Black holes are some of the most intriguing bodies of the cosmos, because they are places in space where gravity is so intense that not even light can escape from them. They usually form when a supermassive star collapses under its own weight. This phenomenon gives birth to stellar-mass black holes. However, supermassive black holes also exist. They can be billions of times more massive than the Sun and are found in the centre of most galaxies.
But, if black holes are invisible because light cannot escape from them, how can we be sure that they really exist, and that they are not a mere invention? Below we will see the two main ways through which we observe the existence of these mysterious objects.
Gravitational influence
One of the methods to detect black holes, which is currently the most used, is the one based on the gravitational influence that they have on their surroundings. For example, at the centre of the Milky Way, there is an empty area that all stars orbit around, as if there were a very dense and massive object in its centre.
By calculating the speed at which these stars orbit and the path they follow we can deduce the mass and location of the mentioned black hole.
An image of the black hole
The second method to detect black holes is observing the matter that falls in them. While the black hole attracts matter towards him, a disk forms around the black hole that can become extremely hot. A part of the energy released in this process is transformed into highly energetic light (for example X-rays) that we can observe with telescopes on Earth.
Using this technique we can obtain an image of the disk of matter that surrounds the black hole. So, we are observing in an indirect way the black hole in itself, and we can make ourselves an image of the size and form of the event horizon. This “horizon” represents the point from which it becomes impossible to escape from the gravity of the black hole.
In May 2022 was obtained the first image of the Milky Way’s black hole, Sagittarius A* (the asterisk is part of the name), using this technique. The following was the obtained image:
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