Black holes have been an object of fascination ever since Einstein predicted them in 1916 using his then newly developed theory of general relativity. We have come a long way since then. Kip Thorne calculated the now famous image of how a black hole would look like, and an international team of astronomers managed to constrain the mass of our galaxy’s central black hole by means of tracking the orbits of stars that rotate around it. In 2019, we managed to image the first black hole using the event horizon telescope, and now we have finally imaged Sagittarius A*, the black hole in the center of our galaxy.
How do scientists observe black holes?
You might wonder how this works, given that not even light can escape a black hole, meaning that pictures of it should be impossible. This is correct, but similar to tracking the orbits of stars around it, we infer information about the black hole by observing the distribution of gas behind it. In fact, the orange gas that we see around the black hole is not actually in that shape, it is a distorted image due to the gravitational effects of the black hole on the surrounding light. This process, called gravitational lensing, can teach us a lot about the characteristics of the black hole.
The observations are made by a coalition of radio telescopes called the Event Horizon Telescope. Together they span a baseline that is roughly equal to Earth’s diameter, combining this data in order to get the highest possible resolution. This data is then processed with a supercomputer, giving the resulting image. The processing power needed to work through all the data can be compared to running 2000 modern laptops at full speed for a year.
Time series analysis of black holes
The image helps us constrain the mass of the black hole, and once more of them are made, it will be possible to create an animated timeseries that shows how matters falls into the black hole over time. However, to create such a video we will need many years of observations, simply because things move so slowly. Sagittarius A* is also a much more representative black hole compared to M87*, which was imaged three years before. The latter being about 2000 times bigger than the former, and relatively rare in the visible universe.
Now that we have two images of black holes it is safe to say that the methods work, and things should only get easier as we get better and better equipment and more and more experience in making these images.