What is a quasar?

Figure: The mystery of quasars unveiled. This Hubble picture of the quasar PKS 2349 at 1.5 billion light-years proves that at least some, and possibly all, quasars are triggered by some dramatic event such as the merger between a quasar and a companion galaxy. The bright central object is the quasar itself, and looks like a brilliant star in the core of the galaxy - the home of a supermassive black hole feeding voraciously. (Credit: John Bahcall, NASA/Princeton)

Quasars were first identified in the early 1960's, but aside from their great distance, no one had a telescope powerful enough to actually see what they were. They all looked like very bright points of light with absolutely no details to them. Theoreticians, meanwhile, were intrigued by the power they produced. By the mid-1970's there were several ideas being proposed to explain quasars as massive black holes devouring entire stars and gas clouds and converting this mass into energy. These black hole models were later supplemented by 'star burst' models where huge numbers of massive stars were going supernova - a much more common process than black holes that had not yet been proven.

Several groups of astronomers in the early 1980's used the latest telescopes and photography to have a look at some of the closest quasars such as 3C273. They reported detecting 'fuzz' in their vicinity. Careful spectroscopic studies of this fuzz showed that it was at the same redshift as the quasar, and that it looked like the light from spiral-type galaxies. Once the Hubble Space Telescope began to photograph the nearby quasars in the early 1990's, it found that nearly all of them were produced during galaxy collisions.

One of these images gives a good idea of what the scientists typically saw. You can easily pick out several bright galaxy cores like the ones you find in the center of spiral galaxies, or you can see fragments of star clouds near the quasar. This led to the idea that a normal galaxy might have a supermassive black hole in its core, which is not eating gas and dust and stars. This black hole contains over 100 million or even several billion solar masses of material and is about the diameter of the solar system. Then, along comes another galaxy. The collision tosses lots of matter into orbits that reach deep into the core of the host galaxy where the black hole lives. The matter is absorbed by the black hole, which is like throwing gasoline on a fire.