What is a quasar and what does it have to do with a black hole?

Asked by: Justin Dopiriak

Answer

In the 1960's radio astronomers knew about a lot of intense sources of radio noise which could not be identified with optical stars or galaxies. Cyril Hazard a British astronomer pointed out in 1963, that the moon would pass in front of one of these sources, number 273 in the Third Cambridge catalogue. This occultation led astronomers to pin point the position of the radio source 3C 273 by timing the moment the moon cut off the radio noise. This enabled astronomers to associate the radio source with an optical star-like object. But this 'star' has a large redshift placing it far outside our own galaxy. Studies showed that these quasars lie at the hearts of distant galaxies to dim to be seen by themselves. It was also discovered that the region producing light was only about a light day across (approx. the size of our solar system) and to be seen at these sorts distances the object must radiate 1000 times more energy than all the stars in the Milky Way put together!

The quest to find a mechanism that could produce this much energy considered almost every idea imaginable. But the most probable and best explanation was that each quasar is in fact a supermassive black hole, equivalent to perhaps 100 million solar masses (1 solar mass is the mass of our sun). These supermassive black holes swallow matter from its surrounding galaxy at a rate of about 1 solar mass per year. The energy released when this occurs can be as much as half the theoretical maximum implied by E=mc^2, this is ample to explain the energetic output of a quasar.
Answered by: Dan Summons, Physics Undergrad Student, UOS, Souhampton