Long theorised and indirectly observed the supermassive black hole at the centre of our galaxy, Sagittarius A*, has recently been imaged for the first time!  

Why all the fuss?

Black holes are some of the strangest and most exotic objects in the Universe, and our galaxy has a supermassive one lurking in the core. We’ve been able to observe evidence of something there in the past, like this image of the Milky Way centre in radio light and this video of stars zipping around an unseen mass, but until recently “seeing” a black hole was science fiction.

Through hours of observation, and some incredible maths and computing, the team at the Event Horizon Telescope have been able to produce an image of Sagittarius A* in radio wave light, only the second ever direct image of a black hole!

Image credit: ESO/José Francisco Salgado, EHT Collaboration

Why not Sagittarius A* first?

As mentioned above, Sagittarius A*’s photo is the second image produced by the team at the EHT. The first photo ever taken was this photo of M87*, a supermassive black hole in the M87 galaxy. At over 2000 times further away, it’s tempting to wonder why Sagittarius A* wasn’t imaged first. Ultimately, it comes down to size.

Despite being 2000 times further away from us than Sagittarius A*, M87* is 1600 times larger! This means that any glowing blobs of gas spinning around M87* have to travel much further, and as such, more consistent images can be taken over a period of several nights. However, Sagittarius A* is so comparatively small that any gaseous blobs orbiting around it may appear visually distinct between nights! This means the team at EHT faced a greater challenge when trying to produce the final image of our supermassive black hole.

Image credit: EHT Collaboration

So what are we actually seeing?

The very nature of black holes, regions of space with gravity so intense that light cannot travel fast enough to escape, makes photographing the “surface” of one impossible. Instead, what we can see in the image is the shadow of Sagittarius A*. The warping of space-time around a black hole causes light to bend around it, resulting in the shadow appearing 2.5 times wider than the actual diameter of the black hole.

Swirling around the black hole, at speeds that are a significant fraction of the speed of light, is an enormous cloud of dust and gas known as an accretion disk. This disk of material is squished together under the gravitational and frictional forces, increasing the temperature to the point of releasing electromagnetic radiation, light. In the case of Sagittarius A*, the accretion disk can be seen as the glowing orange halo around the dark shadow of the black hole. 

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