The stars in our galaxy “orbit” the central black hole? Not really

Since Thursday, May 12, 2022, and the publication of the 1st image of the supermassive black hole at the center of our galaxy, much has been communicated about this strange and mysterious object, whose existence we suspected for a long time time, but which we had never “seen” before. But what is its influence on the billions of stars that surround it? Does its effect extend to our entire galaxy?

Let’s start with introductions. This supermassive black hole that we finally have a picture of is called SgrA*. This name was given to it because it is in the constellation Sagittarius (hence “Sgr”), in a place that emits a lot of radio waves (called “A”), and in this area we see this black hole as a point that emits even more radiation than the rest (hence the star * meaning “point source”).

The production of the now famous image of SgrA* required colossal resources. This huge star is so far from us (about 26,000 light years) that it was necessary to build a network of radio telescopes, distributed all over the planet (in Europe, Greenland, North and South America, in the Pacific Ocean and Antarctica) to reconstruct, from all the signals picked up, this picture, which, however blurred it may seem, is today the most precise and finest ever obtained.

Source: NASA

Although the concept of the black hole has been repeatedly discussed with the general public, both in pop culture and by your favorite science popularizers, this astronomical object challenges our worldview so much that it often creates misunderstandings, including one in particular.

Do the stars really revolve around the black hole?

When we talk about supermassive black holes, of which almost all galaxies have a copy at their heart, it is common to hear that the stars that make up these galaxies orbit them. In a way, it would be thanks to this black hole that the galaxy surrounding it holds together. But is it really true?

Take the example of our galaxy, the Milky Way. It actually turns on itself, and all the stars it contains make 1 revolution in about 250 million years. What we also know is that SgrA* has a mass of about 4 million times the mass of the Sun. This nevertheless classifies it in the category of “small” supermassive black holes. By comparison, the black hole at the center of the galaxy M87, of which we had the first image in 2019, has a mass of more than 6 billion solar masses.

Source: NASA, JPL-Caltech, IPAC, Event Horizon
Source: NASA, JPL-Caltech, IPAC, Event Horizon

But in the universe, gravity tends to make the least massive objects revolve around the most massive objects (the Moon around the Earth, the Earth around the Sun, the Sun in the Galaxy, etc.). We naturally come to assume that if all the stars in our galaxy rotate, it is thanks to the gravity of our supermassive black hole, which attracts all these stars and maintains the integrity of the Milky Way (much like the Sun with the planets in the solar system). But this idea is wrong.

The difference between spinning and circuit is very important

To explain why, it is first of all necessary to distinguish between two terms which seem almost synonymous, but are very different in reality.

  • “To orbit” means to move around an object whose gravity holds you around it. There is a cause, a physical connection, between the central object and the object orbiting it. This link is gravity.
  • Conversely, we can completely “flip” something without gravity binding us to it. For example, if I decide to run around a tree, I “turn around” it. But I am not physically tied to this tree. I do not “orbit” it around it, in the sense that it is not the tree that holds me around it. If the tree disappeared immediately, I could still walk in circles all I wanted, it wouldn’t change anything.

This distinction is very important.

The misconception that the stars of the Milky Way revolve around its central black hole is perhaps reinforced by the fact that it is a so-called “supermassive” black hole. When it comes to its mass, the numbers are staggering. Millions of times the Sun! It is then tempting to assume that such a large and massive object can only have a great influence on the things around it in the universe.

And there are actually a few rare stars that actually “orbit” SgrA*. These are suns that are captured by the black hole’s gravity and whose orbit brings them more or less close to it, depending on where they are in their orbits.

Source: Eisenhauer et al.
Source: Eisenhauer et al.

Below you can see this timelapse, the result of 18 years of observing the neighborhood SgrA*, around which you can see these famous stars orbit around it under the influence of its gravity.

Source: Andrea Ghez et al.
Source: Andrea Ghez et al.

About fifteen stars are therefore in orbit around it, but no more. The reason is that the mass of SgrA*, as large as it is compared to us (or to the Solar System), remains ridiculously small compared to an object as large as our entire galaxy, which nevertheless spans 100,000 light years and contains several hundred billion stars.

Source: NASA, JPL-Caltech
Source: NASA, JPL-Caltech

Why do so few stars orbit a black hole?

If there are no more stars orbiting the black hole, it is because of the properties of gravity. In fact, what we’ve known for centuries is that it decays as the inverse of the square of the distance. This means that if you are 3 times further away, the perceived gravity will be 3²=9 times weaker. If you are 10 times longer, it will be 10² = 100 times lower. If you are 100 times longer, it will be 100² = 10,000 times lower.

In other words, even if its mass were large, the black hole’s gravity quickly becomes so weak that it is no longer able to hold any other body around it. As for SgrA*, its effect gradually becomes so weak beyond a distance of a few light years that it can quickly be considered negligible. After a few light years, you hardly feel its attraction.

But our galaxy is much bigger. We said it above: on the order of 100,000 light years in diameter. Our central supermassive black hole cannot extend its grip to the other end of our galaxy. It is not responsible for the rotational motion of the stars that make up the Milky Way.

We could summarize all this by saying that the stars of the Milky Way trip well around SgrA*, but that they don’torbit not around him. In fact, if it wasn’t there, they would still continue to orbit together around the center of our galaxy without much change.

But then, if the stars in the Milky Way are not affected by the gravity of the central supermassive black hole, why do they spin? What force binds them together? This force is always gravity. But the cumulative gravity of all 250 billion stars in our galaxy. This global mass is much larger than SgrA*. It is she who generates the enormous well of gravitational potential in which all these stars are trapped (and spinning).

In conclusion, to say that the stars in our galaxy “orbit” the central black hole is very misleading, because it would imply that the gravity of this black hole keeps them around it. Which, as we have seen, is far from true.

What holds the Milky Way together is the mass of its stars, but also of all the planets orbiting them, asteroids, gas, dust and all the matter it contains. And for the accounting to be fair, we must also add something 10 times heavier than all the visible matter in our galaxy, whose mass is much higher than anything we see, but which scientists are still trying today to lock up for the secrets: dark matter.

But that will be another story…

Look at the world from space

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