Astronomers think they discovered a small black hole, with such a small mass that it placed it in a unique category. Best of all, it is excitingly close.
About 1,500 light years from our own planet, in a Milky Way coupler called Monoceros, it is the closest black hole candidate to our planet scientists have had the fortune to discover yet.
The team at Ohio State University has named it Unicorn – a hat tip to the black hole’s home and its extremely rare nature.
“When we looked at the data, this black hole – the Unicorn – had just popped out,” said astronomer Tharindu Jayasinghe.
So how did we not see it before? In fact, we had our astronomical blinds on.
From tiny primordials to supermassive giants that power the hearts of galaxies, theory predicts that black holes can exist in a range of masses. However, in terms of black holes formed by falling dead star cores, astronomers have found some ‘mass gaps’ over the years.
If a star falls down to about less than 2.3 times the mass of our Sun, it will be a neutron star instead of a black hole. And, until recently, we had not found any stellar black holes of less than 5 solar masses – leaving us with the mass gap.
Before we found any objects in that gap, their existence had become so questionable that astronomers noticed a nearby red giant star struck by something, they denied the possibility that it was a small unseen companion.
But Jayasinghe looked at it in a different way. As a graduate, his supervisor had told him about the potential for very small black holes, and he wanted to investigate.
Analyzing data from various telescope and satellite systems, he joined a red giant in the constellation of Monoceros, which was in the later stages of his life.
The speed of the star and the way it was drawn by gravity all suggested that a small black hole was rotating. The size of this dark and silent companion was calculated to be roughly 3 solar masses.
“Just as the Moon’s gravity distorts the Earth’s oceans, causing the seas to swell to and from the Moon, producing a high tide, so the black hole distorts the star to a football-like shape with one axis longer than the other, “explains astronomer Todd Thompson, who has helped find other small black holes in the past.
“The simplest explanation is that it’s a black hole – and in this case, the simplest explanation is the most likely.”
For decades, it was unclear if anything existed in the mass gap between two types of dead stars.
The Unicorn is now teaming up with several other little black holes to help solve that mystery. The results have not yet been officially validated, but for the time being, this seems like a strong candidate for another black hole smack in the middle of the mass gap.
“I think the field is pushing toward this, to really map out how much low mass, how much intermediate mass, and how many high-mass black holes there are,” Thompson said, “because every time you come finding one gives you an idea of which stars are falling, which are exploding and which are in between. ”
Who knows how many more black holes there are for us to find. Ready or not, here come astronomers.
The results were accepted for publication in the Monthly Notices of the Royal Astronomical Society and the preface is found here.