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Seeing the black hole at the centre of our galaxy

For the first time, we have an image of the supermassive black hole at the centre of the galaxy. This is a great accomplishment for the scientific community and will open the door to a new understanding of black holes.

You might remember that we had the first image of a black hole a few years ago. What is exciting about this new image? Since they look very similar, what does that tell us about black holes? And what exactly are we seeing here, since the image simply looks like a fuzzy orange ring?

The picture on the left is the supermassive black hole at the centre of the galaxy Messier 87 (or simply M87), located 55 million light-years away from us. This image was released in 2019 and it was humanity's first look at a black hole. On the right, we can see the black hole at the centre of our own galaxy and it's much closer: "only" 27,000 light-years away. For historic reasons based on how it was first detected, the black hole in our galaxy is called Sagittarius A*, or Sgr A* (pronounced Sagittarius A star - which is very confusing since it's a black hole, not a star!)

What do these pictures really represent?

These pictures are radio images: the signal was picked up by radio telescopes. The colours are therefore false - scientists have simply decided to use an orange hue - but the brightness represents the intensity of the signal (the brighter it is, the stronger the radio signal). Astronomers use radio telescopes to study many objects in the universe. It’s a common misconception that we pick up sound with radio telescopes, but radio waves are not sound waves. Radio waves are a type of light with different wavelengths, making them invisible to the eye.

So what are we seeing exactly? The radio waves that form the “doughnut” were diverted by the presence of the black hole in the centre. The black region in the middle, therefore, represents where the black hole is located, although the black centre on the image is larger than the black hole itself.

These black holes are extremely large - we call them supermassive black holes! Sgr A* has a mass of about 4 million suns and would extend all the way to the orbit of Mercury if it replaced the Sun. The black hole at the centre of the galaxy M87 is even more impressive: about 1000 times more massive and it would encompass the whole Solar system! Click on the image above to see the comparison.

What can we learn from this new image?

We had the chance to talk to Professor Julie Hlavacek-Larrondo from the University of Montreal to get the details on what this new image means for her research.

What are black holes?

Black holes are objects with extreme density: a lot of mass in very little space. Because of this, they have such intense gravity that nothing can escape, not even light. That’s why we call them black holes, and why we can’t see them without this new method.

Most galaxies seem to have extremely massive black holes in their centre. We call them supermassive black holes. These have masses equivalent to millions or billions times the mass of the Sun. There are other types of black holes, which are much less massive: only the mass of a few Suns. These are created when stars die. They are also very interesting, but the focus here is supermassive black holes since they're the ones we can view!

How were these images taken?

Taking a picture of a black hole is extremely complex! We have known about the existence of black holes since Einstein first predicted them over 100 years ago, but all observations so far were indirect: we couldn’t see them but we could see the effects of their presence thanks to their immense gravity. For example, we could see stars in very tight orbits around nothing. We could then conclude that this “nothing” was a black hole.

Obtaining the images of the black hole was quite a challenge in itself! It took 8 radio telescopes, spread over 4 continents to succeed in getting it. By combining the light (radio waves in this case) from each of the radio telescopes, scientists manage to simulate an antenna as wide as the Earth! This allows us to obtain the precision needed to photograph the black hole; because it is quite far, it would appear very small in our sky. Combining light with radio telescopes is a process that requires expertise in science, mathematics, and programming. Imaging a black hole is really a celebration of human ingenuity and what we can accomplish when we all work together! Over 200 scientists collaborated on this project, including many Canadians.

To Go Further

The black hole itself is infinitely small, and we call it a singularity. However, we often talk about the size of the black hole by what scientists call the event horizon. This is the point of no return: anything that crosses the event horizon is forever a prisoner of the black hole. In the black hole photo, the black region in the centre, called the shadow, is about 2.5 times larger than the black hole’s event horizon.



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