Quasars - Biggest Questions Answered
What do we know about these bizarre objects?
To the mind that is still, the whole universe surrenders. - Lao Tzu
Greetings, fellow Bohron
The universe is beaming with activity. No matter where we look, our line of sight always ends up clinging to something new and beautiful. But what we are able to see is only a small part of the observable universe. How far have we been able to look to date?
Quasars are one of the farthest known objects shining brightly in our universe.
Quasars are distant superluminous supermassive black holes that radiate the energy of the order of 10⁴¹ W which is a billion times brighter than the sun.
They feed on the matter falling into them which creates an accretion disk. As quasars emit such a high amount of energy and due to their winds they shape their host galaxy.
History of Observation
Now let's have a look at the history of quasars. How were they discovered? It happened in the late 1950s when a group of astronomers with the help of a radio telescope observed a large number of radio waves emanating from an invisible object.
The first-ever quasar to be detected was 3C 273 (273 in its name comes from the fact that it was the 273rd object in the Third Cambridge Catalogue of Radio Sources). It was photographed by the Hubble Space Telescope's Advanced Camera for Surveys. The black region at the centre of the image is blocking light from the central object, revealing the host galaxy of 3C 273.
This observation was possible thanks to "Lunar Occultation".
Lunar Occultation occurs when the moon hides an object from the earth by passing between an object and the earth.
These lunar occultations helped the astronomers to find the location of the radio source with precision, and they were able to relate it to a faint stellar object. By the 1960s many such faint objects were observed.
Why are they called ‘Quasars’?
When these bizarre objects were first discovered, their location was found to be similar to those of stars. The investigations of their electromagnetic spectra, however, indicated the dissimilar chemical composition of these objects from stars. Also, they emit an enormous amount of radiation, of a kind not usually seen in ordinary stars.
This is why we call them quasars or "quasi-stellar(star-like) objects and radio sources".
Formation
It is more interesting to know how quasars are formed. They originate from a massive black hole which intakes a considerable amount of matter in an accretion disk and matter starts to spin around this disk and speeds up with time.
This causes the temperature to rise and it heats up which causes more friction between bits of matter resulting in the production of light which is visible to us. Most quasars are found in the red spectrum which helps us to know how far they are and at what speed they are moving away.
Whenever we see a quasar it means that it is approximately ten billion light-years away and it was there ten billion years ago so might be it’s already gone.
Properties
As quasars, stars and galaxies have some properties in common but here is a broad criterion to distinguish them:
They are a stellar radio source.
They have fluctuating light.
They have a large redshift.
They rotate at a very high speed.
Quasars can be categorized on the basis of their spectra: Spectra of some show narrow emission lines while the others have broad emission lines.
How far are Quasars?
The study of the quasar spectra also shows that the spectral lines of the incoming radiations are redshifted i.e. the apparent frequency of its light that reaches us is lesser than the original frequency emitted by the source. This means that quasars are moving faster and farther away from the earth.
So in the 1960s, scientists used Hubble’s law to tell that quasars they have been observing must have been very distant from the Earth, some of them might be 10-20 billion light-years away. This was the reason we believe that they are the farthest known objects in the universe.
How energetic are these objects?
Quasars are supplied with an enormous amount of gas, possibly due to a collision between two galaxies. The surrounding matter inflows and this increases the temperature resulting in the emission of radiation. This is the reason it is believed that they are the most powerful and energetic objects in the universe.
Comparison with Black Holes
Although both of them are found in galactic centres, quasars differ from usual black holes in the sense that quasars are hot and luminous whereas usual black holes are cold and dark.
The formation of a black hole takes place when a massive star dies and it collapses due to its own gravitational pull while quasars are formed when there is sufficient material around the black hole and it falls into its accretion disk.
Objects that are gulped into a black hole experience very high gravitational force and they reach the centre of a black hole where all the stuff is compressed into a very small point (singularity) where the curvature of space-time becomes infinite. On the other hand, quasars interact with the neighbouring galaxies and feed the supermassive black hole with the gas and dust that is present around the hole.
Theoretically, the lifetime of a black hole is around 2.6 × 10⁶⁹ years whereas that of a quasar is about 10 million years. Black holes can cause time dilation due to strong gravitational force and magnetic fields. In the case of quasars, no such dilation is observed.
Sources:
Quasar vs Black Hole (How Are They Different?) - Scope the Galaxy