How small can a black hole be? For several decades, astronomers have worked to answer this question by tallying the black holes in our corner of the universe.
They’ve found plenty of big and medium-size ones over the years—including a supermassive monster at the heart of our galaxy. But until recently, they’ve seen no signs of small ones, and that’s presented a long-standing mystery in astrophysics.
Now, astronomers have discovered a black hole with just three times the mass of the sun, making it one of the smallest found to date—and it happens to be the closest known black hole, at just 1,500 light-years from Earth.
The discovery “implies that there are many more [small black holes] that we might find if we increased the volume of space that we searched,” says Tharindu Jayasinghe, an astronomer at Ohio State University and lead author of a new paper detailing the discovery in the Monthly Notices of the Royal Astronomical Society. The finding “should create a push to find these systems.”
Jayasinghe and his colleagues have dubbed the object the “unicorn,” in part because it is unique, and in part because it was found in the constellation Monoceros, named by ancient astronomers after the Greek word for unicorn. By studying this unicorn and other objects like it, researchers hope to get a clearer picture of what happens to stars in the final moments of their lives and why some of them collapse to become black holes while others leave behind dense stellar husks called neutron stars.
Searching for the unseeable
Since no light can escape from a black hole, they can only be detected by indirect means. Most known black holes have been found by searching for the x-rays emitted when the invisible object pulls material off an orbiting companion star. As that material heats up in a dense ring around the black hole, known as an accretion disk, it emits radiation that can be detected with x-ray telescopes.
The unicorn, however, was found by a different method. Jayasinghe’s team used data from a number of observatories to measure periodic changes in the brightness and spectrum of light coming from a red giant star known as V723 Mon. These types of observations have been used for several decades to search for exoplanets, which can be extremely difficult to spot directly.
The team deduced that an unseen companion object is tugging at the red giant, distorting it into a raindrop shape. The data give the combined mass of both objects, and if the star is heftier than the team’s estimate, it’s possible the unseen object is a neutron star. But the team believes that companion is most likely a small black hole.
Although the unicorn is changing the shape of the red giant, it isn’t pulling material off it. That means there’s no accretion disk and therefore no x-rays, which is why it went unnoticed until now. This lack of x-ray emissions in such “quiet” black holes may account for why so few small ones have been found so far.
Black holes with more than five times as much mass as our sun appear to be plentiful, but below that figure, they seem to be in short supply. Astronomers refer to the puzzling lack of small black holes as the “mass gap.”
Filling in the mass gap
Prior to the unicorn’s discovery, several other candidates for black holes within the mass gap had been put forward. In 2019, the same team announced they’d found a dark object orbiting a giant star—however, their estimates for the object’s mass were less precise, and they were only able to conclude that it was either a black hole “or an unexpectedly massive neutron star.”
Last year, another team of astronomers found what they believed was a triple system, about 1,100 light-years from Earth, containing a black hole of about four solar masses orbiting with two stars. If the system really contains a black hole, it would be the closest known to Earth, but other research has since cast doubt on the finding.
Further tantalizing results have come from gravitational wave detectors such as the Laser Interferometer Gravitational Wave Observatory, or LIGO. In 2019 astronomers observed a source of gravitational waves known as GW190814, sparked by the collision of two objects. One weighed in at just 2.6 solar masses—meaning it must have been either an extremely heavy neutron star or the lightest known black hole. Additionally, the merger of two neutron stars, observed as a gravitational wave event in 2017, is believed to have created a black hole of about 2.8 solar masses.
Objects detected via gravitational waves are, unfortunately, hard to study in the long-term. They tend to lie far beyond our galaxy, which means that astronomers only learn about them when they emit a brief burst of gravitational waves. After that, they’re out of sight for good.
The unicorn, on the other hand, is in our galactic backyard, and it can be studied for years to come. “The fact that the companion is a red giant, and that it’s close by, makes the observation more accurate and reliable,” says Vicky Kalogera, an astronomer at Northwestern University who wasn’t involved in the new research.
Collapse in space-time
Astronomers hope that the unicorn and other similar objects will shed light on the physics that governs the formation both of black holes and of neutron stars. Both objects form when a star reaches the end of its life, exhausting its nuclear fuel supply. But which fate awaits any individual star depends on its mass.
If the star is a bit bigger than our sun, it blows up in a supernova explosion. The remainder of the star is compressed by gravity to form a neutron star—an object so dense that material is packed together as tightly as an atomic nucleus.
If the object is much heavier, though, then the object collapses further under the force of gravity, creating a black hole. Even though the star may have lived for ten million years, that endgame plays out with incredible speed.
“In a span of one to five seconds, the star decides if it’s going to explode as a supernova and produce a neutron star, or of it’s going to collapse and form a black hole,” says Todd Thompson, an Ohio State University astronomer and a co-author of the unicorn paper. “Or there could be an intermediate case, where it explodes a little bit, but still has material falling back, producing a black hole. All of that gets decided in very short order.”
One dilemma for researchers is that it’s impossible to study the relevant physics directly. “We still don’t fully understand how matter behaves at nuclear densities,” Kalogera says. “That’s the challenge of astronomy: We can’t mimic those densities in the lab.”
The smallest black holes, such as the unicorn, could help scientists piece together this cosmic puzzle.
A clearer picture may emerge when more data are released from the European Space Agency’s Gaia spacecraft, designed to map the positions of stars in the sky with pinpoint precision, perhaps revealing more little black holes tugging on their companion stars.
Astronomers are also eagerly awaiting the next data release from the Sloan Digital Sky Survey, which uses a telescope in New Mexico to provide detailed looks at of millions of celestial objects and so may reveal the motion of stars as they respond to unseen companions. Small black holes may also be found by the Vera C. Rubin Observatory, now under construction in Chile.
As more data become available, astronomers hope to learn whether the shortage of little black holes points to some novel aspect of stellar physics—or if small black holes are in fact peppered throughout the galaxy, uncounted thus far because we’ve only just developed the means to hunt for them.