Basics of Astrophysics | The Secrets of the Universe

Quasar And Its Types

Going by the dictionary definition, a Quasar is a massive and extremely remote celestial object, which emits exceptionally 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 (starlike) 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. So, today's article of The Basics of Astrophysics series aims to explore a bit about them.

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Discovery of Quasars

The discovery of Quasars was a result of one of the most successful joint venture of Radio and Optical astronomers. In 1932, American engineer Karl Janskey discovered the existence of radio waves emanating from 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 the discovery of first quasars in 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 Quasars was assigned to these mysterious objects.

Hubble's image of Quasar 3C 273. At right, a coronagraph has been used to block the Quasar's surrounding light to easily detect the host galaxy.
Image Credits: Hubble Space Telescope, Wikipedia

Formation and Properties of Quasars:

Though, it's a very debatable issue. But, according to most theories, most large galaxies contain a supermassive central black hole. Theoretically, In quasars and other types of Active Galactic Nuclei, a gaseous accretion disk surrounds these black holes. As gas falls towards the black hole, energy is released in 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.

Artist's conception of a Quasar with a Supermassive Black hole in the center
Image credits: ESA/NASA, the AVO project and Paolo Padovani

Quasars are remarkably different from the normal stars as they show a strong ultraviolet emission. The normal main sequence stars do not show ultraviolet excess like Quasars. Quasars 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 values of redshift with a range greater than 3 yield that Quasars are receding with high speeds and lie at very great distance.

Subtypes of Quasars

The different subtypes of Quasars are as follows:

Radio-loud Quasars

These are the quasars with powerful jets that are strong sources of radio-wavelength emission. These 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 emission. 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 levels of dust extinction 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 an extremely distant, bright, and small in apparent size. So, it serves as useful reference point in establishing a measurement grid on the sky. Eventually, Quasars are one of the fundamental basis 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. Quasars this young can reveal information about how galaxies evolved over time. Moreover, energy from quasars takes billions of years to reach the Earth's atmosphere. For this reason, the study of quasars can provide astronomers the information about the early stages of the universe.

Author's Message

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. I hope this article gave you at least a basic insight into what the word Quasar stands for.

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