Image of Sagittarius A*. Credits: Event Horizon Telescope.
What is a black hole ?
A black hole is a place in space where matter has been squeezed so tightly together that its gravitational attraction prevents anything from getting out, not even light, which as we know it, usually happens in the case of dying stars. While the star is alive, the nuclear fusion occurring inside its core manages to balance out its gravitational attraction - preventing it from collapsing. At the final stages of their lives, most stars such as the Sun will expand into Red Giants, lose mass, and calmly turn into White Dwarfs.
In the case of larger stars whose masses reach up to 10-20 times the mass of our Sun, as the core begins to collapse, shockwaves weave through the stellar core, eventually causing a massive explosion called a supernova. In the remnant of the supernova, no forces are left to counteract the force of gravity, causing the stellar core to collapse on itself into an infinitely small point, creating a black hole.
Artist rendition of a black hole. Credits: NASA
However, Sagittarius A* is no ordinary black hole. It is what we call a Supermassive black hole, something scientist believe can be found at the centre of each galaxy. In the 1980's, scientists came to the conclusion that a black hole as heavy as 4 million times the mass of our Sun was located at the centre of our galaxy. Although this black hole is unusually massive compared to stellar black holes that occurs when ordinary stars die, it is of ordinary size and mass when comparing to other supermassive black holes.
If Black holes don't emit light, how can we see them ?
As one can imagine, it is very difficult to see a black hole, as they do not emit any detectable type of light. Despite this, astronomers are able to detect them by measuring the visible light, radio wave and X-rays emitted by the material surrounding the black hole.
X-rays and radio waves are just electromagnetic radiation at other wavelengths than visible light. Although everything we see is in visible light, this is only a small part of the electromagnetic spectrum. In this article, we previously discussed how the James Webb Space Telescope will observe the universe in infrared light, and how much more it will see compared to ordinary visible light telescopes.
The electromagnetic spectrum. Credits: BYJU'S
Therefore, since scientists are not actually able to image the black hole itself but rather the material that surrounds it, the images show how a body with such high gravity affects light and matter. In fact, when looking at the black hole images produced by the ETH (Sagittarius A* and M87*), some of the light visible is actually behind the black hole but the light has been bent by the body's extreme gravitational pull.
Images of the M87* and Sagittarius A* black holes taken by Event Horizon Telescope. Credits: ESO
Looking at the two images of the supermassive black holes above, both images are of the same resolution despite the M87* black hole lying a mere 55 million light years away compared to 25640 light years for Sagittarius A*. The reason for this is the size difference. While the mass of Sagittarius A* sits around 4 million times the mass of our Sun, M87* is measured around 2400 billion solar masses. This image comparison shows that there is nothing extraordinary with our galaxy's black hole, and who knows how big and heavy black holes can really get ?
What is the Event Horizon Telescope ?
The image of the Milky’s Way’s black hole, Sagittarius A*, with the Atacama Large Millimeter/submillimeter Array (ALMA). Credits: ESO/Jos
The Event Horizon Telescope (EHT) is a large global network of radio telescopes consisting of 9 observatories at 7 sites. By allowing each telescope to observe a celestial object simultaneously, the ETH is able to virtually act as a planet-size dish, giving it the power and resolution to observe an object millions of light years away.
One of the complicated aspects of operating such a large network of telescopes is the weather. "A big challenge is just running when the weather is good enough at all of our sites to be able to actually see the source and take data,” said Lindy Blackburn, a member of the EHT collaboration and astrophysicist at the Center for Astrophysics | Harvard & Smithsonian. “So there's a large coordination effort to try to find the night when the weather's pretty decent, and we have a good shot at running the campaign.”.
Location of ETH telescopes. Credits: ESO/O. Furtak