Black holes do not emit or reflect light and are therefore difficult to detect. These objects are so massive that when a certain point (the event horizon) is reached, the escape velocity, that is, the speed required to leave the object’s gravitational field, exceeds the speed of light in a vacuum, and even light cannot leave this region.
Except when black holes are part of binary matter or absorb material that emits electromagnetic radiation, they are difficult to observe because black holes are so massive that even light cannot resist their gravitational grip. Astronomers infer their existence based on the effects of gravity on their surroundings, that is, the behavior of stars and matter in their immediate vicinity.
The black hole “in our stellar backyard”
A team of astronomers led by researchers at the University of Alabama Huntsville (UAH) has observed a black hole with a stellar mass (about 12 times the mass of the Sun), which the scientists placed “in our stellar backyard.” The newly discovered black hole is located about 1,550 light-years from Earth and is part of a binary system that also includes a Sun-like star.
A stellar-mass black hole forms at the end of a massive star’s life and collapses under the influence of its gravity. If this happens in a binary system, the result will be a bright star and a black hole orbiting each other.
It is closer to the sun than any other black hole known to us. It’s 1,550 light-years away, so it’s practically in a backyard, says UAH’s Dr Sukanya Chakrabarti, lead author of the article submitted for publication in The Astrophysical Journal (currently available at arXiv.org preprint database (DOI: 10.48550)/ arXiv.2210.05003).
Simple stellar population calculations suggest that stellar-mass black holes should be abundant. Chakrabarti says there must be 100 million people in the Milky Way alone. “However, black holes are difficult to identify at this mass scale,” he adds.
Unusual binary system
To find a black hole, Dr. Chakrabarti and his colleagues analyzed the data of nearly 200,000 .double stars from a group collected by the European Space Agency’s Gaia mission. “We have been looking for things that are reported to have large companion masses, but whose brightness can be attributed to a single visible star,” Chakrabarti admits. The sources of interest were supplemented by spectroscopic measurements from various telescopes, including the Automated Planet Finder in California, the Chilean Giant Magellan Telescope, and the Keck Observatory in Hawaii.
In this way, the researchers found a sun-like star that is strongly affected by the gravity of an invisible object. New research indicates that this object is a black hole. However, it is so far from its stellar companion that it does not steal its substance and does not form an accretion disk, and therefore is invisible.
Most black holes are found in binaries in the X-ray field. In other words, they are bright under X-rays because of their interaction, because a black hole often devours the second star. Chakrabarti points out that because the second star collides with the black hole, we can see X-rays.
The systems mentioned by the researcher are systems in short-range orbits. But this is not the case. – We are looking at a black hole, but it is in a long-term orbit lasting 185 days, which is about six months. Dr. Chakrabarti says it is quite far from the visible star and is not making any progress towards it.
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