I am sure many of you have seen the images of the black hole at the centre of the Milky Way but what even is a black hole? They are incredibly dense objects in space where there is so much gravity (a great deal more heavier than what we feel on Earth) that nothing can escape its boundary. Not even light can escape – hence why they are called black holes. Their boundary is called the event horizon and it encloses all the matter that it is made of. If you were to get too close to a black hole, you would be stretched and squeezed vertically and horizontally. This is known as spaghettification – you would look like a noodle. You would also be stuck inside as in order to escape you would need to go faster than the speed of light. The image below shows an artist’s impression of the anatomy of a spinning supermassive black hole.

 

An artist’s impression of a rapidly spinning supermassive black hole surrounded by an accretion disc. Image credit https://www.eso.org/public/archives/images/screen/eso1907h.jpg

 

Black holes are usually divided into three categories due to their mass: stellar-mass, supermassive and intermediate mass. Stellar black holes occur when the core of a star collapses in upon itself. The collapse also causes a supernova to occur and if the star’s core collapse was around twenty times the mass of our Sun, it would collapse into this kind of black hole. Supermassive black holes, like the one the one in the Milky Way known as Sagittarius A*, can be thousands to billions of times bigger than the mass of our sun. Their formation is still unknown, but it’s thought that they may have formed the same time as the galaxy they are in. Intermediate black holes are ones in between the mass range of the other two.

 

Black holes are effectively invisible if we wanted to observe them with a telescope. So how are they even found if we cannot see them? We observe them based on their surroundings. They are often surrounded by gas and dust. These make up rings around the black hole known as accretion disks. This matter gradually moves from the outer edges of the disk into the inner until it falls into the event horizon. As the gas spirals into the black hole, it gets heat up to high temperatures and releases x-ray light. This can then be observed by telescopes. However, there are some cases known as isolated black holes where the matter surrounding them has all fallen into the event horizon and they hence do not have accretion disks. These are more difficult to find.

 

The first image of the supermassive black hole Sagittarius A* at the centre of our galaxy by the Event Horizon Telescope. Image credit https://eventhorizontelescope.org/sites/projects.iq.harvard.edu/files/styles/os_files_xxlarge/public/eht/files/avgimage_afmhot_us_edit.png?m=1652355847&itok=3wGKp_XG

 

The image of the first black hole was made using radio telescopes from across the world known as the Event Horizon Telescope. Then they viewed this blackhole in polarised light. The lines in this image mark the orientation of polarisation. The light is essentially has been filtered in a certain direction, similar to polarised sunglasses, which helps astronomers to sharpen their view of the region surrounding the black hole. My studying these polarisation lines, we can learn more about the magnetic field of a black hole.

 

There is still a lot we do not know about black holes and research in this area is ongoing. If you would like to read more information about black holes the following websites are very useful:

 

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