What is a quasar?
A quasar is an active galactic nucleus.
Many galaxies have a supermassive black hole at their centre. Our own Milky Way galaxy has a supermassive black hole which is about 4,000,000 times the mass of the Sun.
If the galaxy is young or has collided with another galaxy, then there will be a large amount of dust and gas at the centre of the galaxy.
This dust and gas starts to fall into the black hole forming a disc called an accretion disc. Contrary to what many people think, black holes are slow eaters. It takes a long time for material to fall into a black hole.
Once an accretion disc has formed, additional material falls into the disc. As it does so it gets heated by friction and gravitational effects. Once the material gets hot enough it emits vast amounts of energy. This is a quasar. This is also known as an active galactic nucleus.
The known quasars are very distant objects which are billions of light years away. The light from them has taken billions of years to reach us. They are the active galactic nuclei from young galaxies formed in the early universe. These galaxies will have run out of material at their centres billions of years ago, so will no longer be quasars now.