Quick explanation
A black hole is a region in space where gravity is so immense that not even light can escape.
And in 2019 astronomers finally succeeded in observing the "shadow" of a black hole!
A star collapses
A black hole is the most fantastic object in the Universe. It is neither matter (like stars, planets, and bicycles), nor radiation (like X-rays, green light, and radio waves). It is, in fact, merely a clot of gravity.
A black hole is formed when a star more massive than approximately 25 Solar masses ends its life.
A star is the result of the equilibrium between two forces: Gravity trying to pull everything together, and energy production trying to push everything apart. When the star has burned all of its fuel, it will collapse. For light stars, this collapse will stop at the state of a white dwarf, while a heavier star explodes in a supernova, leaving back a neutron star.
However, if the neutron star is more massive than about 2–3 Solar masses, nothing is able to prevent the catastrophe: The collapse will continue until all that has not been blown away by the supernova explosion has crumpled to a single point!
Space is warped
The final remnant typically weighs a few Solar masses, and all this mass is located not just in a very small volume, but in a single point!
Near this point, Einstein's theory of relativity predicts that gravity is so extreme that space itself is "curved". That space is curved is a somewhat popular way of saying that if something — e.g. a beam of light — is traveling in a straight path through space, a far-away observer will see the path of the beam as being deflected.
The closer one gets to the point, the stronger the gravity, and thus the more speed is needed to escape. Inside the so-called "event horizon", which for a one Solar mass black hole is 3 km from the point, this necessary speed exeeds the speed of light.
That means that nothing, not even light, can escape.
The definition of the black hole is the sphere inside the event horizon.
Credit: sciencenews.org.
Time slows down
Not only space, but also time gets warped in the vicinity of a black hole, in the sense that the closer one gets to the event horizon, the slower time wil run as seen from far away. An astronaut falling towards the black hole will feel nothing special at the horizon itself, but will eventually be ripped apart by tidal forces, getting crushed in the central point. However, from the point of view of an observer located far away, the astronaut will fall slower and slower, never reaching the horizon.
If the falling astronaut has a powerful jetpack so that he can come close to the horizon, hang out there for a while and then return to his friends, he will find that while the journey took him, say, an hour, a hundred years has passes in the "outside" world. In this way, a black hole can be used as a time machine to travel forward in time (but not backward — for this you must use other means…).
In the movie "Interstellar" this happens to two of the characters — they spend a day on a planet orbiting a black hole, and hwne they return to their spaceship, their friends are a little bitter because they've waited for 26 years (I don't remember all the details, but I believe it was something like this).
Supermassive black holes
I'LL WRITE SOMETHING ABOUT SUPERMASSIVE BLACK HOLES HERE SOON!
Observations of black holes
Seeing a black hole isn't easy; light cannot escape you know. Sometimes you can see the light from gas plummeting into the black hole, being heated till it glows. But that isn't really the black hole.
But now, black holes are no longer "just something theoretical". Now, finally we've seen it!
In 2019, the "Event Horizon Telescope" collaboration announced that, using a network of various telescopes around the world, they had observed the "shadow" of the 6.5 billion Solar mass supermassive black hole in the galaxy M87 (see also) my own image of M87).
Credit: The ETH Collab. (2019, ApJ, 875, L1).
The shadow of a black hole
If you look toward a black hole, all sightlines within the Schwarzschild radius (\(r_s\)) will terminate on the surface of the event horizon. Since it is black, you cannot see it, but if there is something bright behind — e.g. glowing gas swirling around the hole — you can still "see the missing light" as a round disk in front of the luminous background.
But because space curves around the black hole, you don't just see a disk with radius \(r_s\): sightlines a little farther away from the center will curve around and end on the back of the hole! And sightlines even farther away curve all the way around to the front again, or several times around, so you see the front and back several times, all the way out a distance of \(2.6r_s\). Further out, the sightlines will curve around multiple times, but end up "next to" the black hole (in any direction).
This disk with a radius of \(2.6 r_s\) is called the "shadow" of the black hole.
