The Classification of Black Holes

After studying Stellar Astrophysics in detail in the last few articles, we take a step forward to meet pop science's favorite topic: Black Holes. In the previous article, we learnt how black holes form. They were the stellar mass black holes. In theory, black holes can be broadly classified into three main categories. So today, in the twentieth article of Basics of Astrophysics series, let us learn about the classification of black holes. We first start with a brief introduction to these bizarre objects.

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What Are Black Holes?

Black holes are the objects in space that have gravitational field so intense that even light cannot escape. According to the General Theory of Relativity, mass bends space and time. More the mass, more is the curvature of space-time. If the mass of an object is compressed in a sphere of a radius with critical value (Schwarzchild Radius), it becomes a black hole. This radius is different for different objects and is proportional to the mass of object. Its value for Sun is 3 Km. So if Sun is compressed to a sphere of radius 3 Km, it will become a black hole. For Earth, that critical radius is 9 mm.

Read Also: 5 Amazing Facts About The First Black Hole Image

Classification of Black Holes

Broadly, black holes are classified into three main categories. They are:

  • Micro black holes
  • Stellar mass black holes
  • Supermassive black holes

Now let us learn about them one by one

Micro Black Holes

Classification of Black Holes
Micro black holes can be formed in high energy particle smashers

Also known as quantum mechanical black holes, the micro black holes are hypothetical. The fact that black holes smaller than stellar mass can form was first theorized by Stephen Hawking in 1971. These micro black holes have a certain mass limit. According to the concept of Schwarzchild Radius and Compton wavelength, the minimum mass of a micro black hole is 22 micro-grams, also known as the Planck mass.

Click here for the mathematical formula of Schwarzchild Radius

The quantum mechanical black holes must have played an important role in early universe in tremendous energy and density. Such black holes, however, were unstable and must have evaporated through the Hawking radiation. In 1975, Hawking showed that due to quantum mechanical effects, smaller the black hole, faster it will evaporate. So it results in a sudden burst of particles as the micro black hole suddenly explodes.

Mathematics tells us that the energy required to create these micro black holes is of the order of 10^19 GeV. This is far more than the maximum energy we can achieve with current technology.

Stellar Mass Black Holes

Classification of Black Holes

The second in the classification of black holes is the stellar mass black hole. These are one the most studied black holes and unlike the micro ones, they do exist in nature. Their formation mechanism is also known to the scientists. As the name suggests, a stellar mass black hole forms when a massive star collapses. The massive stars have the potential to host full scale fusion of heavy elements in their core. They progressively fuse elements such as carbon, neon, oxygen, silicon, sulphur and so on.

Once this alpha ladder reaches nickel-56, the reaction chain stops. Further fusion of nickel into zinc isn't thermodynamically favorable. This causes the core to shut down. In such a scenario, the star collapses under its own gravity. If the star is quite massive, nothing can halt this collapse and the star is crushed into a black hole. We have already discussed the formation of stellar mass black holes in our previous article. (Read: How are neutron stars and black holes formed)

The critical boundary beyond which nothing, even light cannot escape is known as the event horizon of a black hole. In the reference frame of the in-falling matter, everything is fine. But, to an outside observer, things appear differently because of gravitational time dilation. As the gravitational pull increases, light from the in-falling material starts becoming redshifted and as the material reaches the event horizon, due to tremendous redhsift, it fades away. So, an outside observer can never witness the formation of the event horizon of the black hole.

Supermassive Black Holes

As the name indicates, the supermassive black holes are the largest black holes that are found at the centers of the galaxies. They can be a billion times as massive as the Sun. But these black holes can have density less than that of water. The reason is simple: the Schwarzchild radius of the black hole is directly proportional to its mass and the volume is proportional to the cube of the radius. This makes density to be inversely proportional to the square of the mass. Hence, more the mass, less is the density of the black hole.

Supermassive black hole real image (Credits: EHT)

Also Read: 5 Most Massive Black Holes Discovered So Far

Also, the tidal force of such a black hole is very less. The tidal force on a body at the event horizon is likewise inversely proportional to the square of the mass. Thus a person on the surface of the Earth and one at the event horizon of a 10 million M (10 million solar masses) black hole experience about the same tidal force between their head and feet.

The mechanism through which such bizarre objects take birth is still a mystery and an open field of research in Astrophysics. There are several hypothesis. One hypothesis is that the seeds are black holes of tens or perhaps hundreds of solar masses that are left behind by the explosions of massive stars and grow by accretion of matter. Some scientists also speculate that the stellar mass black holes formed from the death of the first stars in the universe could have given way to such supermassive black holes. The correct explanation, however, is yet to come.

So this was all about the classification of black holes.

Author's Message

While interacting with people and especially students over the past few years, I have realized how influential pop science is. It is good to watch science fiction movies and read science books written by famous scientists such as Stephen Hawking, Michio Kaku etc. These books are excellent as they introduce Astronomy in a simple language. But those who wish to pursue a career in Astrophysics should be careful. No doubt, these books give good knowledge about the Universe, they also project Astrophysics as a very "glamorous" field of study. Reading about these cool topics such as white holes, time travel, black holes, worm holes etc. pumps the students and they go for Astrophysics. Here, they come across Physics and Mathematics: subjects such as Statistical Mechanics, Quantum Mechanics, Electrodynamics, Mathematical Physics, Relativity, Optics, Spectroscopy. These subjects are the very basics of Astrophysics and many students struggle to get hold of them.

So, from my own experience, I want to advice my budding Astrophysicists that it is good to read as many books as you can. However, make sure that you also start with Physics and Mathematics. Keep in mind that the road to Astrophysics is through Physics. If you master the above mentioned subjects, Astrophysics will really be a cake walk for you.

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3 thoughts on “The Classification of Black Holes

  1. Thank you Mr.Rishabh Nakra. I’ve benefitted immensely from your expository articles. I’d like to have your mail so I can tell you more about my master’s program here in Nigeria and get useful suggestions from you.

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