There are an estimated 100 million black holes in our Milky Way galaxy. Yet scientists have never been able to identify an isolated black hole. Until now probably. The Hubble Space Telescope has provided possible evidence of a solitary black hole for the first time ever.

It took the telescope six years to measure the exact mass of the black hole. Until now, the mass has always been statistically inferred from interactions with a binary star system or measured in the core of galaxies. This black hole is not accompanied by a binary star, which is what makes the latest discovery so special.

The recently observed black hole is located about 5,000 light-years from Earth in the Sagittarius Arm, one of the Milky Way’s four major spiral arms. Based on this, scientists suspect that the nearest black hole may be only 80 light-years away.

Wandering black holes in our Milky Way galaxy are born from rare large stars that are at least twenty times more massive than our sun. These stars explode as supernovae and the remaining core is compressed by gravity into a black hole. Because the explosion is not perfectly symmetrical, the black hole could be fired like a cannonball through our galaxy.

Telescopes cannot capture such an unpredictable black hole, because it emits no light. What it does do is slightly distort the space, bending and amplifying the light of whatever is right behind the black hole at that moment. Telescopes on Earth therefore try to detect a sudden lightening of one of the millions of stars when a massive object passes between us and the star. The Hubble then looks at the most interesting cases.

Two research teams, one led by Kailash Sahu of the Space Telescope Science Institute in Baltimore and the other led by Casey Lam of the University of California, Berkeley, used the Hubble data in question. Although the results differ slightly, both teams determine the presence of a compact object. They do this by using a technique called gravitational microlensing: with a microgravity lens it can be observed that the gravitational field of a star bends and amplifies the light of a more distant star. The technique is also used to discover exoplanets.

Multiple observations showing the brightening of a star over time.

Several consecutive observations show how the brightness of a star changes over time. Source: NASA

neutron star
At the current black hole, the activity lasted more than two hundred days longer than usual due to the very strong gravity of the hole. Hubble determined that the star’s image was shifted by a milli arcsecond (the arcsecond is a unit to indicate the magnitude of an angle) from its expected location. Sahu’s team estimates that the black hole weighs seven times as much as the Sun. Lam, however, comes to a lesser mass of 1.6 to 4.4 times the sun. It could also be a neutron star.

“As much as we would like to conclude that it really is a black hole, we must report all possible explanations. It could also be a lower mass black hole and possibly even a neutron star,” said Jessica Lu of the Berkeley team. in the press release about the find. “Whatever it is, this is the first dark stellar remnant we’ve discovered that roams the Milky Way without being joined by another star,” Lam added.

Black hole illustration

Hubble measures the deflection of a star’s light by a black hole. Source: NASA.Six years of research
It is estimated that the black hole shoots through our galaxy at 160,000 kilometers per hour. So it could travel from Earth to the Moon in less than three hours. That’s faster than most neighboring stars. For years, Hubble recorded how the light from the star behind it was deflected as the black hole flew by to arrive at the exact measurements. In total, the light from the star was amplified by the black hole over a period of 270 days.

We have known that stellar black holes exist since the 1970s, but until now there have only been observations in binary star systems. The current discovery of the isolated black hole is very useful. “The discovery of a solitary black hole provides new insights into the population of these objects in our galaxy,” Sahu said. He assumes that more isolated black holes will be found, although it is difficult to find. It is expected that only one in several hundred observations with the microlensing technique will consist of isolated black holes.