Black Hole

 Black Hole

Discover the smallest black hole in the universe

It has only 3.3 times the mass of the sun and orbits a star located 10,000 light years from Earth

Astronomers have discovered the smallest black hole in the Universe hidden behind a giant red star located 10,000 light years from Earth: it has only 3.3 times the mass of the Sun and is the first of a new class of celestial objects.

Black holes are an important part of how astrophysicists make sense of the universe, so important that scientists have been trying to build a census of all black holes in our galaxy, the Milky Way.

But new research from the Ohio State University (USA) suggests that this search could have missed a complete class of black holes that were not known to exist.

In a study published in the journal Science, astronomers show that it is possible that there is a class of smaller black holes than the smallest known black holes in the universe.

Black holes have gravitational pulls so fierce that nothing, neither matter nor radiation, can escape. Black holes form when some stars die, contract and explode.

In addition to black holes, astronomers have also been looking for neutron stars, small, dense stars that form when some stars die and collapse. Both could contain interesting information about how stars live and die.

But to discover that information, astronomers first have to figure out where the black holes are. And to find out where the black holes are, they need to know what they are looking for.

A track

Black holes often exist in something called a binary system. This simply means that two stars are close enough to each other to be joined by gravity in a mutual orbit around each other.

When one of those stars dies, the other can remain orbiting the space where the dead star was, turned into a black hole or a neutron star.

For years, black holes that scientists knew had about five to 15 times the mass of the sun.

Known neutron stars are generally no larger than about 2.1 times the mass of the sun: if they were above 2.5 times the mass of the sun, they would collapse into a black hole.

But in the summer of 2017, an investigation by the Gravitational Wave Detection Observatory (LIGO) discovered two huge black holes merging into a galaxy about 1.8 million light years away. One of those black holes was about 31 times the mass of the sun; the other about 25 times the mass of the sun.

Some astrophysicists had long suspected that black holes could have sizes outside the known range, and the discovery of LIGO showed that black holes could be much larger than estimated until then.

Also smaller?

But the discovery also revealed another question: can black holes be smaller than has been estimated so far?

To solve that mystery, astronomer Todd Thompson's team analyzed data from APOGEE, the Galactic Evolution Experiment of the Apache Point Observatory, which collected light spectra from around 100,000 stars in the Milky Way.

Thompson realized that the spectra could show if a star could be orbiting around another object: changes in the spectra, a change to more blue wavelengths, for example, followed by a change to redder wavelengths, they could show that a star was orbiting an invisible partner.

Thompson began to review the data, looking for stars to show that change, indicating that they could be orbiting a black hole.

Then, it reduced APOGEE data to 200 stars that could be more interesting. His data analysis discovered a giant red star in rapid rotation, located about 10,000 light years from Earth, on the edge of our galaxy (near the constellation Auriga).

That star seemed to be orbiting something, but that something, according to its calculations, was probably much smaller than the known black holes in the Milky Way, but also much larger than most known neutron stars.

After further calculations and additional data from the Echelle Reflector Tillinghast Spectrograph and the Gaia satellite, Thompson's team realized that they had found a low-mass black hole, probably only about 3.3 times the mass of the sun.

 "What we have done here is to find a new way to look for black holes, but we have also potentially identified the first of a new class of low-mass black holes that astronomers did not previously know," Thompson said in a statement.