Sagittarius A*, pronounced “Sagittarius A star”, and abbreviated Sgr A*, is a supermassive black hole that lies in the centre of our galaxy, the Milky Way.
It is a very bright compact source of radio waves. For a long time, scientists suspected it was a black hole, but this couldn’t be confirmed with total certainty until the 12th of May 2022, when the first image of the black hole at the centre of our galaxy was obtained.
With our current knowledge, we know that stellar-mass and intermediate-mass black holes form when a supermassive star collapses under its own weight, leaving behind it a black hole. However, the mechanism that forms supermassive black holes isn’t still clear. One of the theories is that they start as intermediate-mass black holes, and, by absorbing gas and dust from their surroundings, and even other black holes, they reach a gigantic size after billions of years.
Size and mass of Sagittarius A*
The most exact value we have of the mass of Sagittarius A* is 4.3 million the mass of the Sun. It was calculated by the astronomers Reinhard Genzel and Andrea Ghez using the data obtained during 16 years of observations of the orbit of a star around the black hole.
It is estimated that the diameter of Sagittarius A* is about 23.5 million kilometres. However, this isn’t much compared to the diameter of the Milky Way, which is about 100,000 light years.
The observations of Sagittarius A*
This supermassive black hole represents the centre of the Milky Way, and all that belongs to the galaxy orbits around it. The Earth is situated 26.000 light years away from Sagittarius A*.
In fact, it is impossible to see a black hole, because it has such intense gravity that, once the event horizon is crossed, not even light can escape.
So, the only way to be able to “see” a black hole is by observing the disk of dust and gas that surrounds it. This disk is named the accretion disk, and it has a temperature of millions of degrees Celsius. Additionally, the presence of a back hole can also be deduced by observing the gravitational influence it has on the bodies that surround it, for example stars.
This last technique has been of special utility, because between the Earth and Sagittarius A* is a dense dust cloud that prevents us from clearly observing the supermassive black hole.
The first image
On the 12th of May 2022 appeared the first image ever obtained of the black hole at the centre of the Milky Way.
Obtaining an image of a black hole isn’t an easy task. To get it, the Event Horizon Telescope (EHT) was used, which is a global network of telescopes that act like one giant telescope.
The following image shows Sagittarius A*. The red and yellow part is the light emitted by the accretion disk, which acquires an extreme temperature as the gas and dust hurtle towards the centre of the black hole. On the other hand, the dark region at the centre is the vent horizon, from which not even light can escape.