A we look further into space, we look backwards in time. The most distant objects - about 12 billion light years away - we can see are also the most energetic - emitting up to 100 times the energy output of the Milky Way. They are compact sources emitting massive amounts of electromagnetic radiation, and are believed to be as a result of an accretion disk of dust and gas being heated and falling into a supermassive black hole at the centre of a galaxy formed as a result of the collision of smaller galaxies. As it falls, radiation is emitted preferentially along the axes of rotation.

The name 'quasar' is derived from the phrase 'quasi stellar object'. When they were first observed, they appeared as point sources similar to stars but were too energetic to be a star. Now we can be more definite about the size of some quasars by looking at how they vary in brightness, because they cannot be larger than the the speed of light times the period over which the light output varies. We get an estimate of about the size of the solar system. With advances in telescope technology, we can pick out the surrounding host galaxy.

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