Few phenomena in our universe seem to straddle the line between fiction and reality quite like black holes. Indeed, if seeing is believing, then black holes are truly unbelievable. They are objects so massive that light can't even escape their gravitational grip, making them impossible to see directly.

Scientists do know that they exist, however. While black holes are invisible, they still exert a strong gravitational presence, the effects of which can be observed in the behavior of objects that orbit them. In fact, it is now believed that our own galaxy, the Milky Way, has a black hole at its center that is 4 million times more massive than our Sun.

How do black holes form?

The most common type of black hole, a stellar black hole, forms when a sufficiently large enough star collapses under the weight of its own gravity. All stars have a lifespan that depends on the amount of fuel they have to burn. So long as they have fuel, they can fight back against the inevitable crush of gravity. But when that fuel runs out, gravity takes over, and the star implodes, forming a black hole.

Not all collapsed stars become black holes; it all depends on their size. Our own sun isn't large enough.

Not all black holes are created equal

Like with stars, size matters when it comes to black holes. Stellar black holes are just one type. There are also supermassive black holes, like the black hole at the center of our galaxy. These are in the order of hundreds of thousands to billions of solar masses, and how they formed remains something of a mystery. It's hypothesized that they were likely the result of immense gravitational forces that existed during the early stages of the formation of whole galaxies.

There are also believed to be tiny black holes that are no larger than an atom, even though they have the mass of a mountain. Mini-black holes like these have yet to be observed, however.

How many black holes exist?

It's hard to say, but recent discoveries suggest there could be way more of them than previously believed. For instance, scientists have detected "stealth" black holes that are unusually quiet, making them difficult to spot. It is believed that there could be more than 100 million stellar black holes in our galaxy alone, taking into account these silent, hidden ones.

Supermassive black holes are more rare, but there's reason to believe they could be far more numerous than previously believed too. New models of the universe suggest that supermassive black holes may come in clusters, which, if true, will increase our count of them significantly.

What would happen if you fell into a black hole?

The good news is, black holes don't go around looking for planets to swallow. Earth isn't on a crash course with one, and falling into one would require seeking out the experience directly. Having said that, if you did happen to fall into a black hole you wouldn't live to tell the tale. Things really get grim after you cross over a black hole's so-called "event horizon," the point of no return. This is where gravity is so strong that nothing can escape its grip, not even light. Your body would instantly begin to stretch out like goo from a tube of toothpaste, until you were broken down into a stream of subatomic particles.

Eventually, what's left of you would be pressed into a singularity, a place where the laws of physics as we currently understand them don't even compute. What happens there is anybody's guess, and it's obviously not possible to witness firsthand.

What is a singularity?

The mysteries that surround the physics of a black hole's singularity have lead to a lot of speculation about what might happen there. Interestingly, black holes aren't the only places where singularities — places where gravity approaches an infinite point — can be found. The Big Bang itself is believed to have started from a singularity. This common connection between black holes and the Big Bang has led to some wild theories, such as the idea that black holes could be portals to whole other universes. Some scientists have even suggested that our own universe may have formed within a black hole in some larger universe. Again, though, it wouldn't be possible to survive a trip to a black hole's singularity, so we're trapped in our own universe one way or another.

What happens when two black holes collide?

Spacetime itself literally vibrates, like a ripple on a lake. Though these spacetime ripples, or gravitational waves, were first predicted by Einstein's general theory of relativity, they were only first detected here on Earth in 2015 by the Laser Interferometer Gravitational-wave Observatory (LIGO). The waves are believed to have been caused by two colliding black holes 1.3 billion light years away. It's a breakthrough discovery that could allow us to peer deeper and further into the universe than ever before, in the same way that X-rays allow us to see into the human body without opening it up.

By studying these waves, we might finally begin to unravel some of the mysteries that happen inside black holes.

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

What is a black hole?
A rundown of one of the most perplexing and wondrous phenomena in the universe.