In 1963, astronomer Maarten Schmidt trained the 200-inch Hale Telescope at Palomar Observatory onto a faint blue point of light catalogued as 3C 273. What he measured changed cosmology forever. The object was not a nearby star. It was a galaxy 2.4 billion light-years away, blazing with the energy of two trillion suns. Schmidt had identified the first confirmed quasar — and nothing in astronomy was ever quite the same again.

The Discovery That Broke the Rules

The word "quasar" is a contraction of quasi-stellar radio source, a name born of confusion. Early radio surveys in the 1950s picked up dozens of compact, intensely bright objects that looked stellar on photographic plates but emitted vast quantities of radio energy. When optical astronomers turned their telescopes toward them, the objects appeared star-like yet showed redshifts far larger than any known star — or even any known galaxy — could produce.

Schmidt's breakthrough was recognising that the spectral lines of 3C 273, which had seemed nonsensical, were simply hydrogen lines shifted dramatically toward the red end of the spectrum by the expansion of the universe. The object was receding at roughly 16 percent of the speed of light. Its distance — and therefore its extraordinary luminosity — became undeniable.

In the decades since, astronomers have catalogued more than 750,000 quasars. The most distant known, J0313-1806, sits at a redshift of 7.64 — meaning its light left when the universe was just 670 million years old, less than 5 percent of its current age.

How Quasars Work

A quasar is not a star. It is the intensely luminous core of a galaxy — an active galactic nucleus, or AGN — powered by a supermassive black hole consuming enormous quantities of surrounding gas and dust. As matter spirals inward through an accretion disc, it is compressed, heated to hundreds of millions of degrees, and converts a fraction of its mass directly into energy via friction and magnetic forces. The process is staggeringly efficient: quasars can radiate up to 10 percent of infalling mass as energy, compared with less than 1 percent for nuclear fusion inside a star.

The Engine Behind the Light

The black hole at the heart of a quasar is not ordinary. These are supermassive black holes — objects with masses ranging from hundreds of millions to tens of billions of times that of the Sun. J0313-1806's central black hole, for example, masses about 1.6 billion solar masses and must have grown to that size astonishingly quickly after the Big Bang, posing an unsolved puzzle about the formation of the earliest cosmic structures.

The geometry of the accretion disc and the strength of the surrounding magnetic field determine much of what we observe. In many quasars, a pair of powerful relativistic jets — streams of plasma moving at close to the speed of light — punch out perpendicular to the accretion disc and extend for millions of light-years into intergalactic space.

"A single quasar can outshine its entire host galaxy by a factor of a hundred — all that energy pouring from a region no larger than our solar system."

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What Quasars Tell Us About the Cosmos

Quasars are not merely curiosities. They are among the most powerful tools cosmologists possess for understanding the structure and history of the universe. Because they are so bright and so distant, their light passes through billions of light-years of intervening gas, and the absorption patterns imprinted on that light act as a chemical diary of the cosmos — recording the abundances of hydrogen, helium, carbon, and heavier elements at different epochs.

They also illuminate a key question: how did galaxies grow? Most astronomers now believe that virtually every large galaxy, including the Milky Way, harbours a supermassive black hole at its centre. When those black holes were actively feeding in the distant past, they appeared as quasars. The quasars we observe today are essentially looking back at our galaxy's ancestors in a period of frenzied growth.

  • Most luminous quasar known: J0529-4351, discovered in 2024, is 500 trillion times more luminous than the Sun
  • Nearest quasar: 3C 273, approximately 2.4 billion light-years from Earth
  • Most distant quasar: J0313-1806, at a redshift of z = 7.64
  • Typical quasar size: the energy-generating region is often less than one light-day across
  • Peak quasar activity: the universe had the most quasars roughly 10–12 billion years ago

Modern surveys like the Sloan Digital Sky Survey (SDSS) and the Dark Energy Spectroscopic Instrument (DESI) have mapped quasars across the sky in unprecedented numbers, using their distribution to trace how the large-scale structure of the universe — the cosmic web of filaments and voids — has evolved over billions of years.

With the James Webb Space Telescope now able to resolve the host galaxies around distant quasars in infrared light, a new era of quasar science is underway. Webb has already imaged host galaxies around quasars at redshifts above 6, revealing that some ancient galaxies were already far more massive and structured than current models predicted. The brightest beacons in the sky, it turns out, are still full of surprises.