What is a black hole and why it is important that they have photographed ours

Photograph of the black hole Sagittarius A*HARVARD

It has been a long time coming, but we already have the first image of the supermassive black hole that inhabits the center of our own galaxy: Sagittarius A*.

Radiofrequency image of the supermassive black hole Sagittarius A* (upper right corner) located in the center of the Milky Way /NASA

It is a monster with a mass equivalent to four million suns located 26,000 light years from our planet. With a diameter 17 times that of our Sun, it is, however, a thousand times smaller than the supermassive black hole in the galaxy M87, the first to be imaged in 2019.

Since black holes do not emit light, the dark area in the center of the image corresponds to the shadow of Sagittarius A*, the region where light cannot escape (bounded by the event horizon).

The bright region around the black hole is the radio frequency signal emitted by hot gas orbiting the black hole at speeds close to the speed of light.
This new feat has been carried out by the same world consortium of radio telescopes (called EHT for its acronym in English, Event Horizon Telescope) that photographed the supermassive black hole in M87. To date, it is made up of eleven perfectly synchronized radio observatories to detect radio frequencies associated with emissions from black holes.

This novel photograph of Sagittarius A* is the result of the composition of numerous images of this object (after thousands of hours of observation) and processed using advanced computational technology.

The upper image of Sagittarius A* has been obtained by averaging the four lower images. https://iopscience.iop.org/article/10.3847/2041-8213/ac6429

In other words, it is the best image of the black hole in the center of the Milky Way that fits the data collected by the EHT (within the laws established by Einstein’s Theory of General Relativity).
Regarding the bright spots in the Sagittarius A* image, these are due to an apparent increase in emission when the radiation is pointed directly at us (known as the Doppler effect for electromagnetic waves).
But, before addressing why this new discovery has been so relevant, let’s now see how a black hole forms and how they can reach the size of our galactic companion, Sagittarius A*.

A black hole is an astronomical object with an exceptional gravitational attraction. Not even light can escape from it.
Stellar-type black holes form at the end of the life of a massive star. When it runs out of fuel, its core (which is where the thermonuclear reactions that generate its energy take place) collapses in on itself, compressing the dead star into a region of zero size and infinite density: the singularity.
Other lower mass stars will not form a black hole at the end of their lives, evolving into white dwarfs or neutron stars.

The singularity is the center of the black hole and is hidden by the surface that makes up the event horizon (whose approximate radius, for static black holes, is called the Schwarzchild radius).
However, not only black holes can be formed by stellar evolution. British physicist Stephen Hawking proposed the existence of so-called primordial black holes.

Recreation of an atomic-sized primordial black hole

Created in the first moments of the Big Bang, these objects can have a mass less than that of an asteroid. These tiny black holes (unlike their more massive companions) lose mass due to a phenomenon called Hawking radiation and eventually disappear.
Some theories suggest that they were formed by a slow absorption of matter from a star-sized black hole. However, this process is extremely slow to build a giant like Sagittarius A* in a relatively short time.
Other alternative theories suggest that these supermassive black holes originated from the gravitational collapse of huge amounts of interstellar gas (as well as swallowing the smaller stars around them).
The following animation describes in detail this process of absorption of a star by a black hole.

Returning to our supermassive black hole Sagittarius A*, perhaps one of the most significant aspects of this finding is precisely its proximity to us, as it is in our own galaxy: the Milky Way.

Although its existence has been known since the end of the last century (due to the powerful gravitational attraction it exerts on surrounding stars), it has not been easy to obtain an image of it.
Among other reasons for its position in the center of our galaxy (possibly located in an environment with a high concentration of interstellar dust and gas) as well as its small size (compared to M87).

Comparison between the supermassive black holes M87 (left) and Sagittarius A* (right)