LIFE OF A BLACK HOLE

Black holes are captivating both in a sense and as a topic of discussion. Everything about them is bizarre and extreme, and we're always discovering something new. But even the most basic facts about black holes are pretty incredible, and it's possible you've never had a primer on them. So today, we're going back to basics and talking about what black holes are, how they form, and how they die.

Let's start at square one and define exactly what a black hole is: a region in space where the gravitational pull is so strong that anything that enters cannot escape. This includes the fastest thing in the universe—light. And since light can't come back out once it crosses that point of no return, known as the event horizon, observers like us on the outside can't see black holes. They are completely invisible.

You may be wondering how we know they're there. We've even taken pictures of them—it was in the news and everything! The black hole itself may be invisible, but we can still find them because of the effect they have on nearby matter and on light passing close, but not too close. We've spotted the radiation given off by superheated matter swirling around them, forming what's called an accretion disc. It basically looks like space's version of the Eye of Sauron. We can even see the backside of the accretion disk that would normally be hidden behind the black hole's shadow if it weren't for the wild way black holes can alter the path light takes.

According to Einstein's theory of general relativity, black holes warp space and time to the extreme because they pack an enormous amount of mass into an incredibly small space. At the black hole's center, just how much mass the black hole has determines how far away the event horizon is from the center—a distance called the Schwarzschild radius. And the radius can grow if more matter falls into the black hole. The Schwarzschild radius is important in another way—it's also the minimum size an object will have to be squeezed down to for gravity to take over and turn it into a black hole. Everything has a Schwarzschild radius—you, me, the Earth, the Sun. To make a star with the mass of the Sun into a black hole, you would need to compress it down to a sphere with a radius of less than three kilometers. If that were to suddenly happen somehow—let's say via some alien superweapon or something—the Earth and all the planets wouldn't be sucked in and compressed into oblivion. Remember, objects have to be within that Schwarzschild radius for a black hole to devour them, and as long as we stayed more than three kilometers away from our black hole Sun, that wouldn't be our fate. In fact, if the Sun were suddenly replaced with a black hole of one solar mass, the orbits of the planets in our solar system wouldn't change at all. The sudden onset of eternal night would be catastrophic though, so we'd still be doomed, just not in the way you might first assume.

Ruling out alien superweapons, how do black holes form? Well, it depends on what kind of black hole we're talking about. Stellar-mass black holes, with masses between three and dozens of times that of our Sun, dot the galaxies most often. They form when a star of at least 20 solar masses goes supernova, leaving behind a core that collapses below its Schwarzschild radius under its own weight. Black holes ranging from hundreds of thousands to billions of solar masses are found at the centers of galaxies, if not all of them. The famous photo of a black hole is a supermassive one in the galaxy Messier 87. How they form is still a mystery, but we think they get their start when their galaxies are just beginning. Obviously, that leaves a huge gap in size between stellar-mass and supermassive black holes, so scientists have long predicted the existence of intermediate black holes, and we've just started to spot our first candidates recently. There's also the possibility that black holes smaller than stellar masses were created by the early universe, and some speculative theories even suggest black holes could be as tiny as the subatomic scale and could be smashed into existence from collisions inside particle accelerators. We haven't seen any evidence of those yet, but if we did, don't fret. Remember again that just because something is a black hole, that doesn't automatically turn it into an unstoppable, planet-crushing vacuum cleaner. It would need to get close enough to you for it to be a threat.

And because of how black holes die, microscopic black holes will never get the chance to expand to a dangerous size. We think black holes decay by giving off something called Hawking radiation. The oversimplified explanation goes that virtual particles are constantly popping into existence in the vacuum of space, and usually they come together and cancel each other out. But if a pair forms straddling an event horizon, one will escape while the other will fall in, and the process shrinks the black hole until it is no more. Physicist Stephen Hawking predicted the existence of this radiation and also showed that the smaller a black hole is, the faster its rate of decay. That means a microscopic one made from particle collisions would have an unfathomably short lifespan of around 10 to the minus 27 seconds—much too short to balloon to a dangerous size. That's a bit of a relief, but by the same token, this also means that supermassive black holes will be around for a very long time—many orders of magnitude longer than the current age of the universe. Still, it's not like we'll come dangerously close to one of those in our lifetimes. So, really, the only thing about black holes that you won't be able to escape is us talking about them in this article.