2024-07-12 03:29:21
Black holes may be invisible, but astronomers have nevertheless managed to find a new one – a monster in the heart of the globular cluster Omega Centauri.
- Researchers have discovered a medium-mass black hole in the globular cluster Omega Centauri.
- The discovery is based on the observation of seven stars moving at extremely high speeds.
- The black hole is about 18,000 light years away from Earth.
Until now, the existence of medium-mass black holes was only a guess – there was no reliable evidence. An international team of researchers led by Maximilian Häberle from the Max Planck Institute for Astronomy in Heidelberg has presented the best evidence to date for just such a black hole.
The team has discovered seven stars in the center of the globular cluster Omega Centauri that are moving at extremely high speeds in the archive data of the Hubble Space Telescope, the researchers report in the journal Nature.
The rapid movement of the stars can only be explained by the gravitational pull of a black hole, it says. From the data, the astronomers conclude that the black hole at the center of Omega Centauri must have 8,200 times the mass of our sun.
The newly described black hole is located in the direct cosmic neighborhood of Earth. It is about 18,000 light years away, explains co-author Nadine Neumayer. This makes it the closest known example of a massive black hole. The supermassive black hole in the center of the galaxy is about 27,000 light years away.
Black holes are not an immediate threat to the Earth. ESA project manager Uwe Lammers said in response to a t-online query: “The mathematical probability that ‘wandering’ black holes could pose a threat to the Earth is practically zero.” The reason: The dimensions of the Milky Way are so enormous that “a collision is completely impossible.”
In order to find new black holes, astronomers had repeatedly searched for such racing stars – so far without success. Häberle started searching again. In doing so, he used data from the Hubble telescope that had previously been unused for this purpose, which had repeatedly photographed Omega Centauri to calibrate its instruments.
Häberle’s research team had access to 500 archive images from a period of 20 years. The astronomers meticulously measured the movement of around 150,000 stars in these images.
“Searching for fast stars and documenting their movement was like looking for a needle in a haystack,” explained Häberle. In the end, he had not only created the most complete catalog of star movements in Omega Centauri to date, but had also found seven needles in a haystack: seven stars that move at high speed.
Häberle believes that the stars would have to fly out of the star cluster at this speed. His calculations show that only the gravitational pull of a black hole with 8,200 times the mass of the sun can hold the stars in place like this.
Finding a black hole with such a mass is of great importance for astronomers, because until now astronomers only knew of two types of black holes.
Globular clusters are large spherical collections of stars that are gravitationally bound to one another. They usually consist of hundreds of thousands to millions of stars. Globular clusters are a common occurrence.
On the one hand, there are so-called stellar black holes with up to 150 solar masses. These are formed when large stars have used up their nuclear energy reserves and collapse without stopping. On the other hand, there are the supermassive black holes in the centers of galaxies with millions or even billions of times the mass of the sun.
It was assumed that these supermassive black holes were formed by the merging of smaller black holes with a few thousand solar masses. It was assumed that some of these medium-mass black holes still exist in the cosmos today – but there was no direct evidence for this.
This is where Omega Centauri comes into play: With ten million stars, it is the largest globular cluster in the Milky Way. In the southern sky, it can even be seen as a spot with the naked eye. Omega Centauri is probably the former central region of a small galaxy that collided with the Milky Way billions of years ago and lost its outer regions in the process.
The idea was that if this collision had occurred, then a medium-sized black hole that had previously existed in the center of the small galaxy should have survived in Omega Centauri to this day. And thanks to the globular cluster’s proximity to Earth, the movement of stars can also be observed there.
The stars now discovered by Häberle and his team confirm this idea and provide the best evidence to date for the existence of medium-sized black holes. However, the Hubble images only show the movement of the stars in the sky and not the movement towards or away from us.
The researchers now want to measure this radial movement of the seven racing stars with the James Webb space telescope and thus dispel any remaining doubts about the existence of the black hole in Omega Centauri.