The first quasars to be found were detected by their radio emission, and were termed quasistellar radio sources; when other quasars were found which do not emit radio noise, the name was changed to quasistellar object, or QSO. Only one quasar in 200 is actually a radio source. In either case, the name "quasar" can be regarded as a contraction of "quasistellar".
Quasars are very distant galaxies that have extremely active centres, which are so bright that they completely outshine the rest of the galaxy and the other galaxies around them.
Some Quasars radiate as much energy as 100 giant galaxies.
It is thought that they become active when large amounts of gas are sucked into the supermassive black holes (see "black holes" topic page) at their centres.
Some quasars are the most distant objects known. They are seen by light which left them more than ten billion years ago. That's only a few billion years after the Big Bang.
The most luminous Quasar to be seen so far was detected via radio observations by the Whole Earth Blazer Telescope (WEBT). In 2005, the Quasar underwent an outburst that was as bright as 550 billion Suns and "represents the most luminous Quasar state ever observed," according to the WEBT team.