So this is what »our« extremely massive black hole looks like: The image from the center of the Milky Way shows a dark central region, which experts call the »shadow« of the black hole, and a glowing ring with brighter and darker spots of hot plasma. It was recorded by the Event Horizon Telescope Network (EHT for short) over a period of five days. Sagittarius A*, as the object is formally called, is 26,000 light-years from us and has a “mass” of 4.3 million solar masses. The EHT team presented the image today at several simultaneous press conferences around the world.
However, the area in the center called the shadow does not show the black hole itself. According to the experts, it is about 2.5 times the diameter of the event horizon, the outer limit of the black hole. Rather, the dark zone is created by the enormous force of gravity. The light rays follow curved paths around the black hole, so objects directly behind the black hole can also be seen. However, the light must curve around the event horizon by a minimum distance or it will fall into it. This minimum distance – also known as the “last photon orbit” – is visible as a dark center.
Shadow and Ring of the Gravity Monster
The bright ring around the black hole is what is known as the accretion disk. It consists of hot matter from the environment falling into the black hole. Before that, it orbits the black hole, similar to water around a bathtub drain, and heats up enormously due to friction. The lighter and darker areas show that the disk is not uniform but contains hotter and cooler clumps. However, the image only shows a representative mean value: the processes in the disc are running faster than the observation period.
In order to be able to photograph Sagittarius A*, the researchers had to connect eight telescopes around the world and let them work as a single, huge telescope. This technique is called Very Long Baseline Interferometry (VLBI). All individual telescopes observe the same target together. They act like a virtual giant telescope, spatially resolving objects and regions that cannot be achieved with a single telescope. Your measurements are merged and synchronized in a complex manner. For the image of Sagittarius A*, a total of several hundred individual images of the entire telescope network were also required.