So it looks, "our" extremely massive black hole: the picture from the center of the Milky Way shows a dark central region that describes experts as "shadow" of the black hole, and a bright ring with lighter and darker stains made of hot plasma. The event horizon telescopic network (English: Event Horizon Telescope, short EHT) took over a period of five days. Sagittarius A*, as the object is called formally, is 26,000 light years away from us and is 4.3 million solar masses "difficult". The EHT team presented the picture today on several press conferences that take place worldwide at the same time.
However, the area in the center called the shadow does not show the black hole itself. According to the experts, it has about 2.5 times the diameter of the event horizon, the outer boundary 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 that you can also see objects directly behind the black hole. However, the light must curve the event horizon at a minimum distance, otherwise it will fall into it. This minimum distance – also referred to as the "last photon orbit" – is visible as a dark center.
Shadow and gravity monster's ring
The bright ring around the black hole is the so-called accretion disk. It consists of hot matter from the environment that falls into the black hole. Before that, it orbits the black hole, similar to water around a bathtub drain, and heats up enormously due to the friction. The lighter and darker areas show that the disk is not uniform, but contains hotter and cooler lumps. However, the image shows only a representative average: the processes in the disk take place 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 make them work like a single, huge telescope. This technique is called Very Long baseline Interferometry (VLBI). All individual telescopes observe the same goal together. They act like a virtual giant telescope that spatially resolves objects and regions that cannot be created for a single telescope. Your measurements are merged and synchronized in a complex way. In addition, several hundred individual images of the entire telescope array were required for the image of Sagittarius A*.
The Event Horizon Telescope makes the invisible visible
The telescopes observed the black hole at a wavelength of 1.3 millimeters, i.e. in the microwave range. The Event Horizon Telescope thus achieves an angular resolution of less than 20 microarcseconds. An arcsecond is the 3600th part of an angular degree. The resolution achieved with the EHT would be sufficient to be able to detect a two-euro coin on the moon from Earth. However, this also means that the image of the black hole is not a real photo, but a false-color image – the wavelength of 1.3 millimeters is not in the visible range of the electromagnetic spectrum.
The measurements on Sagittarius A* took place at the same time as the measurements for another image, with which scientists of the EHT team already caused a sensation in 2019. It was then that they published the first image of a black hole at all. However, the extremely massive black hole in the center of the galaxy Messier 87 (M87) was visible on it.
This is much further away in the constellation of Jungfrau at a distance of 55 million light years-a little more than 2000 times-than our galactic center, but it does to 6.5 billion sun masses. The blackness of the hole-the event horizon-is around 1500 times larger on site than with SGR A*. Therefore, it appears from Earth to the sky as like Sagittarius A*.
Sagittarius A*: smaller, faster - and more difficult
The data collaboration had already collected the necessary data during a multi-day measurement campaign in April 2017: all eight radio telescopes, from South Pole Telescope at the south pole to the Alma radio Observatory in Chile to the Iram telescope in Spain Both towards M87 and Sagittarius A*. Afterwards, the data evaluation for M87 was extremely complex, but it is a little better for a photo shoot than the extremely massive black hole in the center of our Milky Way.
Another difference between the two images is that the M87 shot is basically a static image. With its larger mass and dimensions, all processes in the immediate vicinity of the hole in M87 are slower than in Sagittarius A*. Therefore, the plasma moves much slower around the black hole, so that the conditions almost did not change over the observation period.
On the other hand, the structures in the Sagittarius A* disk are moving so fast that they are virtually "smeared" in the now released recording. Of course, this made the data evaluation more difficult, and therefore the image is only available today – four years after the scientists of the EHT collaboration turned their telescopes to the center of our galaxy.
