How do we know that black holes exist?

If you have heard only one thing about the black holes, it is likely that within the horizon of the black hole events, nothing, even the light can not escape. At this point it is natural to ask: if nothing can escape from a black hole, how can we ever notice one? How do we know there is a black hole? (video included)

Well, just things in the horizon of events are blocked. The black holes also attract objects outside the horizons of the events, and looking at those objects we can form a very good idea that there is a black hole nearby.

For example, a lot of stars are orbiting in pairs, but we also see stars that orbit cosmic objects that are not ordinary stars, but which in turn emit huge amounts of X-rays, and X-rays in space often come from cosmic dust and gas they overheat as they head toward a very dense object.

However, by finding out the mass and the orbital features of stars whose partners emit X-rays, we can determine the mass of those partners. Some are light enough to be neutron stars; and only the neutron stars can get so great before they collapse; calculations show that the largest are at most 2-3 times the sun's mass.

Yet there are a lot of stars whose orbits make it clear that their X-ray partners are 5-10 times larger than the sun's mass, and we just do not know what else could be than black holes.

Sometimes, there is no need for a star that orbits; only X rays and radio waves from the hot material that collapses can be used to determine the mass of a solitary object that is not a star.

In some cases, these objects prove to be neutron stars, but in others they prove to be too heavy and can be black holes.

There are also objects in the centers of some galaxies (including the center of our galaxy) that emit an enormous amount of X-rays, where radio and infrared radiation, but not much visible light, and we know that these objects are very heavy because of the way in which the nearby stars and heated dust are orbiting them.

These orbits tell us that the objects are so heavy and so small that they can not be stars or a group of stars or a cluster of invisible matter; they can be nothing but supermassive black holes.

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For example, in the middle of the Milky Way there is an object called Sagittarius A * that emits X-rays, where radio and infrared radiation. The stars near this cosmic object have small and fast orbits around it. Sagittarius A * weighs 4 million times more than our Sun!

And finally, we also directly observed, on several occasions, where the gravitational forces that were generated as a result of the collision between two very dense objects.

Some of these have the signature of a collision between objects that are light enough to be neutron stars. But other gravitational stars can only originate the collisions between objects too heavy to be anything other than pairs of black holes that merge, becoming a single gigantic black hole.

And in these cases, the characteristics of the gravitational waves show exactly what indicates all the theoretical calculations that they should look after the collision between the black holes.

So, in many different places in the universe, we detected very dense objects using the gravitational force exerted by them; either indirectly through effects on nearby matter, such as gas and dust discs, either directly through gravitational waves.

Many of these enormously enormous objects are too "black" to be ordinary stars, too compact to be clusters of stars and too heavy to be neutron stars.

So these objects exist, behave just as physics predicts that black holes behave and literally there is nothing else but black holes.

To quote an astronomer: "We have a strong belief that black holes, or at least objects that have many of the features of black holes, exist."

In other words, if an object looks like a black hole and behaves like a black hole, we call it a black hole.

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