A very small black hole, at least when compared with those of supermassive black holes that we are used to discussing, was discovered by a research team that analyzed a catalog of 100,000 stars called Apache Point Observatory Galactic Evolution Experiment (APOGEE). The black hole is characterized by a mass that appears to be only 3.3 times that of the Sun. The study was published in Science.
About 10,000 light-years away from us, this black hole is located in an external region of the main disk of the Milky Way and, if the dimensions were confirmed, it would be the smallest black hole ever identified, so small that it would fall into a new class. The black hole should have a diameter of only 12 miles but further observations will be necessary to confirm this data.
Researchers usually detect black holes thanks to the interception of the accretion disc, mostly composed of gas and dust, which swirls around the object. If there is not this disk of gas and dust, which by swirling it heats up and emits X-rays, the black hole cannot be identified unless it is part of an interacting binary system, i.e. a system consisting of a black hole and another object, for example a star, in which the first sucks material from the second.
In this case, however, there was no disc of gas and dust and, although the black hole was part of a binary system, this seemed to be non-interacting. However, the researchers still managed to locate the black hole thanks to another feature related to the fact that this black hole is in a binary system.
Analyzing the data of the aforementioned catalog, the researchers focused on a particular binary star system, called J05215658, and they realized that one of the two stars is actually a black hole that orbits a giant star every 83 days. In this case, the researchers noticed the presence of the black hole by analyzing the light of the star, a giant star whose light seems to be “shifted.” The brightness of the star, in fact, continues to change, increases and decreases, a sign of the fact that there is something in orbit around it.
Precisely these variations indicate the presence of a binary companion which, since it does not emit any light, must be a black hole. The latter does not seem to swallow any material and has been identified only thanks to this special distorting feature of the starlight.
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