A Staff Report from the Straight Dope Science Advisory Board

Why are quasars always found at the edge of the universe?

May 22, 2001

Dear Straight Dope:

I've read many books on astronomy in my life, and several mention how quasars are the oldest, most distant visible objects in the known cosmos. However, none has come close to offering an explanation as to WHY quasars only hang out at the edge of all that is known. I mean, one could conclude that since quasars are the oldest (or at least most distant, but it's the same thing really) objects, then maybe every celestial body has its roots in a quasar. Any ideas?

Not every celestial body has its roots in a quasar, but many do. A quasar is basically a galaxy, or at least the core of one, that's going though a wild adolescence. Most galaxies (and all large galaxies, it's believed) have a huge black hole in their core, with a mass millions of times the mass of the Sun. As stuff falls into this supermassive black hole, it rubs against other material and heats up tremendously. Short of matter-antimatter annihilation, this is the most efficient process known in the universe for releasing energy: A body falling into a black hole from a great distance can release up to half the energy you'd get by annihilating it. The result of all this energy being released is a quasar, the brightest class of objects in the universe, or if there's not quite so much energy involved, a similar object called a Seyfert galaxy or an active galactic nucleus.

Youth doesn't last forever, though. Sooner or later you run out of material to drop into the hole. In fact, it's believed all galaxies by now have long since used up their supply of quasar fuel. Since there aren't any quasars around any more, if we want to see one, we need to look very far away, to see the light that was produced billions of years ago. Very far away from us = edge of the known universe. The nearest quasar to Earth, which goes by the poetic name of 3c273, is about three billion light years away, so the longest-lasting quasars probably ran out of gas about that many years ago.

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