At a press conference in Munich, which was held simultaneously at six other locations on earth, the European Southern Observatory, together with the "Event Horizon Telescope" project, presented an epoch-making result in astronomical research. It is the first image of the black hole that sits at the center of our Milky Way. This is the first time mankind has seen this mysterious object, scientifically named Sagittarius A*.
The earth, our entire solar system and all the stars in the Milky Way move in circular orbits around this black hole. This view into the center of the Milky Way is a groundbreaking feat of engineering.
As heavy as four million suns
The shadow of the huge object was shown. The black hole itself cannot be imaged because it does not emit light but swallows any light that comes too close to it. With its incredibly strong gravitational pull, it draws light into its interior, but of course also gas and dust and entire stars. Due to this feeding, it is now heavier than four million specimens of our sun and thus the largest and heaviest object in our Milky Way.
Two problems had to be solved
A black hole was imaged in space for the first time three years ago. It was even two thousand times further away in space than the now visible black hole in the Milky Way. So shouldn't the closer black hole in our Milky Way have seen first than the much more distant object in one of our neighboring galaxies? But the discovery story went the other way around.
The reasons for this: The black hole pictured three years ago was a thousand times heavier than our "own" black hole in the Milky Way – and that makes a huge difference. While the object pictured three years ago sits quite still in space due to its huge mass and is therefore comparatively easy to image, the black hole in the Milky Way has less mass, is lighter and therefore behaves much more restlessly, says Michael Kramer, director of the Max Planck Institute for Radio Astronomy in Bonn, which is involved in the "Event Horizon Telescope Project":
Despite trembling unrest, the clever calculation and filtering of enormous amounts of data has succeeded in producing images of "our" black hole. And it wasn't just the tremors that the researchers got under control. They even ignored the fact that the black hole in the center of our home galaxy shouldn't actually be visible from Earth.
Image created with radio radiation
Because the Milky Way is shaped like a spiral with several arms and all these spiral arms lie in one plane. Viewed from the outside, the Milky Way looks like a disk with a spiral pattern. The solar system including the earth is part of this disc. The view from Earth into the center of this disc is mostly blocked by dust. That is why the picture that has now been published is not one that was taken with visible light, but one that was created with the help of radio radiation.
Radio waves can penetrate the dust and can be caught with antennas. The received signals can be used to create images that show what the region in space looks like from where the radio waves were originally sent into space. Although the dust scatters the waves a little, computer programs can be used to filter the scattering out of the image.
Several years of calculation work required
A radio telescope would not have been enough to get a sharp radio view of the center of the Milky Way. To do this, the world's best radio telescopes had to be aimed at the center of the Milky Way at exactly the same time. This has been happening for several years, usually once a year in spring as part of the "Event Horizon Telescope" (EHT) project. Event horizon is what cosmologists call the edge of a black hole. And that is exactly the goal of the project – to map the large black holes in the core of galaxies or their outlines.
The radio telescopes used for this are spread all over the world – one of them even at the South Pole. Linking them with data lines would be too complex and error-prone. Therefore, the huge amounts of data are initially stored on hard drives. These hard drives are then transported to two data centers: one in Germany, at the Max Planck Institute for Radio Astronomy in Bonn, and the other in the USA. It is only in these centers that the data from the individual radio telescopes are combined and the images are calculated from the combined data on a supercomputer.
Calculations took five years
In the case of the picture now published, the calculation work took a full five years. Anton Zensus, director at the Max Planck Institute for Radio Astronomy in Bonn and chairman of the board of the "Event Horizon Telescope" project, explains:
New research opportunities for cosmology
With the super-sharp vision of the radio telescopes associated in the EHT project, it will be possible to observe the behavior of the black hole at the center of the Milky Way. This is to answer questions. How much matter does it devour? So how regularly do radiation bursts occur? How do the stars orbiting the black hole in close proximity behave? Perhaps it will even be possible to assemble individual images into a kind of time-lapse.
In this way one could see whether Einstein's theory of relativity describes all processes correctly, even in areas where extreme masses are gathered and extreme gravitational pull prevails. Because one thing is clear: the physics of the universe is not yet fully understood. There is still something missing in the physicists' theory building. What is not yet correct in the existing theories could most likely come to light or be noticed in extreme situations. And what can be more extreme than the gravity of a black hole weighing millions of suns?