What is Black Hole?

Black holes are cold remnants of former stars. They are the weirdest and most interesting objects present in the outer space. They are so dense and have such a strong gravitational pull that even light cannot escape through them. Black holes consume the gas and dust from the galaxy around them and thus keep increasing in size.

The mass of a black hole is expressed in “solar mass,” one solar mass equals to the mass of the Sun. Some of the most massive black holes are NGC 4889, APM 08279+5255, NGC 1600, OJ 287 primary, and S5 0014+81.

German-born physicist Albert Einstein first predicted black holes in his General Theory of Relativity, published in the year 1916. However the term “black hole” was coined by American astronomer John Wheeler in 1967, and the first black hole was discovered in the year 1971.

Types of Black Holes

According to theory, depending on the mass there are three types of black holes:

  1. Stellar black holes

These are formed by the gravitational collapse of massive stars and have the mass between 3 to 10 solar masses. They are small in size but extremely dense, and have strong gravitational force on objects around it.

  1. Supermassive black holes

Supermassive black holes have the mass equivalent of billions of suns and are located at the centres of most of the galaxies, including Mikey Way. It’s unclear yet how they are formed, but they are too big to have formed from a collapsing star. Being close to gas clouds and tightly-packed stars, supermassive black holes continue to grow in size by consuming matter from the galaxy around them. They are capable of accruing infinite amounts of matter; thus become denser with the increase in their mass.

  1. Intermediate black holes

Intermediate-mass black holes have the mass somewhere between the stellar and supermassive black holes. It is expected to have the mass between 100 and 1000 solar masses.

Black holes are also classified as spinning (possess angular momentum) and non-spinning (static). Spinning black holes are formed from the gravitational collapse of a massive spinning star or from the collection of stars that have a non-zero angular momentum. Whereas, static black holes might be a result of the collapse of stars with zero momentum.

Note: Most of the stars are spinning thus most black holes in nature are spinning black holes.

How is the mass of a black hole calculated?

Black holes often have stars orbiting around them, making it possible to measure their mass by measuring the speed of the orbiting material.

If a star and a black hole orbit around their common center of gravity. Then to calculate the mass of black hole, the size of the orbit and the speed of the star is measured, and by using the law of gravity mass of the black hole is calculated.