Black holes are objects that have long been theorised by scientists for centuries. Just as you have to travel faster to leave Earth than you to do to leave the Moon, it was theorised that such places could exist where you would have to travel faster than light to escape them. Early descriptions of such objects were called ‘dark stars’; a star so large and massive that its own gravitational pull would pull in the light it should be emitting. Albert Einstein made this idea popular in his theory of general relativity, which includes equations to measure such strong gravitational pulls. The object in question would have an edge known as the event horizon, a point at which you can never return if you cross it. Even light, the fastest thing in the universe, would not be able to escape. The problem with ever seeing a black hole is exactly that, they are black. They do not emit or reflect any type of light. The closest known black holes are also very far away and appear tiny in the sky due to their distance from us. How could astronomers possible see an object that was virtually unseeable?
Scientists have been searching for a supposed black hole lying in the centre of our own galaxy known as Sagittarius A* (or Sgr A*) for over 20 years. Using powerful telescopes on Earth, they observed star movement near its centre over a 16-year period. The results showed that the stars were being whipped around in highly elliptical orbits at extremely fast speeds, all orbiting an invisible point. How could these stars possibly be orbiting nothing? This was some of our first indirect evidence for the existence of black holes. Such a black hole in our galaxy would have as much mass as roughly 4 million Suns yet would fit inside the orbit of Mercury. These telescopes were not powerful enough to resolve such a small and distant object.
A super massive black hole (originally named as M87*) has long been theorised to be lying in the centre of another galaxy known as Messier 87. This black hole was calculated to be far larger than the one in our own Milky Way and we’ve observed large jets of plasma being ejected from the core at almost-light speeds. Sgr A* and M87* both became targets of an ingenious network of telescopes that became known as the Event Horizon Telescope (EHT).
No single telescope on Earth is powerful enough to observe either of these black holes alone. The EHT solved this problem by linking up eight different radio telescopes around the world to create a virtually Earth-sized telescope dish. Scientists pointed this network of telescopes at both Sgr A* and M87* back in 2017, observing both for several weeks at a time. The EHT collected so much data that it could not be uploaded and synced over the internet. It had to be shipped around the world using physical hard drives so that it could be processed using supercomputers. The result was the first ever image of a black hole. The image released in April of 2019 shows the super-massive black hole lying in the centre of Messier 87. This super massive blackhole was given an official name; Powehi (pronounced po-ve-hee). This name comes from Hawaiian creation legend, meaning ‘adorned fathomless dark creation’. It is an acknowledgement to the Hawaiian observatory that contributed to the EHT network.
So, what exactly am I looking at? Until this image was released, all black holes that you may have seen are artistic representations. They are renderings of what we thought a black hole would look like from using mathematical models and physics. You may remember seeing the incredible imagery of a black hole in the film Interstellar. The image of Powehi may not look as spectacular as the one in interstellar, but they are very similar. What you are seeing is a ring of matter, rapidly being spun around the black hole while being superheated due to friction. This is known as an accretion disk and its what allowed us to see the silhouette of Powehi. This ring of light is warped around the black hole due to the incredible forces of gravity bending light rays. The brighter spots are matter that is moving towards us, while dimmer regions are moving away. Powehi was chosen to observe because it is so large spanning over 38 billion km. Just to give you an idea of size, Pluto is around 6 billion km away. This black hole would engulf our entire Solar System with room to spare. No need to worry though, as Powehi lives almost 53 million light years away! Results from the observation of Sag A* in our own galaxy are expected in the not too distant future, but even that black hole is roughly 26,000 light years away.
Dr. Katie Bouman of MIT led the team who developed the imaging algorithm that made imaging Powehi possible. The incredible results of the EHT are a testament to the importance of scientific collaboration and transparency. No single organisation or group would have been able to take this incredible image alone. Hundreds of people from dozens of countries worked in unison to achieve something that was once thought impossible through science.
Josh Kirkley, Astronomy Educator
L: The first ever image of a black hole, showing the super massive black hole that lies in the centre of the galaxy Messier 87.
R: A wide view of the region around the super massive black hole, taken by the Chandra telescope.