– January 30, 2020 – Science
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- What is a black hole?
Black holes are among the most impressive and fascinating objects in the universe. Scientists are trying to unravel their mysteries, even though they are invisible and often unpredictable. What’s a black hole made of?
A black hole was imaged for the very first time in 2019. This breakthrough, awaited by scientists, was particularly well-publicized. The ring of fire that stands out against the black background of this first photo reminded us how fascinating and mysterious black holes are, and how much astronomers still have to learn about them.
But what exactly is a black hole? What are the different types of black holes known? Can a black hole die? Is the Earth in danger of crossing a black hole? Here’s what you need to know about these unpredictable astronomical objects.
Although its name might suggest it, a black hole is not a place in space that would be empty. On the contrary, a black hole is a region of space where a very large amount of matter accumulates. To imagine a black hole, we can for example imagine the mass of a star, 10 times more massive than our own Sun, which would be contained in a sphere the diameter of the city of New York, says NASA.
A black hole has a gravitational field so intense that no matter entering it can escape, including light. That’s why black holes are optically invisible objects. Material caught in a black hole, on the other hand, can be detected because it is heated to very high temperatures. This material forms an accretion disc, which looks like a luminous halo on the only black hole photo ever made. The accretion disc rotates (but the black hole itself does not necessarily rotate).
- Read: The first black hole in the photo emits a jet that almost defies the speed of light.
A black hole, artist’s view // Source: YouTube Unveiled screenshot
Black holes are usually distinguished from each other by their mass.
- Stellar black holes: they generally represent between 10 and 24 times the mass of the Sun.
- Supermassive black holes: these are giant black holes whose mass is at least a million times that of the Sun. They can be even more imposing: a black hole of 40 billion solar masses has been measured. M87*, the black hole that was photographed, is also a supermassive black hole.
- Intermediate black holes: as their name indicates, they are halfway between stellar black holes and supermassive black holes. Their mass would be a few thousand times that of the Sun. Their existence has yet to be irrefutably proven: scientists have proposed to use the “symphony” of their waves to try to detect them.
It is accepted that stellar black holes are born when a star collapses on itself. This star must be massive enough to form a black hole: our Sun could not become one, for example. To form a stellar black hole, the residual core left by the star must be greater than three solar masses.
- Read: An “impossible” black hole in the Milky Way? New questions surround the discovery
The formation of supermassive black holes is the subject of scientific debate. It can be assumed that the formation of these giant black holes takes place on long time scales, given their size. Yet we know that supermassive black holes were already present in the still young universe. These objects could not form by star accretion (by “swallowing” star matter, which allows them to grow).
The formation of intermediate black holes would be explained by a chain reaction: a collision of stars located in very dense clusters, which would lead to an accumulation of very massive stars. Their collapse would give rise to intermediate-mass black holes. The fusion of intermediate black holes could even give rise to supermassive black holes.
- Read: A new category of small hidden black holes may have been discovered
Even if it is strongly discouraged to visit a black hole (NASA recalled it in a much too cute video), one can wonder what is inside these strange objects. To know this, one would have to be able to cross the “boundary” of the black hole (in the geometric sense, because it is not a membrane) which is called the event horizon. This is precisely the point of no return beyond which no element entering the black hole can emerge.
If you could fall into a black hole and see what’s there, what would happen? You probably wouldn’t have time to see much because you would be stretched like a spaghetti (scientists call it spaghettification). Since nothing escapes a black hole, it is not possible to know what is at the centre of this object, in what is called its singularity.
Stephen Hawking highlighted a paradox: black holes are not totally black holes because they emit particles and can evaporate, until they disappear completely. This phenomenon occurs in the form of radiation, called Hawking radiation.
Because of this radiation, black holes have a limited lifespan. Nevertheless, for some black holes, the time it would take for them to completely evaporate is more important than the age of the universe.
The fate of stars caught in black holes (or that of that lucky star that was expelled from the black hole in the Milky Way) may raise fears that one day our planet will cross the path of a black hole. Yet the danger is elsewhere.
- Read: What would happen if the Earth was sucked into a black hole?
If anything should frighten us, it is not the encounter with a black hole, but the encounter with another star. It is much more likely to cross the path of a star (the most common star in our galaxy) than that of a black hole.