What is a quasar? What is their importance?
A quasar is an active galaxy.
A quasar is powered by a supermassive black hole at the centre of a galaxy. If there is enough material near to the supermassive black hole it will start to fall into the black hole creating an accretion disc. It takes a long time for material to fall into a black hole. As more material falls into the accretion disc it gets superheated by friction and gravitational effects to the point where it emits huge amounts of energy. This is a quasar.
The quasars we have observed are very distant. They are billions of light years away. This means that they are young galaxies and we are seeing them as they were in the early days of the universe.
An important consequence of this is that we know that young galaxies have supermassive black holes at their centres.
We now think that most galaxies have supermassive black holes at their centres. There is evidence that the supermassive blacks holes control the evolution of galaxies and they may be responsible for the formation of galaxies.