What are black holes?

Black holes are areas of space where the gravitational field is so strong that nothing at all (including light) can escape, which is the reason why black holes are… black. Some black holes are former colossal stars which have been crushed to an extreme density because of a supernova explosion. Other black holes contain the mass of millions or even billions (in some cases) of stars.

However, many people are wondering how or why are we able to see black holes, since light can’t escape them. The answer is simple: We don’t really see black holes, rather we see the effects that they have on their surrounding space.

Hundred year old theory

You might have heard of Albert Einstein. He published his general theory of relativity back in 1915, therefore being the first scientist to suggest that the universe we live in contains such dense, massive and strange objects. Black holes can be traced back to Einstein’s equations of general relativity, as a natural outcome of the death (and adjacent collapse) of massive stars. However,  the first person to mathematically formulate black holes was Karl Schwarzschild, a German mathematician, back in 1916. The term “black hole” was first used in 1967 by theoretical physicist John Wheeler.

The very first black hole ever to be discovered was Cygnus X – 1, and it was observed and studied in 1971.

Types of black holes

There are two types of black holes. The first kind is the stellar – mass type  black hole, which is just a remnant of a huge star. At the end of its life, a star that is over five times more massive than our sun explodes in a spectacular supernova. Then, its core is violently and rapidly compressed under gravity. In concordance to the mass of the star, the collapse might cease and form a neutron star or continue collapsing and ultimately form a black hole. The mass of such a black hole ranges from at least five times the mass of our sun up to about 60 times the mass of our sun. The diameter can vary between 10 and 30 miles.

The other kind of black hole is the supermassive black hole, whose mass can be millions of times bigger than the one of our sun. The colossal mass of such black holes suggests that they can’t be remnants of individual stars. It’s believed that supermassive black holes formed in the early history of our universe thanks to huge collapsing clouds of interstellar hydrogen, but this might be far from the truth, however. Supermassive black holes can have diameters greater than that of the solar system our planet is part of. There is a black hole at the center of most galaxies. The one at the center of our galaxy, Sagittarius A* is about 4 million times more massive than our sun and approximately 37 million miles in diameter.


Many people wonder what would happen if a black hole gets too close to our planet. While we don’t  really know for sure, it’s certain that the gravitational pull of unimaginable proportions won’t do any good to us.


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