Quasar

A quasar (/ˈkweɪzɑːr/ KWAY-zar) is an extremely luminous active galactic nucleus (AGN). It is sometimes called a quasi-stellar object, or QSO for short.

The energy of an AGN comes from material falling into a huge supermassive black hole. This black hole can be millions to billions of times the mass of our sun. The material forms a spinning accretion disc around the black hole. As it falls in, the material heats up and gives off lots of electromagnetic radiation. The brightness of quasars is amazing; some shine thousands of times brighter than a whole galaxy like our Milky Way.

The word quasar comes from a shortenning of "quasi-stellar [star-like] radio source". This is because when they were first found in the 1950s, they looked like faint stars in pictures but gave out strong radio waves. Better telescopes, like the Hubble Space Telescope, show that quasars are found in the centers of galaxies.

Scientists have found about a million quasars by looking at their spectroscopic colors, and many more in photometric lists. The closest known quasar is about 600 million light-years away. The farthest known quasar is very far—about 31.6 billion light-years away, and we see it as it was 13.2 billion years ago.

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 mysterious, so people used a long name: "quasi-stellar radio sources." Chiu suggested using "quasar" to make the name shorter and easier.

History of observation and interpretation

Background

Main article: Galaxy § Distinction from other nebulae

In the early 1900s, astronomers learned that some objects in the sky, called "nebulae," were really faraway galaxies like our Milky Way. When radio astronomy started in the 1950s, they found strange objects among the galaxies. These objects gave off lots of energy at many different wavelengths but looked very small and faint. Their light had unusual patterns that were hard to explain. Some of these objects changed brightness quickly, which meant they were very small. Because of these mysteries, 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 saw 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 objects far away in space. Because they are so far, their light looks redder when it reaches Earth. This happens because the universe is expanding.

Quasars are found in the centers of active galaxies. They 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. This light comes in many forms, 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. Most big galaxies, including the Milky Way, have these giant black holes, but only a few become bright enough to be seen as quasars.

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

The material falling toward a black hole usually forms a flat disk around the black hole. Quasars can become active again when galaxies crash into each other. In about 3 to 5 billion years, when the Andromeda Galaxy collides with the Milky Way, a quasar might form.

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.
• Radio-quiet quasars are quasars without strong jets and have weaker radio waves. Most quasars 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.
• Type 2 (or Type II) quasars are quasars where the bright center and special light lines are hidden by thick gas and dust.
• Red quasars are quasars that look redder because of dust in their galaxies.
• 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 far away, very bright, and look small in the sky. This makes them good reference points for measuring the sky. The International Celestial Reference System (ICRS) uses many quasars as fixed points all around the sky. Because they are so far away, they seem to stay still. 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, which is much more precise than the best optical measurements.

Multiple quasars

When two or more quasars appear close together in the sky, it might be by chance, or they might really be near each other. Sometimes gravity bends the light of one quasar and makes it look like there are several images.

The first case found was called the Twin Quasar in 1979. Other examples include the Einstein Cross and the Cloverleaf Quasar. Sky surveys keep finding more of these special cases.

When quasars look very close together from Earth and move in similar ways, they might 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. 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.