What Is a Quasar?

Quasars (quasi-stellar radio sources) are the brightest sustained objects in the universe. They occur when a supermassive black hole at the centre of a galaxy actively accretes enormous quantities of gas and dust. As material spirals inward through an accretion disk, friction heats it to millions of degrees, emitting intense radiation across the electromagnetic spectrum. A single quasar can outshine its entire host galaxy by a factor of 100 to 1,000, producing as much energy as a trillion Suns. Quasars were most common in the early universe (10-12 billion years ago) when galaxies had more fuel to feed their central black holes.