The stars that orbit close to the Milky Way's supermassive black hole are already tough to explain – but there also seems to be some stars missing
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| There seem to be some stars missing near the centre of the Milky Way Shutterstock/structuresxx |
There is a strange and unexplained “zone of avoidance” for stars near the centre of the Milky Way. Stars at any given distance from the supermassive black hole at the galactic centre, called Sagittarius A*, should have a random distribution of shapes to their orbits, but one group of stars is mysteriously missing from that distribution.
The group of about 200 stars that reside near Sagittarius A* are called S-stars, and their very existence is somewhat unexpected because the environment in that area is extremely hostile. Andreas Burkert at University Observatory Munich in Germany and his colleagues examined the orbits of the 50 or so of these stars for which we have solid data to search for patterns.
They found a close correlation between each star’s distance from the galactic centre and the shape of its orbit – the closer a star is to the middle of the galaxy, the more stretched out, or eccentric, its orbit is. Unexpectedly, in the area immediately around Sagittarius A*, there are no stars with circular orbits, a phenomenon the researchers call a zone of avoidance. If those stars existed, they would be the easiest to spot, so it is unlikely that this strange zone comes from a problem with the data itself.
“The zone of avoidance has a very sharp upper boundary,” says Burkert. “It’s as if something forbids stars from going into the zone, or the ones that do go into the zone get destroyed.” But given that there are plenty of stars close to Sagittarius A*, there is no known reason there shouldn’t be any within the zone of avoidance.
“It seems that the presence of a supermassive black hole creates an underlying mechanism that forces objects to arrange in a non-randomised pattern,” says Florian Peissker at the University of Cologne in Germany. “The big and important question is: why is that?” Figuring it out will be crucial to our understanding of how black holes shape their environments, not just in their immediate surroundings but further out as well, he says.
There are two main hypotheses to explain the shapes of the S-stars’ orbits and how they can be so close to Sagittarius A*. The first is that there could be another, smaller black hole in the same area changing the stars’ orbits, and the second is that the S-stars used to be binaries, but one of the pair was flung away into deep space and the other remained in a tight orbit at the centre of the galaxy.
Neither of these ideas can explain the zone of avoidance and the other, more subtle patterns the researchers spotted. “These are the competing theories, and they don’t have to compete anymore because they don’t work,” says Burkert. “We don’t really know what it is yet. I think that’s the most exciting part.”
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