Around the 1950s radio engineers were looking out for any missile launch or bombs by tracking radio signals at the time of world war 2 they received some radio signals but surprisingly they were not from our surroundings but from very far (coming from the depths of the space-time) which are actually coming from the space.
But the problem was radio telescopes are having a less angular resolution that they cannot actually pinpoint the location at which these radio signals are coming from.
This problem was solved in the 1960s with Parkes radio telescope. Parkes telescope was pointed toward the brightest radio source in the sky which is 3C273 (third Cambridge list (273rd source))
These objects were called as Quasi-Stellar Radio Sources. Abbreviated as quasars
When looked with the largest optical telescope (Palomar telescope) they found the exact location of 3C273 which is in the Virgo constellation. The first quasars discovered were 3C48 and 3C273.
Many similar radio sources were discovered and these sources were at high redshifts (redshifts gives us how far is the source located). If these sources are very far then how can they be visible brightest to us? They must have luminosity equal to the luminosity of more than 500 trillion stars. which means it must be having great brightness. Another peculiar thing is, the luminosity of quasars is changing, for some quasars this change is in days and for some in weeks. Therefore, the size of the object should be small for this rapid change of luminosity (days and weeks in the cosmological scale are rapid) astronomers were surprised, how can a very far object of small size can have large luminosity?
Therefore, it cannot be a star for sure. When viewed in visible light with coronagraph (the process in which the light from the central quasar is blocked to look at the surroundings) they found out that the quasar is having a host galaxy around the quasar which shows that the quasar is at the centre of a host galaxy. Due to the great luminosity of the quasar, it outshines the host galaxy.
Soon astronomers found out that these quasars cannot exist separately they need some host galaxy. And the question is how quasars are so bright? the only possibility which is accepted by all astronomers is that quasars contain a supermassive black hole (supermassive black hole is equal to some billion suns in mass) at the centre and an accretion disk is present around the black hole. An accretion disc is nothing but the fast-moving matter (dust and gas)
When dust and gas are moving with speed nearly equal to the speed of light they produces a large amount of friction and they emit radiation in the entire electromagnetic spectrum.
The accretion disk is responsible for the brightness. Quasars are hungry, violent supermassive black holes that are getting fed with the dust and gas around them. Normally in nuclear fusion, only 0.7% of the mass is converted into energy (according to the Einstein famous equation E=MC2) but in the accretion disk, 6% to 35% of the mass is converted into energy.
Very powerful Jets are produced from the quasars. These jets are seen in x-ray and radio spectrum. These jets extend to some million light-years and are caused due to the strong magnetic field of a quasar. These jets are very powerful, they release a lot of energy to the surroundings.
Soon it was figured out that all the large galaxies undergo this active phase. When the galaxy is young there is a lot of dust and gas present near the galactic centre i.e., near supermassive blackhole this dust and gas form an accretion disk. When there is no more dust and gas to form an accretion disk, the black hole gets calm and stays forever until it gets triggered again.
The milky way galaxy also went through this active phase. The supermassive black hole at the centre of our galaxy is called Sagittarius A* which is calm now. and some x-ray images of the galactic centre show that there are two lobes of gas and dust at the two opposite ends perpendicular to the rotation axis (which may be the jets produced by the quasar)
This indicates that Sagittarius A* was also a quasar back then. how can it get triggered again to become a quasar?
The merging of galaxies triggers quasar activity. When two galaxies collide, we know that black holes get merged and become an even bigger black hole. This is what exactly going to happen with the milky way. Our neighbour andromeda galaxy is heading towards us with a speed of 300,000 miles per hour. In 3 to 5 billion years, both the galaxies are going to collide and these black holes will merge to form a bigger supermassive black hole. Due to the collision, there will be a lot of dust and gas present and the formation of an accretion disk is possible. So, for some distant galaxy, our galaxy looks like a quasar.
Soon it was classified that quasars are one of the categories of active galactic nuclei (AGN). Other types of AGN are radio galaxies, blazers, etc.
Actually, radio galaxies and blazars are quasars themselves the only difference is the point of view of the observer.
When viewed through the jet it is a blazer. When observers see jets are perpendicular to the rotation axis it is a radio galaxy and when the observer sees that jet is at some angle to the rotation axis it is a quasar.
Astronomers found out that not all quasars are radio sources. Some don’t give radio spectrum. Therefore, quasars are classified into radio-loud and radio-quiet quasars. Therefore, astronomers prefer to say QSO (quasi-Steller objects) instead of quasars. ULAS J1342+0928 is the most distant quasar known with the redshift of 7.5 (which means this quasar is formed after some million years after the big bang) and 3C273 is the brightest quasar in the visible sky.