Quasars (QUAsi-Stellar Radio Source)

Last Updated May 27, 2023 by . First Published in 2010.

Astronomical Objects Series
  1. Constellation Guide and Associated Mythology
  2. What are Asteroids, Meteors and Comets?
  3. Binary Stars and Double Stars
  4. What are Variable Stars and How to Observe Them
  5. Supernova and Supernovae
  6. What Nebula and Nebulae, What are the Types of Nebula?
  7. What Is a Black Hole? Black Holes Explained - From Birth to Death
  8. Quasars (QUAsi-Stellar Radio Source)
  9. Pulsars - Natures Lighthouses Key to Astronomy
  10. What is a Neutron Star? What is Inside a Neutron Star?
  11. Gamma Ray Bursts
  12. Kuiper Belt
  13. What is an Exoplanet? How Can We Detect Exoplanets?
  14. What is a Galaxy? How Does Galaxy Formation Work?
  15. The Messier Catalogue of Objects To Observe
  16. The Caldwell Catalogue
  17. 25 Stunning Sights Every Astronomer Should See
Quasars (QUAsi-Stellar Radio Source)

Quasars are very energetic and distant galaxy with an active galactic nucleus. They are the most luminous objects in the universe.

Quasars (QUAsi-Stellar Radio Source) are a very energetic and distant galaxies with an active galactic nucleus. They are the most luminous objects in the universe.

Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended sources similar to galaxies.

While there was initially some controversy over the nature of these objects - as recently as the early 1980s, there was no clear consensus as to their nature - there is now a scientific consensus that a quasar is a compact region in the centre of a massive galaxy surrounding its central supermassive black hole. Its size is 10-10,000 times the Schwarzschild radius of the black hole. The quasar is powered by an accretion disc around the black hole.

An artist's impression of a growing quasar.
An artist's impression of growing quasars 

A growing black hole can be seen at the centre of a faraway galaxy in this artist's concept. Astronomers using NASA's Spitzer and Chandra space telescopes discovered swarms of similar quasars hiding in dusty galaxies in the distant universe. The quasar is the orange object at the centre of the large, irregular-shaped galaxy. It consists of a dusty, doughnut-shaped cloud of gas and dust that feeds a central supermassive black hole. As the black hole feeds, the gas and dust heat up and spray out X-rays, as illustrated by the white rays. Beyond the quasar, stars can be seen forming in clumps throughout the galaxy. Other similar galaxies hosting quasars are visible in the background.

Quasars show a very high redshift, which is an effect of the expansion of the universe between the quasar and the Earth. They are the most luminous, powerful, and energetic objects known. When combined with Hubble's law, the redshift implies that the quasars are very distant, thus from much earlier in the universe's history. The most luminous quasars radiate at a rate that can exceed the output of average galaxies, equivalent to one trillion (1012) Suns.

More than 200,000 quasars are known, most from the Sloan Digital Sky Survey. All observed quasar spectra have redshifts between 0.056 and 7.085. Applying Hubble's law to these redshifts, it can be shown that they are between 600 million and 28.85 billion light-years away. Because of the great distances to the farthest quasars and the finite velocity of light, they and their surrounding space appear as if they existed in the very early universe.


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