Quasar

A quasar (/ˈkweɪzɑːr/ KWAY-zar) is an extremely luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous; the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way.

The term quasar originated as a contraction of "quasi-stellar [star-like] radio source"—because they were first identified during the 1950s as sources of radio-wave emission of unknown physical origin—and when identified in photographic images at visible wavelengths, they resembled faint, star-like points of light. High-resolution images of quasars, particularly from the Hubble Space Telescope, have shown that quasars occur in the centers of galaxies.

About a million quasars have been identified with reliable spectroscopic redshifts, and between 2-3 million identified in photometric catalogs. The nearest known quasar is about 600 million light-years from Earth, while the record for the most distant known AGN is at a redshift of 10.1, corresponding to a comoving distance of 31.6 billion light-years, or a look-back time of 13.2 billion years.

Naming

The word quasar was first used in May 1964 by an astrophysicist named Hong-Yee Chiu in a journal called Physics Today. At that time, these objects were very mysterious, and people used a long name to describe them: "quasi-stellar radio sources." Because scientists didn’t fully understand what these objects were, they needed a shorter, easier name. So, Chiu suggested using the word "quasar" to make it simpler.

History of observation and interpretation

Background

Main article: Galaxy § Distinction from other nebulae

In the early 1900s, astronomers realized that some objects in the sky, called "nebulae," were actually distant galaxies like our own Milky Way. But when radio astronomy began in the 1950s, they found some strange objects among the galaxies. These objects gave off lots of energy at many different wavelengths, but they looked very small and faint through telescopes. Their light showed unusual patterns that were hard to explain. Some of these objects changed brightness very quickly, which suggested they were very small. Because of these puzzles, astronomers called them "quasi-stellar objects" or QSOs, and later shortened this to "quasar."

Early observations (1960s and earlier)

The first quasars, called 3C 48 and 3C 273, were found in the late 1950s during surveys of radio waves from space. At first, they were only seen as radio sources without any visible object. Using telescopes, astronomers discovered they were very small in the sky. In 1963, astronomers found that the radio source 3C 48 matched a faint blue object in pictures. Its light showed many unusual patterns.

Another quasar, 3C 273, was studied closely in 1962. Astronomers found that its light matched ordinary light from hydrogen but stretched to longer wavelengths, called redshift. This suggested the quasar was moving away very fast or was very far away. Its brightness changed over years, showing that the light came from a very tiny area.

Development of physical understanding (1960s)

Main articles: Redshift, Universe, and Expansion of the universe

At first, astronomers weren't sure why quasars showed this redshift. Some thought they were very close but very massive, while others thought they were far away and extremely bright. The idea that their light came from matter falling into a giant black hole was suggested in the 1960s but wasn't widely accepted then because black holes weren't well understood.

Over time, many observations showed that quasars were indeed very far away and extremely bright. This included studying their host galaxies, their light patterns, and how their position appeared in telescopes. By the 1970s, the idea that quasars were powered by matter falling into supermassive black holes became the accepted explanation. This also matched the idea that many galaxies have giant black holes at their centers.

Modern observations (1970s and onward)

Later studies showed that not all quasars gave off strong radio waves—only about 10% did. They are also grouped into "radio-loud" and "radio-quiet" types. In 1979, astronomers saw the first example of Einstein's idea that light could bend around massive objects, using a quasar whose light appeared doubled.

More recent studies have looked at the light from quasars in new ways, helping us understand more about black holes and the early universe.

Current understanding

Quasars are very bright and faraway objects in space. Because they are so far, the light from them appears redder when it reaches Earth. This happens because the universe is expanding.

Quasars are found in the centers of active galaxies and are some of the brightest and most powerful objects we know. They can shine up to a thousand times brighter than our entire Milky Way galaxy, which has between 200 and 400 billion stars. This light comes in many forms, from X-rays to infrared, and some quasars also give off strong radio waves and gamma rays. Big telescopes have helped scientists see the faint galaxies around some quasars.

Scientists believe quasars get their energy from huge black holes at the centers of galaxies. As material falls toward these black holes, it heats up and gives off lots of energy. This process can turn a surprising amount of mass into energy. Most big galaxies, including the Milky Way, have these giant black holes, but only a few become bright enough to be seen as quasars. This happens when there is enough material around the black hole to create this bright energy.

Quasars were more common long ago because there was more material around the black holes to create energy. As the black holes used up this material, the quasars faded away. It’s possible that many galaxies, including our own, went through a bright phase like this in the past.

The material falling toward a black hole usually doesn’t fall straight in. Instead, it spins and forms a flat disk around the black hole. Quasars can become active again when galaxies crash into each other, sending fresh material toward the black hole. In about 3 to 5 billion years, when the Andromeda Galaxy collides with the Milky Way, a quasar might form.

In the 1980s, scientists developed ideas that help explain how quasars are different from other active galaxies. They now think that how we see a quasar depends on our viewing angle.

The farthest known quasar is UHZ1. It appears about 31.7 billion light-years away from Earth.

Properties

More than 900,000 quasars have been found, mostly from the Sloan Digital Sky Survey. They are very far away, some being as distant as 30 billion light-years away from Earth. Because of their great distances, they look as they did when the universe was very young.

Quasars get their power from huge black holes at the centers of galaxies. These black holes pull in material and heat it up, shining brightly. Even though they look dim from Earth, quasars are some of the brightest objects we know. The brightest one, 3C 273, is in the constellation of Virgo. It is bright enough to see with a telescope, and if it were closer to us, it would shine as brightly as the Sun.

Quasars were more common when the universe was younger. They help scientists understand how galaxies and black holes grow and change over time. Their brightness can change quickly, showing that their energy comes from a very small area. This makes black holes the best explanation for their powerful light.

Subtypes

Quasars come in different types, each with special features.

• Radio-loud quasars are quasars with strong jets that give off powerful radio waves. They make up about 10% of all quasars.
• Radio-quiet quasars are quasars without strong jets and have weaker radio waves. Most quasars, about 90%, are radio-quiet.
• Broad absorption-line (BAL) quasars are quasars that show special lines in their light, caused by gas moving away from the bright center toward us. These are usually radio-quiet and make up about 10% of quasars.
• Type 2 (or Type II) quasars are quasars where the bright center and special light lines are hidden by thick gas and dust. They are brighter versions of certain types of galaxies.
• Red quasars are quasars that look redder because of dust in their galaxies. They are a big part of all quasars.
• Optically violent variable (OVV) quasars are radio-loud quasars where the jet points toward us, making the quasar’s brightness change quickly. They are also a type of blazar.
• Weak emission line quasars are quasars with very faint special lines in their light.

Role in celestial reference systems

Quasars are very far away, very bright, and appear small in the sky, making them great reference points for measuring the sky. The International Celestial Reference System (ICRS) uses many of these quasars as fixed points all around the sky. Because they are so far away, they seem to stay still, and scientists can measure their positions very accurately using a special technique called very-long-baseline interferometry (VLBI). Most of their positions are known to within a tiny fraction of an arcsecond, much more precise than the best optical measurements.

Multiple quasars

When two or more quasars appear close together in the sky, it can happen by chance, by actually being near each other, or because gravity bends the light of one quasar to make multiple images. Figuring out which of these is true can be tricky.

Sometimes, a quasar's light gets bent by gravity, creating two, three, or even four images of the same quasar. The first such case found was called the Twin Quasar in 1979. Other examples include the Einstein Cross and the Cloverleaf Quasar. Ongoing sky surveys keep finding more of these special cases.

When quasars look very close together from Earth and also have similar movements, they might actually be pairs of quasars. Since quasars are rare, finding three or more together is unusual. Studying these helps scientists learn about how galaxies interact and change, especially long ago when the universe was younger.

Recent sky surveys suggest some quasars thought to be single might actually be pairs. Observations with powerful telescopes confirmed this for one such case. In 2007, the first true triple quasar was found, with three separate quasars close together. Later, a second triple quasar system was discovered. The first true quadruple quasar system was found in 2015.