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In order to understand what is a quasar, it helps to know what ‘quasar’ stands for. The word quasar stands for quasi-stellar radio source. They are the brightest and most distant objects in the universe as we know it. The term quasar has fallen out of common use by astronomers and they are now known as quasi-stellar objects(QSOs) because we now know that they are not true stars.
Quasars have a very high redshift. They are the most luminous, powerful, and energetic objects known of at this time. They seem to inhabit the centers of active young galaxies and can emit up to a thousand times the energy output of our entire galaxy. According to Hubble’s law the redshift shows that quasars are very distant and, because of their distance, much older than our universe. The most luminous quasars radiate in excess of the output of one trillion Suns. This radiation is emitted across the spectrum from X-rays to the far-infrared, but have a peak in the ultraviolet-optical bands. Some quasars are strong sources of radio emission and gamma-rays. In early optical images, quasars looked like single points of light. Infrared telescopes and the Hubble Space Telescope have identified the galaxies surrounding the quasars. These galaxies are normally too dim to be seen against the glare of the quasar otherwise.
Quasars are believed to be powered by the accretion of material into centralized supermassive black holes. This makes them luminous versions of active galaxies. Light cannot escape the super massive black holes, so the escaping energy is generated by gravitational stresses and intense friction outside of the event horizon. Large central masses, some over 1 million solar masses, have been measured in quasars. Several dozen large galaxies, with no sign of a quasar nucleus, have been shown to contain a similar central black hole in their nuclei, so it is thought that all large galaxies have one with only a small fraction emitting powerful radiation. Quasars may ignite(re-ignited) from normal galaxies with a fresh influx of matter. Some scientists have theorized that a quasar could form as the Andromeda galaxy our own galaxy in approximately 3 to 5 billion years.
Scientists have a very minute understanding of ‘what is a quasar?’. What is known today is 10-fold what was known in the 1960′s and will grow another 10-fold in the coming decades. That could truly be a great time to be an astronomer.
We’ve also recorded an entire episode of Astronomy Cast all about Quasars Listen here, Episode 98: Quasars.