Astronomers have long thought that black holes come in just two kinds, the “stellar” and the “supermassive.” The type depends on the weight of the black hole, or more specifically, its mass.

Stellar black holes form when the center of a very massive star collapses in upon itself. This collapse also causes a supernova, or an exploding star, that blasts part of the star into space. Scientists think supermassive black holes formed at the same time as the galaxy they are in.

However, in the event that a pair of black holes were to merge, an immense amount of energy should be given off as gravitational waves, with distinctive waveforms that can be calculated using general relativity. This stage is called binary black hole inspiral. The black holes will merge once they are close enough.

A black hole is where gravity has become so strong that nothing around it can escape, not even light. The mass of a black hole is so compact, or dense, that the force of gravity is too strong for even light to escape. Can we see them? Black holes are truly invisible.

In short, black holes are massive pits of gravity that bend space-time because of their incredibly dense centers, or singularities.. When a star dies, it collapses inward rapidly. As it collapses, the star explodes into a supernova—a catastrophic expulsion of its outer material.

So far, astronomers have identified three types of black holes: stellar black holes, supermassive black holes and intermediate black holes.

Of all the species in the world, the largest—and most dangerous—is the saltwater crocodile. These ferocious killers can grow up to 23 feet in length, weigh more than a ton, and are known to kill hundreds each year, with crocodiles as a whole responsible for more human fatalities annually than sharks.

The Milky Way is still gulping small galaxies and their hydrogen gas. Sloane telescope can see distant galaxies but only as they were billions years ago. And scientists discovered that spiral galaxies similar to the Milky Way are also dying.

The Milky Way Galaxy

Orion Arm

Earth is located about halfway between the center of the Milky Way and its outer edge. Light at the galaxy’s center takes 25,000 light-years to travel from Earth. (A light-year is the distance light travels in one year.)

Earth is the third planet from the Sun at a distance of about 93 million miles (150 million km).