Artist's impression of a black hole -

© D. Mark / Pixabay 2020

  • Supermassive, stellar, intermediary ... according to a study published by our partner The Conversation, there are several varieties of black holes, these celestial objects popularly assimilated to "planet eaters".

  • They are said to be "black" because no light can emanate or escape from these objects!

  • The explanation of this phenomenon for the attention of the youngest was led by Yaël Nazé, astronomer at the Institute of Astrophysics and Geophysics of the University of Liège.

To define a black hole, we must first speak of

escape

 : throw a ball in the air: it falls.

Throw it harder: it will rise higher, but will fall anyway.

If it is launched with a speed of 40,000 km / h (almost 1,000 times faster than the speed of a car driving in town!), It will not, however, fall again: this is called the speed of liberation: what it takes to free yourself from the force of gravity of the Earth.

This speed depends on the mass and size of the object considered: the more massive the object, the higher the speed.

Whether to 40,000 km / h to leave Earth, do

as

8,600 km / h to exit the Moon (much lighter than the Earth) and more than 2 million km / h to leave the sun.

If you have a lot of mass concentrated in a very small radius, then the speed of release can exceed the speed of light - yet our theories tell us that nothing can go faster than light.

We then have

a black hole

 : a hole because if we fall towards such an object, there is no way to escape from it;

black because no light can come from or even escape from the object!

Where to find a black hole?

It depends on what we're talking about!

Indeed, there are several types of black holes.

First, stellar black holes: they come from the death of a massive star (several dozen times heavier than our sun) - it was its heart that collapsed.

These objects are found scattered throughout galaxies (large group of stars).

Then, at the heart of galaxies, we have

supermassive

black

holes

, with masses of a few million to a few billion times the mass of the Sun.

Their formation is still hotly debated: combination of small black holes over time or rapid direct formation?

We do not know…

Finally, between these two extremes, there could be intermediate mass black holes, but their very existence is still debated today.

Does it really exist?

Yes, we really think we have detected black holes.

  • In

    "

    cannibalistic"

    systems

    : we have to imagine a normal star accompanied by a black hole - the latter sucks up part of the matter of its companion.

    In this kind of system, we can detect from Earth the dance of the star around its companion, and given the properties that we find, it can only be a black hole.

    On the other hand, the matter, falling towards the black hole, starts to shine very brightly.

    Because of the intense gravity of the black hole: by studying the deformation, we can tell if it is a black hole and even how fast it is spinning!

  • At the heart of the Milky Way (our galaxy): we followed a small star, called S2, which revolves around the center of our Galaxy.

    It is interesting because knowing an orbit makes it possible to know the mass of the central object - mass of the Sun by observing the planets, mass of the center (which we do not see) for S2.

    We come to 4 million times the mass of the Sun concentrated in a very small area - there is only one black hole to respond to such properties.

  • Almost everywhere in the universe: If couples of stars exist, couples of corpses of stars also exist - therefore couples of stellar black holes.

    But they cannot turn around forever.

    They distort space-time (the fabric of the universe) around them so much that they gradually come closer.

    Finally, they merge and then emit a particular signal, a burst of

    gravitational waves

    which propagate in the universe and which one can receive on Earth.

  • At the heart of the galaxy we call

    M87

     : by combining the light recorded by several telescopes scattered over the surface of the Earth, we were able to build a very precise instrument and take the first photo of a black hole.

    It's this one :

Image of the supermassive black hole of the galaxy M87 / © EHT - CC BY 4.0

This is a very large black hole - 6.5 billion times the mass of the Sun - and it is

eating

its surroundings.

As with small black holes, this material that falls towards the black hole shines: it is the ring that we see in the photo.

In the center, on the other hand, it's black: that's where the black hole is!

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"

This analysis was written by Yaël Nazé, astronomer at the Institute of Astrophysics and Geophysics of the University of Liège.

The original article was published on The Conversation website.

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