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The Darkness and the Light

Week of May 10, 1999

One aspect of the Universe that has always fascinated me is its paradoxical qualities. For example, the hydrogen atom is so tiny, yet vastly dominates all the matter in the Universe. The ghostly neutrino-- so low in mass that scientists argued for decades (and still do argue) over whether it had mass at all-- is at the heart of the main mechanism that causes stars to explode as supernovae.

It's the same with black holes. A black hole is an object with so much mass squeezed into so tight a space that the escape velocity-- the velocity you would need to be able to get away from the black hole-- is higher than the speed of light. For that reason, black holes are, well, black. If even light cannot escape them, then you cannot see them. The paradox comes in because as it happens, even though black holes are completely dark, they are responsible for the brightest known objects in the Universe!

How can this be? Actually, it's not so hard to understand, and as a matter of fact, the basic principle is familiar to anyone who has tried to warm up their hands on a bitter winter's day.

Imagine a huge black hole, one that masses millions of times the mass of the Sun. Black holes this size may be common in the centers of galaxies; our own Milky Way sports one. As matter falls into the black hole, it tends to form a flattened disk, called an ``accretion disk'' (the reasons behind this are varied and a bit complex for this particular Snack, so this is a rare case where I will ask you to Take My Word For It. Next week's Snack will be about accretion disks!). Anyway, the material in this disk orbits the black hole just like planets around the Sun: the closer in you get, the faster you orbit.

Quick change of scene: it's winter, and you come inside after making a bunch of snowballs. How do you warm your hands up? You rub them together! The friction of the skin rubbing warms up your hands. The faster you rub, the warmer you get. Well, the same thing happens in the disk around the black hole. The stuff closer in moves faster. However, it's also rubbing against the stuff just farther out, the stuff moving a bit slower. Near the black hole's ``surface'', the orbital speed is very close to the speed of light, so as you can imagine the rubbing gets pretty hard.

Because it's rubbing so hard, the material in the disk gets very hot, easily into the millions of degrees. Hot objects glow, and very hot objects tend to glow very brightly. The cores of some galaxies get so bright from this that they can outshine all the other stars in the rest of the galaxy! We call these types of galaxies ``active'', which is typical astronomer's understated lingo. As a matter of fact, we think that quasars, intensely luminous objects that are extremely far away, glow from this very mechanism. Our own Milky Way Galaxy may have been a quasar billions of years ago, but lucky for us it's calmed down since its active youth.

Next week: how the accretion disk shapes affects the environment around it. I'll have a couple of nice images for that Snack too!

©2008 Phil Plait. All Rights Reserved.

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