Sagittarius A – as imaged by the team at the Event Horizon Telescope.
Imagine taking over 4 million copies of our Sun and cramming the combined mass into a region of space no bigger than the orbit of Mercury.
That’s Sagittarius A, the supermassive black hole at the centre of our own Milky Way galaxy. Evidence for Sagittarius A has been growing since the 1970s but now in 2022 the team at the Event Horizon Telescope have actually imaged it.
The term ‘supermassive’ when attributed to black holes is very misleading as black holes are incredibly low volume but dense regions of space. To give you a feel for this, if you took our Moon and somehow compressed it into a black hole, the resulting anomaly would have a diameter of 0.2 millimetres! That’s probably less than the size of a single pixel on the screen you’re reading this article on.
As black holes grow they can devour more mass and will slowly get bigger with the event horizon radius r defined by the famous Schwarzschild equation:
r = 2GM/c^2
In this equation G is the universal gravitational constant, c is the speed of light and M is the mass of the black hole. This is a simple linear relationship, so for example doubling the mass of a black hole will double its radius.
The bright central region of our Milky Way galaxy where Sagittarius A is located. Telescopes of the Atacama Large Millimeter/submillimeter Array in the foreground. Credit: EHT Collaboration
Given the relatively small volumes and areas of space involved, detecting even the most massive black holes in challenging to say the least. Sagittarius A, despite containing millions of solar masses, occupies a volume smaller than a single star in its giant phase of evolution. This gets compounded by the incredible distances involved. The centre of our Milky Way where Sagittarius A is located is a staggering 26,000 light years away. How then did the team capture the image?
The key was using multiple detectors spread across the planet, effectively constructing an Earth sized telescope. The data collected from these widely spaced arrays was then gathered together, producing many terabytes of data, and processed by banks of supercomputers called ‘correlators’. The final image was constructed using advanced algorithmic and statistical imaging techniques.
Clearly by their nature black holes do not allow any light to escape so what we see in the final image is the infrared signature of super-heated gas rotating close to the black hole. Black holes therefore reveal themselves by their indirect influence on nearby objects rather than direct observation.
Indeed, Sagittarius A’s existence was originally inferred by its influence on nearby stars, which are being thrown about at fantastically high speeds due to its intense gravitational influence. Fast enough for us to produce time lapse images over several years (see animation below).