The Smallest Black Hole was Discovered

Black holes are one of the most fascinating and enigmatic powers of the Universe, and concurrently, crucial to our appreciation of astronomy.​

They are the outcome of notably gigantic stars that end up as supernovas at the end of their lives, and also fundamental to our comprehension of General Relativity. Black holes are also theorized to have played a massive part in cosmic evolution.

Due to this appreciation, astronomers have been attempting to create an inventory of black holes located in the Milky Way for a lot of time now. Even so, new research suggests that scientists may have missed an entire class of black holes: a team of astronomers spotted a black hole that is located over three Solar masses. The new findings make this particular force of nature the smallest black hole discovered until today.

The Smallest Black Hole Ever Found

The research, called “A noninteracting low-mass black hole–giant star binary system,“​ was published in the journal Science. The team of astronomers responsible for the discovery included scientists from Ohio State University, the Harvard-Smithsonian Center for Astrophysics, The Observatories of Carnegie Institution for Science, the Dark Cosmology Center and many other observatories and universities.

The finding was worth mentioning because it detected an object that astrophysicists didn’t even know it existed. Therefore, scientists have to now reconsider what they believed they knew about the black holes in our galaxy.

“We’re showing this hint that there is another population out there that we have yet to really probe in the search for black holes. People are trying to understand supernova explosions, how supermassive black stars explode, how the elements were formed in supermassive stars. So if we could reveal a new population of black holes, it would tell us more about which stars explode, which don’t, which form black holes, which form neutron stars. It opens up a new area of study,” ​Ted Thompson, a professor of astronomy at the Ohio State University and lead author of the research said.

Artist’s impression of a star orbiting with a black hole that produces powerful jets. [Image: ESO/Chandra]

New Type of Low-Mass Black Holes

As of date, all the black holes discovered in the Milky Way by astronomers were between five and fifteen solar masses. Neutron stars are usually about 2.1 solar masses, as anything wider than 2.5 solar masses would fail to form a black hole. When LIGO and Virgo​ identified gravitational waves generated by a black hole merger, they were 31 and 25 solar masses, respectively.

This made Thompson and his team take into consideration the probability that there might be uncovered cosmic objects located between the largest neutron stars and the smallest black holes. To determine whether it is so, they started merging data from Apache Point Observatory Galactic Evolution Experiment (APOGEE)​, a survey that gathers spectra from approximately 100,000 stars all over the galaxy.

The astronomers then analyzed this spectra for signs of changes that would point out if a star might be rotating around another object. This technique is among the most efficient and renown methods of determining if a star has an orbiting system of planets.

Soon enough, Thompson started narrowing the APOGEE data down to 200 objects. He then handed over the data to Tharindu Jayasinghe​, a graduate research associate at Ohio State University, to assemble thousands of images of each candidate.

This unveiled a massive red star that seemed to be rotating around something that was a lot smaller than any so-far-discovered black hole, but much wider than any known neutron stars. After merging the results with more data from the Tillinghast Reflector Echelle Spectrograph (TRES)​ and the Gaia satellite,​ they grasped that they discovered a black hole almost 3.3 times the mass of the Sun.

The result confirms the existence of a new type of low-mass black holes, and it also provided a new technique for spotting them.

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