Explained: What are supermassive black holes, the one which was photographed at the centre of The Milky Way

For the first time, astronomers have captured an image of the supermassive black hole or SMBH, at the centre of our galaxy. It's the first direct observation confirming the presence of the black hole, known as Sagittarius A* (pronounced as A-Star), as the heart of the Milky Way galaxy.

Black holes don't emit light, but the image shows the shadow of the black hole surrounded by a bright ring, which is actually light being bent by the gravity of the black hole. Astronomers said the black hole is 4 million times more massive than our sun and falls under the supermassive category. So what exactly are supermassive black holes?

Black holes fall into 4 basic categories: stellar, intermediate, supermassive, and miniature.- Of these, intermediate and miniature black holes have only been hypothesised about. 

As stars reach the end end of their lives, they tend to inflate, lose mass and then all of a sudden cold down to form white dwarfs. A stellar black hole is formed when a huge star burns up all its fuel and collapses in on itself. These stars are often more than 10-20 times bigger than our Sun. 

Stellar black holes grow by taking control of other stars or merging with other black holes and becoming a supermassive black hole. Basically, when a black hole consumes multiple stars or many smaller black holes. As stated, a stellar black hole has a mass of about 10-20 times of our sun. A supermassive black hole can have masses equal to billions of suns, so the one that is at the centre of the Milky Way is a fairly small one.

Usually, supermassive black holes are found at the centre of a galaxy. It is postulated that most galaxies have at least one supermassive black hole at their centre. If there are more than one supermassive black hole, they likely collapse into one another and form a bigger supermassive black hole. 

No matter their starting size, black holes can grow throughout their lives, slurping gas and dust from any objects that creep too close, so it is practically impossible to determine what is the exact mass of a particular supermassive black hole.

Although humans cannot see black holes even with the best and most powerful telescopes, what we can do is observe the effect a black hole has on light rays. When normal stars are sucked in by black holes, they accelerate and heat up, emitting x-rays which are detected by x-ray enabled space telescopes. This is how we came to photograph Sagittarius A*, or any other black hole for that matter.

Because it is stipulated that almost all galaxies have at least one supermassive black hole, it is stipulated that there must be at least 100 billion supermassive black holes in just our corner of the universe. Of these billions, we have been only able to directly image just two including the Sagittarius A* of the Milky Way.

The first one was the black hole at the centre of the giant elliptical galaxy, Messier 87, a SMBH that is at least 1000 times larger in mass than Sagittarius A*.

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