Sunday, September 12, 2021

Types of Black holes – an overview

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Black holes are defined as the astronomical bodies from which nothing not even light can escape. Concept of black hole was a topic of debate since 18th century but no one was sure at that time whether such objects exist or not. However, after the proposal of Einstein’s theory of relativity, different theories on black holes got support.

After that hundreds and thousands of research papers and articles have been published till now explaining what a black hole is, what are its characteristics, what are its type, how it was formed, how it behaves with its surrounding material and many more.

In this article we will see different types of black holes that are categorized till now. Basically black holes can be categorized based on their size, rotational state and charge. So let us have a look at some of these types of black holes.

Types of black holes based on their mass

First we will see the different types of black holes based on their mass.

Microscopic black holes

Microscopic black holes which are sometimes also called primordial black holes, may have existed during the early stages of the Universe, a few moments after the Big Bang. At that time the temperature and pressure were so gigantic that they could have squeezed matter down to a singularity. 

However only primordial black holes of mass about a billion tons could  have survived until the present day and the lighter ones would all have evaporated.  

Stellar-mass black holes

The stellar mass black holes are estimated to about three to twenty times massive than the sun. Astronomers believe that about one billion stellar-mass black holes exist in our own Milky Way galaxy. 

Some of these stellar mass black holes are isolated while other such black holes lie close to normal stars. Such coupled systems are called black hole X-ray binaries, and if they emit a relativistic pair of jets, they are known as Microquasars.

Supermassive black holes

The core of many galaxies is exceedingly luminous, and these brilliant central regions are called Active Galactic Nuclei (AGN). The active galactic nuclei extend only over a few lightminutes or light-days and they are less than one ten-millionth the size of their host galaxy, but they are very much brighter than the whole galaxy.

Astronomers believe that these active galactic nuclei are powered by a central supermassive black hole. Supermassive black holes are not isolated, they attract the matter in their vicinity with their strong gravitational field. Due to this attraction, the gas rotates towards the event horizon, building an accretion disc.

While the gas spirals in, its energy is converted into heat and starts to shine brightly, producing the observed extreme luminosities. Some part of these gas and matter escapes from being gulped down into the black hole, so they are carried away in two radio jets that emerge close to the inner edge of the disc. 

Based on rotation and charging state

Based on the rotational state and charge of black holes, they are categorized into four groups.

Schwarzschild black hole

Schwarzschild black hole are the simplest type of black holes. They are stationary black holes (means non-rotating) and they have no charge. The Schwarzschild solution of Einstein field equation basically describes and explains what a Schwarzschild black hole is.

Kerr black hole

Schwarzschild black hole describes the exterior of an isolated, spherically symmetric black hole and this solution is quite simple. However, things are not simple as it was assumed because nothing in the universe remains stationary so how can a black hole is assumed to be a stationary body. 

To solve this problem Kerr solution of Einstein field equation was derived which basically describes what actually a Kerr black hole is. Kerr black holes are rotating black holes but like a Schwarzschild black hole they are also uncharged. 

Reissner-Nordstrom black holes

Reissner-Nordstrm black holes are just another type of Schwarzschild black hole but Reissner-Nordstrm black holes are charged black holes unlike that of a Schwarzschild black hole.

Kerr-Newman black hole

The charged spinning solution or Kerr-Newman solution of Einstein field equation defines what a Kerr-Newman black hole is. Basically it is another type of Kerr black hole but Kerr black holes are uncharged whereas Kerr-Newman black holes are charged.

The Kerr-Newman solution is the most general static black hole solution to the Einstein-Maxwell equations. And it is of great importance for theoretical considerations within the mathematical framework of general relativity and beyond.

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