Black holes are sinkholes of the Universe. Everything that goes in its way gets devoured and eaten up. What’s inside a Blackhole is itself a mystery. Black holes are said to be everywhere in the universe, varying in size, mass, and age.
Black holes are a hot topic for modern theoretical physics today. If you want to time travel, if you want to unlock the secret of the early stage of the big bang, if you want to go to a parallel universe (at least according to some theories), just jump into a black hole.
Well, don’t really! We can’t be sure if you would survive that! Black holes despite having a common story can exist in different forms. So, let’s see how many types of black holes are there in the universe. Generally, Black holes are classified into 3 categories:
- Stellar Mass Blackhole
- Supermassive Black holes
- Intermediate Mass Black holes
Let’s talk about the stellar-mass black holes first.
Stellar Mass Blackhole
These are black holes that are formed due to the gravitational collapse of a single massive star. Gravitational collapse is a process under which a cosmic body continues to contract under the effect of its own gravity when there is no other force remaining to counter its effect.
This stage starts after a star has completely consumed its hydrogen fuel and is no longer able to generate energy that would balance the inward gravitational force. Hence the stars having mass up to 10-20 times solar mass, continue to collapse till they form a compact stage of a super-dense neutron star.
Neutron stars consist of neutrons as due to huge gravitational force, protons and electrons get fused to produce neutrons. Further, when the neutron star becomes unstable, it explodes as a supernova leaving only a compact core behind and this compact core is known as a stellar-mass Blackhole.
Scientists estimate that there are millions of stellar-mass black holes present in the Milky Way galaxy itself. These types of black holes are also known as Collapsars because they are formed by collapsing.
Supermassive Black hole
Every galaxy has a very unique feature. At its center, there is a compact region that is highly luminous and releases energy in the high-frequency spectrum such as X-rays or Gamma rays.
These compact regions are called Active Galactic Nuclei and it is thought to be a position for a supermassive black hole to exist. Active galactic nuclei or AGN are thought to be in abundance since the very beginning of the universe, hence these normal black holes had sufficient time to gather enough mass and eventually become supermassive.
In 2015, NASA scientists observed an X-ray flare coming from the center of our Milky Way galaxy. The flare was around 400 times brighter than normal signals coming from the region. It was clear that this was produced when some cosmic body fell into the black hole present there and the accretion disk of the black hole increased.
These Black holes are about a few million to billion times the mass of our sun and are generally believed to exist at the center of galaxies, just like our Milky Way contains a black hole named Sagittarius A* of mass 4 million times our Sun. These black holes are thought to be formed from the merger of many small black holes or the collapse of an entire stellar cluster. Or maybe they are formed completely of dark matter.
Just think about it, they are big, massive, gravitationally dominant but invisible. Do you see some similarities? It is not weird to think of this proposition but it is also not proven yet, so I will leave it to your imagination.
Intermediate Mass Blackhole
These Black holes are in the range of 100-10000 times the solar mass. They are neither as big as the one in the galactic center nor as small as stellar-mass black holes. This is a recently proposed category and very little evidence has been observed regarding such black holes. But in 2019, a gravitational wave signal was observed from the merger of two black holes, 85 and 65 times the solar mass respectively.
So we do have evidence of Intermediate categories now. They are generally formed when several stars, present in a dense region of the galaxy, collide with each other in a sort of chain reaction. Such black holes are present in large quantities in the arms of spiral galaxies.
These intermediate black holes, when falling into each other, can form a supermassive black hole as we have discussed before. So we have discussed all three general categories of black holes.
Primordial Black holes
Few Russian physicists have proposed one more category of the black holes known as Primordial Black holes. Stephen Hawking was very much fascinated with the concept of Primordial Black holes. Think about the initial seconds after the Big Bang — the universe was not homogenous.
Some regions had high concentrated energy, while some had low concentration. What if during those initial few seconds, high densities and heterogeneous conditions could have led sufficiently dense regions to undergo gravitational collapse, forming mini-black holes. These primordial black holes, in the range of the smallest possible mass of 10^-8 kg to 10^5 times the solar masses, were spread throughout the universe.
They could have simply popped up into existence in that radiation-dominated universe. They are not formed of normal baryonic matter and hence they can be a very good candidate for Dark matter particles. But we have no solid evidence of primordial black holes till now, although Stephen Hawking was very much confident about it.
Also Read: Different Types of Stars in the Universe
These cosmic monsters of the universe are even more mysterious than they seem and many studies are going on throughout the physics community to know the exact nature of black holes. What if I tell you there are still many scientists who propose that there cannot be an actual black hole in our universe because science does not allow such objects to exist at all.
We will surely discuss about their proposals in our future articles. So what do you think might happen inside the black hole? Let us know in the comments section.