This article is a part of the series on ‘Basics of Astrophysics’. Read all the articles here.
Going by the dictionary definition, a quasar is a massive and extremely remote celestial object that emits large amounts of energy. A quasar typically has a star-like image in a telescope. But, contrary to showing stellar phenomenon, they are strong sources of radio waves. As a result, they were named Quasi-Stellar (star-like) Radio Sources, later abbreviated as QUASARS. They shine very brightly, and sometimes they are bright enough to eclipse the ancient galaxies that contain them. These powerful dynamos have fascinated astronomers since their discovery.
Discovery of Quasars
The discovery of quasars resulted from radio and optical astronomers’ most successful joint venture. In 1932, American engineer Karl Jansky discovered radio waves emanating beyond the solar system. By the mid-1950s, an increasing number of astronomers sought explanations for mysterious radio emissions from optically dim stellar sources.
Consequently, this resulted in discovering the first quasars in the late 1950s. Earlier, these were observed as radio sources with no corresponding visible object. However, in 1963, an identification of the radio source 3C 48 with an optical object was made, and the name quasar was assigned to these mysterious objects.
Image: NASA
Formation and Properties of Quasars
Although it’s a very debatable issue, most large galaxies contain a supermassive central black hole, according to most theories. Theoretically, in quasars and other types of Active Galactic Nuclei (AGNs), a gaseous accretion disk surrounds these black holes. As gas falls towards the black hole, energy is released in the form of EM radiations, which we can observe across the EM spectrum.
The observed properties of a quasar depend on many factors, including the mass of the theoretical black hole associated with it, the rate of gas accretion, the orientation of the accretion disk relative to the observer, the presence or absence of a jet, and the degree of obscuration by gas and dust within the host galaxy.
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Quasars are remarkably different from normal stars as they show a strong ultraviolet emission. The normal main-sequence stars do not show ultraviolet excess like quasars. These powerful dynamos exhibit many of the same properties as active galaxies. These are visible in many parts of the electromagnetic spectrum, including radio, infrared, optical, ultraviolet, X-ray, and even gamma rays.
All observed spectra have shown considerable redshifts, ranging from 0.06 to the recent maximum of 6.4. These observed redshift values with a range greater than 3 yields that quasars are receding with high speeds and lie at a very great distance.
Subtypes of Quasars
The different subtypes of quasars are as follows:
- Radio-loud Quasars: These are the quasars with powerful jets with strong radio-wavelength emission sources—they make up about 10% of the overall quasar population.
- Radio-quiet Quasars: These quasars lack powerful jets, contrary to Radio Loud quasar. They possess relatively weaker radio emissions. The majority of quasars (about 90%) are radio-quiet.
- Broad absorption-line (BAL) Quasars: In these quasars, the blue-shifted spectra relative to the quasar’s rest frame exhibits broad absorption lines. Broad absorption lines are visible in about 10% of quasars, and BAL quasars are usually radio-quiet.
- Type 2 (or Type II) Quasars: In these quasars, the dense gas and dust highly obscure the accretion disk and broad emission lines.
- Red Quasars: These are the quasars with optical colors that are redder than normal quasars. These are probably the result of moderate dust extinction levels within the quasar host galaxy. Infrared surveys have demonstrated that red quasars make up a substantial fraction of the total quasar population.
- Optically Violent Variable (OVV) Quasars: These are radio-loud quasars in which the jet directs toward the observer. Relativistic beaming of the jet emission results in strong and rapid variability of the quasar brightness.
- Weak emission line Quasars: These Quasars possess unusually faint emission lines in the ultraviolet/optical spectrum.
Importance of a Quasar
A quasar is extremely distant, bright, and small in apparent size. It serves as a useful reference point in establishing a measurement grid on the sky. Eventually, quasars are one of the fundamental bases of The International Celestial Reference System (ICRS). In December 2017, scientists spotted the most distant quasar sitting more than 13 billion light-years from Earth. This quasar, known as J1342+0928, appeared only 690 million years after the Big Bang.
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Quasars this young can reveal information about how galaxies evolved. Moreover, energy from quasars takes billions of years to reach the Earth’s atmosphere. For this reason, the study of these beasts can provide astronomers the information about the early stages of the universe.
No doubt that still we don’t know much about these powerhouses of radio waves. Though, it may be years before we actually know the proper purpose of these bright, beautiful celestial objects and the physics behind them. But, they sure are something to look at. These can reveal a lot about our universe’s secrets, and hence, they are an evolving species of research these days.
