"Unicorn" may be smallest and closest black hole to Earth

"Unicorn" may be smallest and ...
An artist's illustration of the small, close "Unicorn" black hole and the stretching effects it has on its companion star
An artist's illustration of the small, close "Unicorn" black hole and the stretching effects it has on its companion star
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An artist's illustration of the small, close "Unicorn" black hole and the stretching effects it has on its companion star
An artist's illustration of the small, close "Unicorn" black hole and the stretching effects it has on its companion star

Astronomers have discovered a black hole that may set a new record or two – it seems to be both the smallest black hole ever detected, and the closest one to Earth found so far.

Nicknamed The Unicorn, the black hole is located right next to a red giant star called V723 Mon in the constellation of Monoceros. That’s only 1,500 light-years from Earth, and the object appears to have a mass just three times that of the Sun.

Both of those are possible records for black holes. The previous record-holder for smallest black hole is 3.3 solar masses, while the previous closest black hole is twice as distant as the Unicorn. The latter record is a little contentious, however – last year it was proposed that the star system HR 6819 housed the closest known black hole, just 1,120 light-years away, but follow-up studies throw doubt on there being a black hole there at all.

Either way, the Unicorn is an intriguing oddity nonetheless. It’s smaller than most of the smallest black holes, a group known as stellar mass black holes, which range between five and about 30 solar masses. Until recently astronomers didn’t really think they came any smaller than that.

Of course, black holes by their nature are tricky to see. Instead the astronomers noticed the Unicorn through its effects on its companion star. Its light appeared to be changing in intensity at different points in its orbit, suggesting it was being stretched into an odd shape by the gravity of something nearby. Since it didn’t have a visible star buddy, a black hole seemed to be the most likely candidate.

“Just as the moon’s gravity distorts the Earth’s oceans, causing the seas to bulge toward and away from the moon, producing high tides, so does the black hole distort the star into a football-like shape with one axis longer than the other,” says Todd Thompson, co-author of the study. “The simplest explanation is that it’s a black hole – and in this case, the simplest explanation is the most likely one.”

By analyzing the star’s gravitational distortion, velocity and orbit period, the astronomers were able to calculate that the black hole had the mass of three Suns.

The researchers say that other black holes in this mass gap may be discovered in the next few years, as telescopes become more powerful and astronomers get better at analyzing data.

The research was published in the journal Monthly Notices of the Royal Astronomical Society.

Source: Ohio State University

Because black holes are defined by having a gravitational force large enough that photons closer than the event horizon cannot escape it I always assumed their mass would have to be really high compared to other stars. For context the largest star by size in the universe (UY Scuti) is 1,708 solar radii and 7-10 solar masses but it's not a black hole. The largest known black hole is 66 billion solar masses.
Pierre Collet
The artist who made the illustration got it wrong, as Galileo did at his trial... Galileo could not explain why tides happened approximately twice a day and not only once, when facing the Moon. In fact, when two masses rotate around each other, on the side they face each other, gravity attracts matter and elongates the spheres one towards the other but on the other side, centrifugal forces operate and do the same... So the picture of the red star attracted by the black hole should be an ellipsoid and not an ovoid (i.e. stretched both towards and outwards of the black hole...
Brent Studer
The mass of a black hole does not have to be large—the limit at which neutron degeneracy cannot withstand gravitational collapse is about 3 solar masses. At the event horizon, the escape velocity (Vesc) is equal to the speed of light (c) and Vesc is proportional to the square root of mass/radius. The Schwarzschild radius of a black hole's event horizon is 3 km x the black hole's mass in solar masses. This black hole candidate is just 9 km in radius. That's an extremely small radius for a 3 solar mass object. In comparison, UY Scuti has a very low Vesc—about 0.00014c—due to its enormous radius. A 66 billion solar mass black hole is "only" 165 times larger than UY Scuti but about 7 billion times more massive.