Quasars
Introduction
A quasar (or quasi-stellar object) is an extremely luminous active galactic nucleus (AGN), powered by the accretion of material onto a supermassive black hole at the center of a galaxy. This process releases vast amounts of energy across the electromagnetic spectrum, from radio waves to X-rays. Quasars are among the most distant objects in the universe, and their immense luminosity allows us to observe them even billions of light years away.
Discovery and Identification
The term "quasar" was coined by the astrophysicist Hong-Yee Chiu in 1964, in a paper published in Physics Today. The first quasars were discovered with radio telescopes in the late 1950s. Many early quasars were discovered as radio sources in all-sky radio surveys. They were initially identified as redshifted galaxies due to their strong redshifts - a measure of the stretching of light to longer wavelengths due to the expansion of the universe.
Properties
Quasars are characterized by their high redshifts, indicating that they are located at a significant distance from the Earth. They are also characterized by their luminosity, which can outshine the entire galaxy in which they reside. This luminosity is generated by the accretion disk around the central black hole, as matter falling into the black hole releases energy.
Quasars also often have jets of matter shooting out from the region of the black hole. These jets can extend for thousands of light years, and are a major source of cosmic rays.
Formation and Evolution
Quasars are believed to be an early stage in the evolution of galaxies. The intense luminosity of a quasar is thought to be a temporary phase, and a quasar will eventually burn out and become a more ordinary galaxy. This is supported by observations that quasars are more common in the distant, early universe.
The formation of a quasar begins with a galaxy's central black hole accreting matter from its surroundings. This accretion process can be triggered by the collision or interaction of galaxies, which causes gas to be funneled towards the black hole.
Observations and Studies
Quasars are important objects of study in astronomy because they are unique laboratories for studying the physics of black holes and galaxy formation and evolution. Observations of quasars have provided key insights into the large-scale structure of the universe, the age and size of the universe, and the history of star formation.