Quasar (Quasi-Stellar Radio Source)

FIELDS OF STUDY: Extragalactic Astronomy; Cosmology; Theoretical Astronomy

ABSTRACT: Quasars are powerful astronomical objects characterized by huge bursts of energy from small points in space. They are among the most distant objects observed in the universe. They are also the most luminous known objects in the universe. It is believed that quasars are generated by the accretion disks of supermassive black holes in forming galaxies.

Active Galactic Nuclei

Quasars, or quasi-stellar radio sources, are the brightest known objects in the universe. They can be ten to ten thousand times brighter than the entire Milky Way galaxy. They are also among the furthest objects from Earth ever observed. Quasars are a type of active galactic nuclei (AGN), or a central area of a galaxy that is much brighter than normal. This extreme brightness is caused by the heating of materials around the edge of a supermassive black hole at the center of a galaxy. While scientists believe that most galaxies have central black holes, not all produce AGNs. Only when the black hole is actively "feeding" on matter can an AGN be formed.

There are several types of AGNs in addition to quasars. Most notable are blazars, radio galaxies, and Seyfert galaxies. However, most scientists believe that these classifications are in fact all variations of the same phenomenon. It is only the angle at which these distant objects are viewed from Earth that makes them appear different. Quasars and other AGNs give off huge amounts of electromagnetic radiation, which may include visible light, radio waves, gamma rays, and other emissions.

Discovery of Quasars

Quasars were discovered in 1963 by the astronomer Maarten Schmidt (b. 1929) using the telescopes of Mt. Palomar Observatory. Schmidt was examining sources of radio emissions, focusing on an unusual object named 3C 273. The object did not follow the patterns of known phenomena. It looked like a star through normal telescopes. However, when viewed with a radio telescope, the object was seen to emit incredibly powerful radio signals, unlike normal stars. It also produced other strange spectral emissions.

Schmidt then realized that the mysterious emissions were simply those of hydrogen gas whose wavelengths had been shifted. This phenomenon, known as redshift, happens to all types of radiation over extremely long distances. Objects moving farther away shift toward the red end of the spectrum (longer wavelength), while objects moving closer shift toward the blue end (shorter wavelength). Schmidt used this observation to calculate that 3C 273 would be billions of light-years from Earth. To be visible at that distance, 3C 273 must be brighter than a million galaxies.

For this reason, Schmidt concluded that 3C 273 could not be a star. However, the object and others like it failed to match the profile of any known celestial object. For this reason, they became known as "quasi-stellar radio sources." This was later shortened to the word "quasars." Over time, many more quasars were discovered. Although it was found that not all quasars produced radio emissions, the name stuck. Yet it would not be until the 1980s that scientists reached a consensus on what quasars actually were.

Black Holes and Quasars

Scientists eventually determined that quasars are linked to another type of object: black holes. Black holes are infinitely dense areas of space-time formed when incredibly massive stars burn all their fuel. The nuclear reaction sustaining the star ceases, and the gravity from the star’s dense core quickly pulls the entire star into the center. This causes the core to become even denser, increasing its gravitational pull and causing nearby objects to be pulled into it. As yet more objects are pulled into the core, the process increases in speed and power. Eventually, the area becomes so dense that not even light can escape its gravitational pull. At this point, the absorption and compression process speeds up so much that the area becomes infinitely dense, which creates a small area with an infinitely strong gravitational pull. This area is known as a "singularity."

Black holes are sometimes surrounded by glowing clouds of gas and compressed matter called "accretion disks." This matter orbits the black hole as it spirals toward the singularity at the center. The intense, inescapable gravity near this point compresses the spiraling matter, which causes huge amounts of friction. The friction generates so much heat that the entire accretion disk begins to glow, sometimes glowing more brightly than a star. Some of these accretion disks are even visible from Earth.

Most astronomers believe that a supermassive black hole is located at the center of almost every galaxy. It is believed that the supermassive black holes found in the center of young galaxies are the sources of quasars. Scientists theorize that matter is condensed much more in young galaxies than in their older counterparts. For this reason, far more matter is pulled into the supermassive black hole at the center of the galaxy, creating an AGN. Most of this material is compressed inside the black hole’s accretion disk.

Because of the friction associated with huge amounts of matter being compressed into a relatively small area, the accretion disk begins to emit large amounts of energy. Up to 10 to 20 percent of the captured matter’s mass is converted directly into energy. For comparison, nuclear fusion reactors, such as Earth’s sun, convert less than 1 percent of their mass to energy. This extremely efficient energy conversion explains how such comparatively small objects can produce more energy than entire galaxies. When that energy is emitted in a luminous jet that is oriented generally toward Earth, its point of origin is classified as a quasar. If the jet aims directly at Earth, it is seen as a blazar. Seyfert galaxies are thought to be oriented sideways relative to Earth, so that no jets are observed.

Studying Quasars

Because light has a finite speed, the quasars that are detected on Earth probably existed billions of years ago. For this reason they are valuable to scientists studying the origins of the universe and the life cycle of galaxies. In 2021, astronomers announced the discovery of the oldest, and most distant, known quasar. The object, named P172+18, is about 13 billion light years away, meaning scientists are observing it as it looked just 780 million years after the big bang.

It is theorized that older galaxies, such as the Milky Way, may have had quasars at their centers at one point. As these galaxies grew, matter would have become less concentrated, eventually cutting off the fuel supply for the AGN. Astronomers believe that because of how spread out matter is in the current universe, it would usually be impossible for new quasars to form in established galaxies.

However, galaxy collisions might be one exception to this rule. When two galaxies collide, sometimes the supermassive black holes at their centers merge together. This might pull large chunks of the galaxies into the black hole, creating an accretion disk large enough to cause a temporary quasar. Scientists speculate that this could occur when the Milky Way collides with the Andromeda galaxy in about four billion years.

PRINCIPAL TERMS

• active galactic nuclei (AGN): a central region of some galaxies that emits more electromagnetic radiation than normal, thought to be caused by supermassive black holes.
• black hole: an area of space-time with infinite density, from which light cannot escape.
• gamma rays: a high frequency form of electromagnetic radiation composed of energetic protons.
• radio emission: electromagnetic radiation with a longer wavelength than infrared light.
• redshift: the phenomenon in which the wavelength of electromagnetic radiation increases, moving towards the red end of the spectrum. Calculations of redshift are used to determine the distances of distant astronomical objects.

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