiring careful correction to recover its real brightness.

What Powers a Quasar's Extreme Brightness?

A quasar's brightness is powered by a supermassive black hole at the centre of a galaxy, surrounded by a superheated disk of infalling matter that converts gravitational energy into radiation with extraordinary efficiency. As gas and dust spiral inward, friction and compression heat the material to millions of degrees, causing it to glow brilliantly before crossing the point of no return. This process turns the black hole's gravity into one of the most efficient engines for producing light known to physics.

The Supermassive Black Hole Engine

At the heart of every quasar sits a supermassive black hole, an object so dense that not even light can escape its gravitational grip. A black hole forms when a massive star or cluster of matter collapses under its own gravity: compression is first resisted by temperature and radiation pressure, then by the density of neutron-star matter, but if the mass is large enough, even neutron matter cannot hold, and collapse continues. Einstein's relativity predicts the final barrier — in extreme gravitational fields, time slows, and at a critical field strength time effectively stops for an outside observer.

Consider a radio transmitter inside a collapsing object set to signal every five minutes. To a distant observer, the first signal arrives on time, the second a minute late, the third five minutes late, the fourth half an hour late, the fifth several months late, the sixth millions of years late — and the seventh never arrives at all. This time-dilation effect is the final obstacle to runaway compression. Calculations show that every mass has a minimum volume described by the Schwarzschild formula; once matter is squeezed inside this "Schwarzschild sphere," time practically stops, all internal processes halt, and the matter becomes invisible and undetectable because radiation depends on motion and there is none.

Such an object cannot even be found by a locator, since any ray reaching it is absorbed and never returns. A spaceship, asteroid, or interstellar wanderer caught by its powerful gravitational field would be crushed, compressed, and fused to the collapsing mass.

Accretion Disk Mechanics and Energy Generation

The light of a quasar comes overwhelmingly from its accretion disk, a flattened, swirling ring of gas and dust that forms as material falls toward the supermassive black hole. Because the infalling matter carries angular momentum, it cannot plunge straight in; instead it orbits, spreading into a disk where layers moving at different speeds rub against one another. This friction heats the inner regions of the accretion disk to temperatures of millions of degrees, causing it to radiate intensely across ultraviolet and X-ray wavelengths.

Accretion is remarkably efficient as an energy source — far more so than nuclear fusion. Up to around 10 percent of the rest-mass energy of infalling matter can be converted into radiation before it disappears past the event horizon, compared with less than 1 percent for the fusion that powers stars. This efficiency is the fundamental reason a quasar's central engine, only as large as our solar system, can outshine the billions of stars in its surrounding galaxy.

Black Hole Mass and Energy Output Measurements

The supermassive black holes powering quasars typically contain millions to tens of billions of times the mass of the Sun, and their energy output scales with how rapidly they consume matter. Astronomers estimate these masses from the speed of gas orbiting the black hole and from the total luminosity, since there is a physical ceiling — the Eddington limit — on how brightly an object of a given mass can shine before radiation pressure blows the infalling material away. The most extreme quasars sit near or at this limit, devouring enormous quantities of gas. The black hole behind J0529-4351 is estimated at roughly 17 to 19 billion solar masses, and it grows by swallowing the equivalent of one Sun every single day.

The History of Quasar Discovery

Quasars were first identified in the early 1960s, when astronomers traced mysterious radio signals to tiny, star-like points whose spectra revealed they lay at enormous cosmic distances. The discovery overturned expectations, because no ordinary star could produce such powerful radio emission, and no known galaxy could appear so compact. Within a few years, quasars became one of the central puzzles of modern astronomy.

Discovery by Maarten Schmidt and Cyril Hazard

Quasars were discovered in the early 1960s, with Dutch-American astronomer Maarten Schmidt at the forefront — somewhat earlier than the discovery of pulsars. Australian-based astronomer Cyril Hazard pinned down the precise position of a powerful radio source and found a tiny star-like point in its place. When that object's spectrum was examined, it proved to be extremely far away and receding from us very rapidly, a hallmark of the expansion of the universe. A single star could never be seen at such a distance, so the object had to be the bright core of a distant galaxy in the Universe. The famous early example 3C 273 became the first quasar whose redshift Schmidt decoded.

Early Soviet and American Brightness Observations

In 1963, Soviet scientists Yuri N. Efremov and A. S. Sharov, simultaneously with American astronomers, found that the "Hazard Galaxy" periodically changes its brightness, with a period of variation of just one week. This was extraordinary, because astronomers had never encountered galaxies that could "wink" so frequently. Our own Galaxy spans a hundred thousand light-years and contains some 150 billion stars, and the idea that all of them could synchronously change brightness is impossible. Nearly sixty years after the first quasar was found, the rapid variation pointed clearly toward a small, central engine rather than a galaxy-wide effect.

How Black Holes Form

Black holes form when a large mass collapses under its own gravity until nothing can halt the compression, ultimately concentrating matter into a region from which light cannot escape. This process is the foundation of the quasar phenomenon, because it creates the gravitational engine that lights up an active galactic nucleus. Understanding black hole formation explains why quasars can be so small yet so luminous.

Gravitational Collapse and Neutron Stars

The collapse that produces a black hole begins when a dying star "falls into itself," its matter shrinking under a powerful gravitational field. This compression is first stopped by temperature and radiation backpressure, then by compaction to the density of neutron stars. If the collapsing mass is greater still, and the gravitational field more powerful, the resistance of neutron matter becomes insufficient and the compaction continues beyond it.

Time Dilation and the Schwarzschild Sphere

Beyond neutron-star density, the final barrier to collapse is set not by matter but by time itself, an effect predicted by Albert Einstein. In strong gravitational fields, time moves more slowly, and above a critical field strength it stops altogether — though only from the perspective of an outside observer. Calculations show that every material formation has a minimum volume given by the Schwarzschild formula; once matter is squeezed inside this Schwarzschild sphere, time practically stops and all internal processes cease.

Why a Black Hole Is Invisible

A black hole is invisible because matter inside the Schwarzschild sphere emits no radiation — radiation requires motion, and within the sphere there is effectively none. It cannot be detected by a locator either, since any ray that reaches it is absorbed and never returns. The only way to detect such a fantastically dense clot of matter is through its super-powerful gravitational field, which is precisely how astrophysicists infer the presence of the black holes that power quasars.

The Mystery of Quasar Pulsation

The weekly brightness changes that astonished early observers are explained by the tiny size of a quasar's central engine, not by any galaxy-wide pulsation. An object can only vary in brightness as fast as light can cross it, so rapid weekly variation means the emitting region must be only light-days or light-weeks across — comparable to our solar system rather than a galaxy. This realization was a key clue that quasars are powered by a compact source, the supermassive black hole and its accretion disk, embedded in a much larger host galaxy.

Most scientists agree the quasar phenomenon is tied to the collapse of matter into a state we are unlikely to reproduce in Earth's laboratories even in the distant future. The variability, combined with the immense energy output, pointed researchers toward gravitational accretion onto a black hole as the only mechanism capable of explaining both the brightness and its rapid changes.

Active Galactic Nuclei and Quasar Classification

Quasars belong to a broader family called active galactic nuclei (AGN) — galaxy centres powered by accreting supermassive black holes, classified by their brightness, orientation, and radio emission. All AGN share the same basic engine, but they look different depending on how much they radiate and from what angle we view them. This classification ties quasars to several related types of active galaxy.

• Seyfert galaxies — relatively low-luminosity AGN whose host galaxy remains clearly visible around a bright nucleus.
• Quasars — the most luminous AGN, so bright that the nucleus outshines the entire host galaxy.
• Blazars — AGN whose relativistic jet points almost directly at Earth, producing rapid, dramatic variability.
• LINER galaxies — low-ionization nuclear emission regions, the faintest end of the AGN family.
• Radio-loud and radio-quiet quasars — distinguished by whether they launch powerful jets producing strong radio emission, as only a minority do.

Some quasars are also reddened by surrounding cosmic dust, producing "red quasars" best studied in the infrared; researchers including Dr Vicky Fawcett of the University of Newcastle and the University of Portsmouth have examined how dust and quasar winds shape these objects.

Blazars and Relativistic Jets

Blazars are active galactic nuclei viewed end-on, so that a relativistic jet of particles launched near the black hole points almost straight at the observer. This alignment beams and amplifies the jet's radiation, making blazars appear extremely bright and highly variable, sometimes changing on timescales of hours. The BL Lac object class is one well-known type of blazar, named after the prototype object that first revealed this behaviour. Relativistic jets demonstrate that quasars and their relatives do not only swallow matter — they also fling it back out into intergalactic space at near light speed.

Quasars in the Early Universe

Quasars were far more common in the early universe, and because their light takes billions of years to reach us, the most distant quasars show us conditions when the cosmos was young. Observing them is effectively looking back in time, revealing how the first supermassive black holes grew and how galaxies assembled. This makes quasars essential tools for studying cosmic history.

Galaxy Mergers and Quasar Activation

Quasar activity is often triggered when galaxies collide and merge, funneling vast quantities of gas toward the central black hole and igniting the accretion disk. These mergers disrupt the orderly orbits of gas clouds, sending material inward where it can be captured and consumed. The intense radiation and quasar winds that result then push back on the surrounding galaxy, ejecting and heating gas in a process called feedback that can shut down star formation. In this way quasars do not just light up their host galaxies — they actively shape how those galaxies evolve.

Early Universe Formation and Galaxy Evolution

The existence of extremely distant quasars poses a deep puzzle, because their black holes grew to billions of solar masses within the first billion years after the Big Bang. Objects such as J0313-1806, one of the most distant known quasars, show that supermassive black holes assembled startlingly fast in the young universe. Theorists including Priyamvada Natarajan have studied how such rapid early black hole growth might have occurred, while observations from the James Webb Space Telescope continue to probe how early quasars and their host galaxies co-evolved. Light travel time turns these objects into snapshots of an era long before the Sun and Earth existed.

The Brightest Quasar Ever Discovered

The brightest quasar ever discovered is J0529-4351, announced in 2024 as the most luminous known object in the universe, radiating with the power of roughly 500 trillion Suns. It was found by a team led by Christian Wolf and Christopher Onken at the Australian National University, using data from the Schmidt Southern Sky Survey, the European Space Agency's Gaia satellite, and follow-up observations with the Very Large Telescope. The discovery was published in Nature Astronomy, with key observations made from Siding Spring Observatory.

Remarkably, J0529-4351 had been hiding in plain sight. Its sheer brightness caused automated systems and machine-learning classifiers to repeatedly misidentify it as a nearby ordinary star rather than a record-breaking quasar — a striking example of how the most extreme objects can be overlooked precisely because they defy expectations.

The Fastest-Growing Black Hole Observed

The black hole powering J0529-4351 is the fastest-growing black hole ever observed, consuming the equivalent of one Sun's worth of matter every day. Its accretion disk is estimated to span around seven light-years across — an enormous structure that is itself a powerful source of light and heat. Researcher Samuel Lai contributed to characterizing this object, whose voracious appetite explains both its record luminosity and its rapid mass growth. Future instruments will let astronomers refine measurements of its mass and feeding rate, offering rare opportunities to study black hole growth at its physical limits.

Quasars as Cosmic Beacons

Quasars act as cosmic beacons, their extreme brightness allowing astronomers to study the most distant reaches of the universe and to probe the matter lying between us and them. Because a quasar's light passes through intervening gas clouds and galaxies on its long journey, the light carries information about everything it encounters. This makes distant quasars uniquely valuable for mapping cosmic structure and history.

Gravitational Lensing and Magnification Effects

When a massive galaxy lies between Earth and a distant quasar, its gravity acts as a gravitational lens, bending and magnifying the quasar's light. This lensing can brighten an otherwise faint quasar and, in dramatic cases, split its image into multiple copies — including rare quadruple images arranged around the lensing galaxy. While magnification helps astronomers study fainter, more distant quasars, it also complicates brightness measurements, since the true luminosity must be recovered by correcting for the lens's amplification. Lensed quasars therefore serve both as natural telescopes and as probes of the mass distribution in the lensing galaxies.

Other Contenders for the Brightest Objects in the Universe

While quasars are the brightest persistent objects in the universe, a few transient events can briefly outshine them before fading. These short-lived phenomena reach extreme luminosities for moments rather than ages, which is why quasars retain the title for sustained brightness.

• Gamma-ray bursts — the most energetic explosions known, releasing in seconds more energy than the Sun will emit in its entire lifetime, but lasting only moments.
• Supernovae and superluminous supernovae — exploding stars that can briefly rival a whole galaxy in brightness before dimming over weeks.
• Tidal disruption events — flares produced when a star is torn apart by a black hole, brightening sharply and then fading.
• Blazars during flares — beamed AGN that, while a form of quasar, can spike in apparent brightness when their jets brighten.

Among all of these, quasars such as J0529-4351 remain the benchmark for the brightest object in the universe, combining record luminosity with the ability to shine steadily across cosmic time. For more on astronomy and the wider cosmos, explore our Astronomy articles.