Quasar

• 1. The Brightest Point

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  A quasar named 3C 273, in the constellation of Virgo, is the brightest of its kind in Earth's sky. Quasars are extremely luminous cores of distant galaxies, and this one offers a way to feel their scale. Move 3C 273 to a distance of about 33 light-years, and it would shine in the sky about as brightly as the Sun. Its true luminosity is about 4 trillion times that of the Sun. From Earth, a medium-size amateur telescope is enough to see it. Most other quasars cannot be seen through small telescopes at all. Three questions run through this documentary. What engine can pour out that much light? Why did astronomers in the 1950s and 1960s take these objects for odd stars with unreadable spectra? And why do quasars belong mostly to the distant past?

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• 2. A Spectrum Nobody Could Read

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  Between 1917 and 1922, the work of Heber Doust Curtis, Ernst Öpik and others showed that many so-called nebulae were distant galaxies like the Milky Way. When radio astronomy began in the 1950s, a few puzzling objects turned up among them. They gave off radiation at many frequencies, yet optical telescopes found nothing at those positions, or at most a faint point like a distant star. Their spectral lines, which reveal the chemical elements in a source, made no sense. Some brightened and faded quickly in visible light, and even faster in X-rays. That set an upper limit on their size, perhaps no bigger than the Solar System.

  The first two of these objects, 3C 48 and 3C 273, turned up in the late 1950s in all-sky radio surveys. Small telescopes, together with the Lovell Telescope used as an interferometer, showed that both had a very small angular size. By 1960, hundreds of such sources had been logged in the Third Cambridge Catalogue. In 1963, Allan Sandage and Thomas A. Matthews published a firm match between 3C 48 and a faint blue star. Its spectrum was full of broad emission lines that nobody could identify.

  In 1962, the British-Australian astronomer John Bolton, who made many of the early quasar observations, helped crack the second case. Astronomers predicted that 3C 273 would be hidden by the Moon five times. During one of these occultations, Cyril Hazard and Bolton used the Parkes Radio Telescope to pin down its position. That let Maarten Schmidt find the visible counterpart and record its spectrum with the 200 in Hale Telescope on Mount Palomar.

  The same baffling lines appeared. Then Schmidt recognised them as ordinary hydrogen lines, shifted toward the red by 15.8 percent. At the time that was a high redshift; only a handful of much fainter galaxies were known to exceed it. If the shift came from motion, 3C 273 was receding at a speed far beyond that of any known star. Even such a speed could not explain its huge radio emission.

  Old photographic plates showed 3C 273 as far back as the 1900s, and it varied over yearly timescales. That meant much of its light came from a region less than 1 light-year across, tiny next to a galaxy. Schmidt, Greenstein and Oke soon read the strange spectrum of 3C 48 as hydrogen and magnesium redshifted by 37 percent. Two more quasar spectra in 1964 and five in 1965 were confirmed as ordinary light stretched to an extreme degree.

  In May 1964, the astrophysicist Hong-Yee Chiu put the word quasar into print, in the magazine Physics Today. He admitted the difficulty of naming something so mysterious. In his words: "Because the nature of these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them." He then added: "For convenience, the abbreviated form 'quasar' will be used throughout this paper." Schmidt himself offered two possible readings of 3C 273, and neither was comfortable.

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• 3. Years of Doubt

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  Schmidt concluded that 3C 273 was either a single star about 10 km wide, inside or near our own galaxy, or a distant active galactic nucleus. He judged the distant, extremely powerful option more likely. Most of his colleagues disagreed. If quasars were far away, they would have to radiate more energy than any commonly accepted mechanism could supply. Nuclear fusion, the process that powers stars, fell far short.

  In 1964, Edwin E. Salpeter and Yakov Zeldovich suggested the answer now accepted: matter in an accretion disc falling into a supermassive black hole. Many astronomers rejected it, since black holes were then widely treated as a purely theoretical idea.

  The rival explanations of the 1960s and 1970s each carried their own problems. One camp held that quasars were nearby, with their redshift coming from light climbing out of a deep gravitational well. A star massive enough to do that would be unstable, exceeding what is known as the Hayashi limit. Quasars also show forbidden spectral emission lines. Before this, those lines had been seen only in hot, low-density gaseous nebulae, which are too thin to produce the power or fit inside such a well.

  Some proposals were stranger still. One imagined a stable, unknown form of antimatter. Another cast quasars as the white hole end of a wormhole. A third pictured a chain reaction of many supernovae. The core dilemma was hard to escape. Small and close objects would easily explain the apparent power, but not the redshifts or the lack of visible movement against the background sky.

  The tide turned from about the 1970s, as evidence piled up from several directions. Early X-ray space observatories, better knowledge of black holes and modern cosmology all pointed the same way. Absorption lines from intervening gas explained several spectral oddities, and gravitational lensing added more support. In 1971, Gunn found that galaxies containing quasars shared the quasars' redshift. In 1973, Kristian showed that the fuzzy glow around many quasars fit a dimmer host galaxy. The accretion-disc mechanism was modelled in the 1970s. By 1987 it was described as well accepted, and almost all researchers took the great distances and energies as real. What those black holes actually do with their fuel turned out to be a matter of startling arithmetic.

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• 4. The Black Hole Engine

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  A quasar's central black hole can weigh anywhere from millions to tens of billions of solar masses. Surrounding it is a gaseous accretion disc. The energy does not come from inside the hole, since nothing escapes the event horizon. It comes from gravitational stresses and immense friction in the disc, as material orbits and spirals inward. The result can outshine the Milky Way, with its 200 to 400 billion stars, by as much as a thousand times.

  The efficiency is the key. Infalling matter near a black hole can turn roughly 6 percent to 32 percent of its mass into energy. In a Sun-like star, fusion manages just 0.7 percent. Supernovae also release enormous power, but they last only for days. Matter and antimatter annihilation would also work, yet the universe does not appear to have held large amounts of antimatter at the relevant times.

  The fuel bill is staggering. To reach 10 to the power 40 watts, the typical brightness of a quasar, a black hole must consume the equivalent of 10 solar masses every year. The brightest known quasars devour 1,000 solar masses a year, which is equivalent to 10 Earths per second.

  Size follows from speed. Quasar brightness changes over time scales from months down to hours. Every part of a source must be able to coordinate such changes, so a quasar that varies over a few weeks cannot be much more than a few light-weeks across. A source that small, yet that powerful, demands exactly this kind of efficient engine.

  Gas rarely plunges straight in. It carries angular momentum around the black hole, so it collects into a disc first. Detailed simulations reported in 2021 found that structures such as spiral arms apply gravitational brakes to gas that would otherwise orbit a galaxy centre forever. That braking lets the gas fall toward the black hole. A merger between two galaxies can also ignite or re-ignite a quasar by delivering fresh matter. It has even been suggested that a quasar could form when the Andromeda Galaxy collides with the Milky Way, in approximately 3-5 billion years.

  In March 2021, a team connected to the Event Horizon Telescope presented the first polarized-light image of a black hole. It showed the one at the centre of Messier 87, an elliptical galaxy about 55 million light-years away in the constellation Virgo. The image revealed the forces that give rise to quasars.

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• 5. Many Faces

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  Only about 10 percent of quasars are radio-loud, meaning they have powerful jets and strong radio emission. The other 90 percent or so are radio-quiet, with weaker radio output. This split explains why astronomers use the broader term QSO, for quasi-stellar object, alongside the word quasar.

  Broad absorption-line quasars, found in roughly 10 percent of the population, show dark bands shifted toward blue. These bands come from gas streaming outward from the nucleus toward the observer. Ionized carbon, magnesium, silicon and nitrogen can all be detected this way. Such quasars are usually radio-quiet.

  Type 2 quasars hide their accretion disc and broad emission lines behind dense gas and dust. They are the higher-luminosity counterparts of Type 2 Seyfert galaxies. Red quasars look redder than normal because of moderate dust inside their host galaxies, and infrared surveys show they make up a substantial fraction of the total. Weak emission line quasars simply have unusually faint lines in their ultraviolet and visible spectra. Iron quasars, such as IRAS 18508-7815, show strong lines from low-ionization iron.

  The fastest changes come from jets pointing at us. In optically violent variable quasars, relativistic beaming of the jet produces strong and rapid brightness swings. These are a type of blazar. Seen head-on, blazar jets often contain blobs that seem to move faster than light. That is an optical illusion produced by the properties of special relativity.

  In the 1980s, researchers built unified models. A quasar's look depends on the black hole's mass, its feeding rate, the tilt of the disc, whether a jet exists, and how much gas and dust blocks the view. In many cases the viewing angle alone separates quasars from blazars and radio galaxies.

  The emission lines of hydrogen, helium, carbon, magnesium, iron and oxygen are the brightest in quasar spectra. They span atoms from neutral to highly ionized. Stars cannot produce such a wide range, so the gas must be bathed in radiation from the quasar itself. Most quasars shine brightest in the ultraviolet hydrogen line at 121.6 nm. Because of extreme redshift, that peak has been observed as far red as 900.0 nm, in the near infrared.

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• 6. Light From Deep Time

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  The nearest known quasar lies about 600 million light-years from Earth, and the most distant known active nucleus sits at a redshift of 10.1. That record belongs to UHZ1, whose comoving distance is more than 31 billion light-years. Its look-back time is 13.2 billion years. Distances can exceed the universe's 13.8-billion-year history because space itself is expanding.

  About a million quasars have reliable spectroscopic redshifts, and 2-3 million more appear in photometric catalogs. By July 2023, more than 900000 had been found, mostly by the Sloan Digital Sky Survey. All observed quasar redshifts fall between 0.056 and 10.1, as of 2024. Because of the finite speed of light, the farthest quasars and their surroundings appear as they were in the very early universe.

  In 1967, Maarten Schmidt found that quasars were far more common in the early universe than today. This was early strong evidence against steady-state cosmology and in favor of the Big Bang. Surveys place the peak of quasar activity approximately 10 billion years ago. The reason is fuel. A quasar fades when the gas and dust near its black hole run out, and the galaxy turns ordinary.

  Several dozen large nearby galaxies, including the Milky Way, are confirmed to host a similar black hole without showing quasar activity. So most large galaxies may have had a bright phase. Black holes and their host galaxies appear to co-evolve, though not in a way fully understood. One idea is feedback, in which quasar jets, radiation and winds shut down star formation. Jets in clusters of galaxies are known to have enough power to keep hot cluster gas from cooling and falling onto the central galaxy.

  Quasars also record the end of reionization after the Big Bang. The oldest, at a redshift near 6, show a Gunn-Peterson trough, which points to neutral gas between galaxies. Younger quasars show a spiky Lyman-alpha forest instead, meaning that gas had become plasma, with neutral gas left only in small clouds. Current theories say quasars were not the main cause. The earliest stars, called population III stars, may account for 70 percent, and dwarf galaxies for 30 percent. Heavier elements seen in quasars imply a massive earlier round of star formation. NASA's Spitzer Space Telescope may have caught light from those stars in 2005, though that remains unconfirmed.

  Groups of several quasars, called large quasar groups, may rank among the largest known structures in the universe, if they trace the distribution of mass. A study published in February 2021 counted more quasars toward Hydra than in the opposite direction. That would suggest Earth is moving toward Hydra, yet the direction sits about 28 degrees from Earth's motion measured against the cosmic microwave background.

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• 7. Doubles and Mirrors

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  The Twin Quasar, Q0957+561, was found in 1979 and was the first gravitationally lensed quasar. It was also the first observational confirmation of the lens effect predicted by Albert Einstein's general theory of relativity. A lensed quasar shows up as double, triple or quadruple images of one object. PG1115+08 is an example of a triple. Quadruple-image quasars include the Einstein Cross and the Cloverleaf Quasar, first found in the mid-1980s. The Gaia all-sky survey has already turned up a dozen or so Einstein crosses.

  APM 08279+5255, discovered in 1998, was given an absolute magnitude of -32.2. Imaging with the Hubble Space Telescope and the 10 m Keck Telescope showed that it is lensed. Its light is magnified by a factor of about 10. Even so, it is substantially more luminous than nearby quasars such as 3C 273.

  Multiple images now serve cosmology. In time delay cosmology, astronomers measure the delay between flickers in different images of one source. Each image has a different path length, and combined with a model of the deflecting mass, that gives an absolute distance independent of the standard cosmic distance ladder. The method aims to probe the Hubble tension. Some results from 2020 claimed a precision of 2-3 percent, but later proved to suffer from systematic errors caused by overly strong assumptions. In 2025, the TDCOSMO collaboration weakened those assumptions and published a value of 71.6, with around 5 percent precision.

  Not every pair of quasars is a lens. Two quasars within a few arcseconds of each other are called a double quasar. If their similar redshifts show they are also close in space, they form a quasar pair. A binary or dual quasar is close enough that the host galaxies are likely interacting. Quasars are rare, so finding three or more near the same place is unlikely, and few physical multiples are known.

  The first true triple quasar was found in 2007 at the W. M. Keck Observatory on Mauna Kea in Hawaii. LBQS 1429-008, also called QQQ J1432-0106, had first been observed in 1989, when it looked like a double. Once astronomers found the third member, they confirmed the sources were separate and not a lens effect. The redshift is 2.076, and the components lie an estimated 30-50 kiloparsecs apart, roughly 97,000-160,000 light-years.

  In 2013 came the second true triplet, QQQ J1519+0627, at a redshift of 1.51. The whole system fits within 25 kpc, about 80,000 light-years. The first true quadruple quasar system followed in 2015, at a redshift of 2.0412, spanning about 200 kpc, roughly 650,000 light-years. Gaia also flagged the Sloan Digital Sky Survey quasar J0749+2255 as a likely dual quasar. Hubble, Keck, Gemini, Chandra and the VLA confirmed an interacting galaxy pair at a redshift of 2.17, and future Gaia data releases are expected to add many more.

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• 8. A Fixed Grid

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  The International Celestial Reference System rests on hundreds of extragalactic radio sources spread across the whole sky, mostly quasars. Their distance makes them appear stationary to current technology. Astronomers measure their positions with very-long-baseline interferometry, and most are known to 0.001 arcsecond or better. That is orders of magnitude sharper than the best optical measurements.

  Quasars suit this job because they are distant, bright and small in apparent size. Objects that once defied every explanation now give astronomers the points against which other positions on the sky are measured.

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