https://www.livescience.com/black-holes.htmlWhat are black holes? Everything you need to know about the darkest objects in the universe
April 9, 2021
Find out what black holes are, who discovered them, how we know they exist and what would happen if you fell into a black hole.
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.
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.
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.
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 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.
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 following images have credits where possible:
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)
July 14, 2015, 03:37:58 PM
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