Astronomy

Definition of a Quasar



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A quasar, or quasi-stellar radio source, is a distant, high-energy source of radio waves. Although they were relatively recently discovered (and have been studied only since the 1950s), astronomers now generally agree that quasars are the extremely dense nucleus regions of certain large galaxies, home to a supermassive black hole.


DISCOVERY

In the late 1950s, as scientists began to monitor the various natural radio wave sources of the universe, they began to record extremely strong radio sources which, bizarrely, did not seem to have any corresponding visible light. By the 1960s, a very small number of these had been linked to visible objects in space, like 3C 48, what seemed to be a blue star with a corresponding high emission of radio waves.

Two years later, John Bolton and Maarten Schmidt, analyzing a second of these mysterious radio sources, confirmed that the source was also experiencing high redshift - the process by which light shifts towards the red end of the spectrum as a result of distant galaxies speeding away from us. However, it was not until 1964 that American physicist Hong-Yee Chiu identified these objects by their current name, quasar, mostly because "quasi-stellar radio source" seemed to be about the only characteristics that everyone would agree upon.

The substantial redshift observed by Bolton and Schmidt suggested that these objects were actually a long ways away from the Earth, despite the strength of their detectable radio emissions. However, it was really only the 1980s when scientists began to piece together what exactly was going on with this strange phenomena.


CHARACTERISTICS OF QUASARS

Quasars are, at least in terms of their radio waves, the brightest and most powerful objects we have detected in the universe. They tend to be the nuclei, of centres, of young, active galaxies, which put out many times the light energy of our own Milky Way galaxy. The most powerful also seem to be extremely small: some, for example, emit X-rays in such a way that scientists have estimated they probably less than the size of our solar system. At the centre of each of these is an unusually large black hole; it is the accretion disk of this black hole which actually forms the quasar.

How precisely quasars actually form is less clear. Most or even all significant galaxies are believed to have a black hole in their core - but not one as massive as those found within a quasar. Some have suggested that quasars form when existing galactic cores are bombarded with an enormous amount of new matter - as will happen, for example, when the Milky Way and Andromeda galaxies fuse together in several billion years.

Given their redshift and distance, we can infer that they are also far older than our own galaxy: at a minimum of nearly a billion light-years from Earth, these objects were behaving as we now observe them at a time when life on Earth was still in its most primitive forms.


EXAMPLES OF QUASARS

Scientists have found several hundred thousand quasars through projects such as the Sloan Digital Sky Survey. All of them are quite distant from Earth, usually several billion light-years at least. The brightest quasars in the sky include 3C 273, within the Virgo constellation. In general, quasars are too distant to be detected except with powerful, specialized astronomical equipment; moreover, their strongest emissions are in the radio rather than visual spectrum, further complicating detection. However, 3C 273 is detectable by amateur astronomers - and, at nearly 2.5 billion light-years from the Earth, it is farther away than virtually any other object that can be observed by amateurs.

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