Astronomers believe they have discovered a small black hole so small in mass that it falls into an exclusive category. Best of all, it’s thrillingly close.
About 1,500 light-years from our home planet, in a Milky Way constellation known as Monoceros, this is the closest black hole candidate to our planet that scientists have been able to find so far.
The Ohio State University team has dubbed it the Unicorn – a hat for the home of the black hole and its extraordinarily rare nature.
“When we looked at the data, this black hole – the Unicorn – just appeared,” says astronomer Tharindu Jayasinghe.
So how have we not seen it before? As it turns out, we were wearing our astronomical blinders.
From tiny primeval to supermassive giants powering the hearts of galaxies, the theory predicts that black holes can exist in a range of masses. However, when it comes to black holes formed by the collapsing cores of dead stars, astronomers have discovered some ‘mass fissures’ over the years.
When a star collapses to about 2.3 times the mass of our Sun, it becomes a neutron star, not a black hole. And until recently, we hadn’t found stellar black holes smaller than 5 solar masses – leaving us with the mass gap.
Before we found objects in that aperture, their existence had been so questionable that when astronomers noticed a nearby red giant star being pulled, they initially dismissed the possibility that it was a tiny invisible companion.
But Jayasinghe looked at it in a different way. As a graduate student, his supervisor had told him about the possibility of extraordinarily small black holes, and he wanted to investigate.
He analyzed data from various telescope systems and satellites and focused on a red giant in the constellation Monoceros, which was in its final phase of life.
The star’s speed and the way it was pulled by gravity all suggested that a small black hole orbited it. The size of this dark and silent companion was calculated to be about 3 times of the Sun
“Just as the moon’s gravity warps the Earth’s oceans, causing the seas to bulge to and from the moon, causing high water, so the black hole warps the star into a football-like shape with one axis longer than the others,” astronomer Todd Thompson, who has helped find other small black holes in the past.
“The simplest explanation is that it is a black hole – and in this case, the simplest explanation is the most likely.”
For decades it was unclear whether anything existed in the mass gap between two forms of dead star.
The Unicorn is now joining several other small black holes to solve that mystery. The results have yet to be officially verified, but for now this looks like a strong candidate for yet another black hole in the middle of the mass divide.
“I think the field is pushing here, to really map out how much low mass, how much intermediate mass, and how many high mass black holes there are,” says Thompson, “because every time you find one, it gives you a idea about which stars collapse, which explode and which are in between. ”
Who knows how many more small black holes there are that we can find. Ready or not, here come astronomers.
The results have been accepted for publication in the Monthly Communications from the Royal Astronomical Society and the preprint can be found here.