A black hole is formed when a large enough star, dying as it runs out of fuel, collapses upon itself and creates an almost infinitely dense point in space.

With so much mass packed into a small space, a black hole will pull things around it towards itself by the force of gravity, even light itself. Because light cannot escape a black hole, it is not possible to actually observe a black hole. Instead, astronomers tend to locate them based on the motion of celestial bodies around it whose path is being affected by the gravitational pull of something invisible.

A black hole is a formation in space that is so dense and heavy that the pulling force of its gravity is strong enough that nothing can escape it, not even light. Therefore, it is called a black hole.

This super-strong gravity occurs because a lot of mass is compacted into a very tiny space. In fact, some black holes are believed to be the size of an atom with the weight of an entire mountain. This extreme compression can occur at the end of a star's life.

Stellar black holes form when a very heavy star collapses in upon itself. The star explodes and some parts of the star are blown into space while others are compacted and form a black hole. This event is called "supernova".

Other black holes, so called supermassive black holes, form at the same time as the galaxy they are in. In fact, scientists believe that every large galaxy has a supermassive black hole in its center. The one in the center of our Milky Way galaxy is called Sagittarius A. However, our sun does not have enough mass to ever collapse into a black hole.

A black hole used to be a star (a star large enough). Gravitational force always "attracts", so without any balance force, a cluster of mass will collapse into the center of the cluster due to gravity. A star balances the tendency of collapse through constant nuclear reaction (explosion). But after the star burns out its nuclear materials, the star will become a red giant, and then a supernova, then the gravitational collapse starts. After the collapse, a small star (like our sun) will become a white dwarf; a medium mass star will become a neutron star; and a heavy mass star will become a black hole. So the black hole is the inevitable end of a heavy star.

A black hole forms whenever enough mass is packed into a small enough region of space that the gravity from that mass is enough to prevent light from escaping. There are 3 known types of black holes, and possibly at least 1 more. The known types are stellar-mass, supermassive and intermediate-mass black holes; the potential type is primordial black holes.

Stellar-mass holes form from individual stars, specifically stars that are ~8-20 times the mass as the Sun. During their lifetime, the size of a star is dictated by a balance between the outward pressure of radiation resulting from nuclear fusion and the inward pressure of gravity from the material of the star. Once the star runs out of fuel, the fusion process slows down and the outward pressure decreases. Then, the inward pull of gravity overcomes this weakened outward push and all of the matter of the star is pulled toward the center and crushed together into a (relatively) tiny volume. As this happens, the gravity near the region increases until the velocity to escape the vicinity is above the speed of light. Since nothing can move faster than light, nothing can escape, and a black hole now exists.

Supermassive black holes exist at the centers of galaxies and have masses millions or billions of times that of the Sun. Their formation is still a mystery, but some possibilities are the collapse of immense clouds of material very early in the formation of the galaxy, the growth of a stellar-mass black hole as it consumes enormous amounts of material over millions of years, or the merging of many stellar-mass holes.

In recent decades though, astronomers have recorded X-rays jets and streams of gas like those around stellar-mass black holes, but which would require more mass than those. Astronomers therefore take these as potentially being signals of intermediate-mass black holes.

Primordial black holes are the final type and are the only type on the list without any observational evidence at all - but from theory astronomers think that they might exist. These would have formed very soon after the Big Bang before any of the other structures of the universe (e.g., star and galaxies) had formed. Basically, space right after the Big Bang was not perfectly uniform, with some pockets potentially being dense and high-energy enough to form black holes. Depending on when they formed, the mass of a primordial black hole could vary immensely - from 1/100,000th of a paperclip to 100,000 times that of the Sun. As stated though, these are completely theoretical at present, with no observational evidence.

I'm not sure what is meant by "activating" a black hole, but jets of gas and radiation can be shot out from near a black hole when material falls into it. These emissions "light up" certain detectors, and might be referred to as activity. Watching for these events is one method of locating black holes.

Hi, great question! A black hole is a challenging concept to wrap your mind around. In simple terms, it is a place in space & time where the gravity is so strong that *absolutely nothing*--not dust, not even light (which is why it's called "black")--can escape from it.

How does this happen? Well scientists suspect that it happens when a large amount of matter has been squeezed into a very small space. For example, when a star bigger than the Sun (~865,000 miles wide) is squeezed into a sphere about as wide as New York City (~320 miles), that compression (more than 2700x) results in a ball so dense that it forms a VERY strong gravitational field around it. I hope that helps!

A black hole is what happens when you have so much mass that the gravitational force that the mass exerts is strong enough to overpower the other three fundamental forces of nature (electromagnetism, the weak nuclear force, and the strong nuclear force). The mass collapses into an even denser state, enclosed within an event horizon, the point at which gravity becomes strong enough that not even light can escape.

The exact nature of the mass inside of the black hole is unknown; the theory that describes black holes is Einstein's theory of general relativity, but the answer that this theory gives disagrees with Heisenberg's uncertainty principle in quantum mechanics, and the two theories cannot both be correct.

Black holes aren't "activated". They exert gravitational force because they have mass. Because this mass is very concentrated, it is possible to get very close to the center of mass, and so experience very strong gravitational fields. However, at a distance from a black hole, the gravity that it exerts will be the same as the gravity from any other massive body. For example, a black hole with the mass of the sun would have an event horizon about 3.6 km in diameter, but if you were on a planet orbiting said black hole at the same distance as the Earth orbits the sun, the gravity that you would experience would be the same as the sun's gravity that you feel on Earth.