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Black holes: The darkest objects in the universe

An artist's impression of the Cygnus X-1 system, which comprises a stellar-mass black hole orbiting with a companion star located 7,200 light years from Earth.
An artist's impression of the Cygnus X-1 system, which comprises a stellar-mass black hole orbiting with a companion star located 7,200 light years from Earth. (Image credit: International Centre for Radio Astronomy Research)

Black holes are regions of space where the gravitational pull is so strong that nothing — not even light — can escape. Rather than empty space, black holes are chock full of matter that gets squeezed into a teensy space. 

Who discovered black holes?

Physicist Karl Schwarzschild accidentally discovered black holes in 1916, when he was figuring out a particular solution to Einstein's general theory of relativity. He was trying to find the solution to the gravitational pull of a single, solitary, symmetric ball of matter — such as the sun at the center of our solar system. But that solution contained a peculiar feature: the theory behaved strangely at a specific radius, known today as the Schwarzschild radius.

It was later realized why this radius was so special. If you compressed the mass of an object into a space smaller than that radius, its gravitational pull would overwhelm every known force and nothing could escape. Early physicists assumed that this situation would never be found in nature. But in the late 1930s, it became clear that nature could indeed allow black holes to exist when Indian physicist Subrahmanyan Chandrasekhar found that above a certain density, no force can overwhelm gravity. However, black holes can only form under the most extreme conditions.

How do black holes form?

Stars produce light and heat due to the engines at their cores where a process called nuclear fusion occurs. There, two lightweight atoms fuse together to form a heavier atom, a process that releases energy. Those heavier atoms then fuse to form even heavier atoms, and so on to keep the star churning out light and heat. 

In fusion, two or more particles collide to form a more massive product. In this illustration, deuterium and tritium combine to make helium with the emission of a neutron. This is how stars make their energy.  (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images)

As such, when stars that are more massive than eight times that of our sun near the end of their lives, they fuse heavier and heavier elements in their cores, like silicon and magnesium. Eventually they start forming iron. The problem? Fusing iron requires more energy than that reaction produces, and so at that point nothing can counterbalance the inward gravitational pull of the star's own mass. And so the hefty star collapses in on itself. With all that crushing gravitational weight, the star's core gets squeezed beyond the Schwarzschild radius, at which point a black hole is formed.

Since no known force can stop the collapse, once material forms a black hole it keeps on squeezing down until it becomes a singularity — a point of infinite density. Surrounding that singularity is the event horizon, the invisible spherical boundary that marks the entrance to the black hole. Once anything crosses the event horizon, it can never, ever leave. In order to escape, one would have to travel faster than the speed of light, and since nothing can travel faster than the speed of light, that black-hole meal is doomed.

Supermassive black holes, which are millions of times the sun's mass, form over hundreds of millions of years by both feeding on material around them and by merging with other black holes.

What happens inside a black hole?

Black holes are anything but empty space; inside, one would find loads and loads of mass squished down to an infinitely small point. The gravitational pull of that singularity would inevitably carry any mass toward it. No matter what direction you face or how hard you resist, you are guaranteed to reach the singularity in a finite amount of time, as explained by JILA, a joint institute of the University of Colorado Boulder and the National Institute of Standards & Technology.

Physicists don't know what happens at the singularity. It's such an extreme environment that all of our current knowledge of physics breaks down.

How do scientists know black holes are real?

Despite the lack of insight into the innards of a black hole, physicists do know that black holes exist. The first evidence came in the form of Cygnus X-1, a bright source of X-rays about 6,000 light-years away, NASA explained. Observations of that system revealed a small, dense, dark companion — a black hole — funneling off the atmosphere of an orbiting companion. Astronomers can't see the black hole itself, but, as the gas falls to its doom, it heats up and emits energy in the form of X-rays.

The black hole Cygnus X-1 is pulling material from a massive blue companion star. That "stuff" forms an accretion disk around the black hole. (Image credit: NASA/CXC)

How big are black holes?

The black hole in Cygnus X-1 has a mass about 20 times that of the sun, which is pretty typical for black holes throughout the universe. In our own galaxy, scientists have identified anywhere between 10 million and a billion black holes, NASA reported.The closest known black hole is Cygnus X-1, which lurks just over 6,000 light-years away (although there are unconfirmed black holes as close as 1,000 light-years away).

But in the center of the Milky Way — and at the center of almost every other galaxy — sits a monster, a supermassive black hole. Supermassive black holes are millions of times more massive than the sun, and some can even reach hundreds of billions of times more massive than the sun. These giants reach stupendous sizes by feeding on surrounding matter and merging with other black holes over the course of hundreds of millions of years.

What do black holes look like?

Here, the first ever direct image of a black hole. (Image credit: Event Horizon Telescope Collaboration)

Black holes are just that, they're "black" in that they do not emit any light., But astronomers can still detect them through both the gravitational effects they have on other objects and their messy eating habits. 

For some black holes, primarily the supermassive ones, astronomers can see them because of the quasars they produce. Quasars are intensely bright sources of radio emission. When matter falls onto a black hole, it gets compressed and heats up in a souped-up version of Cygnus X-1. The disk of material surrounding the black hole can glow brighter than its entire host galaxy, and is capable of launching jets of super-heated, nearly-light-speed particles out for tens of thousands of light-years, NASA said.

Another way to "see" black holes is when they merge. When two black holes collide, they send out ripples in space-time known as gravitational waves. These waves are incredibly weak, but sensitive instruments on Earth are capable of detecting them. To date, astronomers have identified 50 black hole merger events.

The only true "image" of a black hole ever created came out in 2019, when astronomers used the Event Horizon telescope — a network of dishes spanning the entire Earth — to snap an image of this lit-up disk of material swirling around a black hole called M87*, Live Science reported at the time. Weighing 3 billion times that of the sun and sitting in a galaxy over 50 million light-years away, M87* looked like a distorted orange donut in that image. Since it's impossible to take a picture of the black hole itself (because no light can escape), what the astronomers instead saw was its "shadow," the hole in the glowing material surrounding it.

What if you fell into a black hole?

It's a good thing that the nearest black holes are thousands of light-years away from us. From a distance, black holes act like any other massive objects in the universe. In fact, if you were to replace the sun with a solar-mass black hole, the orbit of the Earth would remain completely unchanged (all the plants would die, but that's a different problem). But near a black hole, the gravitational forces are so strong that you would be stretched head-to-toe into a long, thin strand of particles before even reaching the event horizon, a terrible fate quaintly called "spaghettification."

Originally published on Live Science.

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Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe, to the earliest moments of the Big Bang, to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space," and frequently appears on TV.