Quasar

A quasar is an extremely luminous active galactic nucleus (AGN). You might also hear them called quasi-stellar objects, or QSOs. These objects are among the most powerful energy sources in the entire universe. In fact, some quasars are thousands of times more luminous than a whole galaxy like our Milky Way.

Because they are so incredibly far away, they often appear as tiny, faint points of light in photographs. This star-like appearance is why the name "quasi-stellar" was originally used.

The immense energy of a quasar comes from a specific physical process. At the center of the quasar sits a supermassive black hole. These black holes have a mass ranging from millions to tens of billions of solar masses. Around the black hole, gas forms a structure called an accretion disc. As the gas in this disc falls toward the black hole, it heats up. This process releases massive amounts of energy as electromagnetic radiation. This radiation includes visible light and high-energy X-rays.

Many different factors can change how a quasar looks to us. The observed properties depend on the mass of the central black hole. The rate of gas accretion, or how much matter is falling in, also matters. The orientation of the accretion disc relative to the observer can change the view. Some quasars also feature a jet of energy. Finally, the amount of gas and dust in the host galaxy can obscure the light.

Astronomers began discovering these mysterious objects in the 1950s. They were first detected as radio-wave sources of unknown origin. In 1963, Allan Sandage and Thomas A. Matthews identified the radio source 3C 48 with an optical object. They found a faint blue star-like object with a very strange spectrum. Later, in 1963, Maarten Schmidt made a major breakthrough. He studied the radio source 3C 273 using the Hale Telescope on Mount Palomar. He discovered that the spectral lines were redshifted by 15.8%.

This redshift was a revolutionary discovery for science. Redshift occurs when light is stretched by the expansion of the universe. Schmidt's work suggested that 3C 273 was not a nearby star. Instead, it was a very distant and incredibly powerful active galactic nucleus. While many scientists were skeptical at first, evidence grew throughout the 1960s and 1970s. They realized that the high energy output was possible if the objects were very far away. This helped confirm that quasars are indeed cosmological in origin.

We have identified many quasars throughout our surveys. About one million quasars have reliable spectroscopic redshifts. Between 2 and 3 million more have been identified in photometric catalogs. The nearest known quasar is approximately 600 million light-years from Earth. Some are much further away. The most distant known AGN has a redshift of 10.1. This corresponds to a distance of 31.6 billion light-years. Looking at these distant objects is like looking back in time 13.2 billion years.

Quasar activity tells us a story about the history of our universe. Surveys show that quasar activity was much more common in the distant past. The peak epoch of this activity was approximately 10 billion years ago. This is likely because there was more matter available to fuel the black holes. As a quasar consumes the gas in its accretion disc, the fuel eventually runs out. Once the gas is gone, the energy production falls off. The quasar then becomes an ordinary galaxy.

Quasars also help us understand how large-scale structures form. Some quasars are found in concentrations known as large quasar groups. These groups may be some of the largest known structures in the universe. They can act as tracers for how mass is distributed across space. Studying quasars connects our understanding of black holes, galaxy evolution, and the expansion of the cosmos. They remain a vital tool for exploring the deep history of the universe.