Black holes
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When a big enough star collapses it becomes so dense that its _________________ won't let objects or light escape.
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Black holes seem to occur in the Universe in two distinct types. First, there are those formed when some massive stars collapse at the end of their lives in supernova explosions. The resulting black holes that are produced have a mass of typically ten times that of the Sun, and are referred to as 'stellar mass black holes'.

Although we can’t see them directly, astronomers have good evidence that they exist in certain binary star systems where an unseen massive object appears to be pulling material off its companion star. Several dozen such systems are currently known within our Galaxy.

The second type are referred to as 'supermassive black holes' and they lie at the centres of many (if not all) large galaxies, including our own Milky Way Galaxy. The supermassive black hole in the centre of our Galaxy is calculated to have a mass of more than four million times that of the Sun.

Astronomers see stars in the centre of the Galaxy rapidly orbiting around this invisible object, and although it’s not ‘feeding’ at the moment, any stars that come too close to the black hole will be torn apart before being swallowed.

The supermassive black holes in the centres of other galaxies have masses up to many billions of times that of the Sun, and not all of them are as dormant as the one in our Galaxy. When astronomers look at very distant galaxies, they are looking back in time, because the light from them has taken millions, or even billions, of years to reach us.

Some of these distant galaxies, seen at a time when the Universe was much younger than now, are intense sources of X-rays, radio waves, and other radiation. These are so-called 'Active Galaxies' and are believed to be galaxies in which the central supermassive black hole is feeding on material that strays too close to it.

As the material falls towards the black hole and is torn apart, it will end up in a flattened structure surrounding the black hole, known as an accretion disc. Then, as this disc material circles the black hole prior to being swallowed, frictional forces in the disc cause it to heat up to millions of degrees, such that it emits X-rays.

For reasons that are not fully understood, some active galaxies also produce enormous jets of material, which are presumed to be emitted perpendicular to the accretion disc, and which can extend for millions of light years into space. Radio waves emitted from these jets can be detected, allowing the jets to be mapped in great detail.

Because more active galaxies are seen in the distant (early) Universe that in the nearby (present day) Universe, it is possible that the active galaxy behaviour is simply an adolescent phase that most large galaxies go through at some point in their lives. By the time galaxies reach middle age, like our own Milky Way, their supermassive black holes have quietened down somewhat!
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