A single black hole emits no light, yet its gravity alters the course of light passing through it. Ute Kraus, Institute of Physics, Universität Hildesheim (background Milky Way panorama: Axel Mellinger).

As a big star collapses under its own weight, a whole new baby black hole is born somewhere in the universe every second.

However, black holes are invisible. Astronomers have previously only been able to identify these stellar-mass black holes when they are acting on a companion.

A team of scientists has now confirmed the first-ever finding of a stellar-mass black hole that is absolutely alone. The discovery opens the door to the possibility of discovering more, which is an intriguing prospect given that there should be roughly 100 million such “rogue” black holes drifting around our galaxy unnoticed.

Relying on the neighbors

Because black holes do not shine like stars, they are difficult to locate. Anything having mass warps space-fabric, time’s and the higher the mass, the more extreme the warp. Because black holes pack so much mass into such a small space, space collapses in on itself. That is, if anything, including light, gets too close, its path will always curve back toward the black hole’s center.

Astronomers have discovered just few hundred of these ghostly goliaths by observing how they influence their surroundings. They’ve discovered roughly 20 tiny, stellar-mass black holes in our galaxy by observing stars being devoured by invisible companions. As the black hole pulls matter from its neighbor, the material forms a swirling, glowing accretion disk that signals the black hole’s presence.

Astronomers have discovered an isolated stellar-mass black hole after decades of seeking. The yet-to-be-named rogue black hole is located around 5,200 light-years away in the center of our galaxy and weighs little more than seven times the mass of the Sun. It’s travelling faster than almost all of the visible stars in its neighborhood, which suggests how it formed.

Scientists believe that when a massive star runs out of fuel and collapses, the resulting supernova explosion may be uneven. “This black hole seems to have gotten a natal kick at birth that sent it speeding away,” says Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore, who led the study. The results of the team have been submitted to The Astrophysical Journal.

Gravitational lensing occurs when a massive foreground object bends and magnifies the light of a background object far behind it. When the lensing object is small (a star, planet, or black hole), this phenomenon is called microlensing.

Seeing the Unseeable

To find the black hole, the team used two cosmic techniques: gravitational lensing and astrometry. The first works because gravity warps space-time, altering the route light takes when it passes nearby. When a celestial object passes very close to a more distant star in the sky from our perspective, the starlight bends as it passes through the closer object. If the foreground object bending is small — say, a planet, star, or black hole — rather than a full galaxy or galaxy cluster, the effect is known as microlensing.

Microlensing causes the nearby object to act as a natural magnifying glass, temporarily enhancing the light of the distant star – an effect that telescopes can detect. Astronomers can estimate the mass of a nearby object based on how long the spike in starlight lasts; larger objects produce longer microlensing events. As a result, a long microlensing event triggered by something we can’t see could indicate the existence of a rogue black hole.

However, microlensing alone cannot confirm the existence of black holes. A small, faint star traveling slowly could pass for a black hole. Because of its sluggish speed, it would also produce a long signal, and if the star is dim enough, astronomers might not see it, only detecting light from the background star.

This is where astrometry comes into play. This process requires taking precise measurements of an object’s position. Astronomers can determine the mass of a nearby object extremely precisely by observing how much the location of the background star appears to shift during a microlensing event.

“That’s how we knew we found a black hole,” Sahu says. “The object we detected is so massive that if it were a star, it would be shining brightly; yet we detected no light from it.”

This discovery is the result of seven years of research. Microlensing signals can disclose small, isolated black holes for nearly a year. The event was detected by two ground-based telescopes, the Optical Gravitational Lensing Experiment (OGLE) and the Microlensing Observations in Astrophysics (MOA). It lasted long enough that astronomers suspected the lensing object could be a black hole.

That’s when they began making astrometric measurements. The deflection of light created by the intervening object was so slight that it could only be seen by the Hubble Space Telescope. The team spent several more years analyzing the astrometric signal, which in general can last five to 10 times longer than its microlensing counterpart.

“It’s extremely gratifying to be part of such a monumental discovery,” Sahu says. “I’ve been searching for rogue black holes for more than a decade, and it’s exciting to finally find one! I hope it will be the first of many.”

Establishing the cosmic norm

It’s still feasible that the object isn’t a black hole after all. A separate team’s examination of the same event places the item between 1.5 to 4 solar masses — light enough to be either a black hole or a neutron star (the crushed core of a dead star that wasn’t massive enough to become a black hole). Given that astronomers have never previously identified an isolated neutron star, this would be a stunning discovery. The results of both teams are still being peer-reviewed.

Regardless of the outcome, some astronomers believe the stellar-mass black holes discovered in binary systems are a skewed sample. Their masses only range from about 5 to 20 times the Sun’s mass, with most weighing in at around 7 solar masses. But the true range may be much broader.

“Stellar-mass black holes that have been detected in other galaxies via gravitational waves are often far larger than those we’ve found in our galaxy — up to nearly 100 solar masses,” Sahu says. “By finding more that are isolated, we’ll be better able to understand what the true black hole population is like and learn even more about the ghosts that haunt our galaxy.”