15 May, 2008
Spotting a black hole can be quite difficult. As the name implies, they’re black, and against the darkness of space this gives them a bit of camouflage. In addition they have this well known property of being so massive that close to the black hole nothing can escape from their gravitational field, not even light. So if the black hole itself isn’t emitting anything that can escape from it, how on earth do we spot them? Fortunately, black holes sometimes leave a bit of a mess behind.
A black hole acts like any other object with mass. It attracts other things with mass, and this gravitational interplay works the same for a black hole as it would for, say, a star. If our Sun was suddenly replaced by an equal mass black hole then (apart from it getting very dark during the day and some altered tidal forces) things would continue as normal on the Earth – and the Earth would stay in its orbit around the black hole.
When a mass is affected by a nearby black hole it has one of three fates. It could just be deflected from its course. Dragged closer to the black hole the object will have been affected, but will head away into the depths of space once again. Or, if the object is on a different path it may be sucked straight in to the black hole, if it is already traveling in its direction.
Finally the object could head into an orbit around the black hole. This may well be an unstable orbit, slowly falling towards the black hole, circling (ellipticing?) many times before finally being sucked in. If there are enough other objects out there, then chances are some of them will be in orbit as well. And the more there are, the greater the chances of collisions. For a black hole in a gas cloud, or near a star and stripping it of its outer layers there will be a *lot* of material and a lot of collisions. All this falling matter gets backed up, pushed and squashed together.
Due to angular momentum the infalling matter flattens out into what is called an accretion disc. The friction generates very high energy X-rays which, not being within the event horizon, can escape out into space. They are so bright that we can see the glare from ancient active galactic nuclei, black holes who pour out more energy than the billions of suns in orbit around them.
Black holes may be difficult to spot out in the depths of space, but once they start eating, the crumbs that they can’t stuff into their event horizons can outshine almost anything else in the Universe.