Astronomers have discovered an enormous black hole that formed in the wake of an exploding star just 2,000 light-years from Earth.
BH3 is the most massive stellar black hole ever found in the Milky Way and revealed itself to researchers through the powerful tug it exerts on a companion star orbiting the object in the constellation Aquila, the Eagle.
The serendipitous discovery is so important that scientists released details of the object earlier than planned to enable other astronomers to conduct further observations as soon as possible.
“It’s a total surprise,” said Dr Pasquale Panuzzo, an astronomer and member of the Gaia collaboration at the Observatoire de Paris. “It is the most massive stellar-origin black hole in our galaxy and the second closest discovered so far.”
Stars form black holes when massive stars collapse at the end of their lives. Dozens have been found in the Milky Way, most weighing about 10 times the mass of the sun.
The most impressive black hole in the Milky Way, Sagittarius A, has the combined mass of several million suns. It is hidden in the heart of the galaxy and did not form from an exploding star, but the collapse of large clouds of dust and gas.
Researchers spotted BH3 in the latest set of data collected by the European Space Agency’s Gaia mission. The space telescope was launched in 2013 with the aim of compiling a 3D map of a billion stars.
As researchers reviewed the Gaia observations, they noticed a distinct wobble in one of the stars in Aquila, a constellation visible in the summer sky in the northern hemisphere. The motion suggested that the star was being pulled around by a black hole 33 times larger than the sun.
Further observations from the European Southern Observatory’s Very Large Telescope in Chile’s Atacama Desert confirmed BH3’s mass and the star’s orbit, which circles the black hole once every 11.6 years. “Only the central black hole in the Milky Way is more massive than this one,” Panuzzo said.
While BH3 is more massive than other stellar black holes in the Milky Way, it is similar to some of those revealed by gravitational waves, or ripples in spacetime, generated when black holes collide in distant galaxies.
“We’ve only seen black holes of this mass with gravitational waves in distant galaxies,” Panuzzo said. “It makes the connection between the stellar black holes we see in our galaxy and those gravitational wave discoveries.” Details are published in Astronomy and Astrophysics.
There may be 100 m stellar black holes in the Milky Way, but despite their huge mass and the powerful forces they generate, they can be extremely difficult to spot. “Most of them don’t have a star orbiting them, so they’re almost invisible to us,” Panuzzo said.
Measurements of BH3’s companion star found no sign that it was contaminated with material ejected from the stellar explosion that formed the black hole. The finding suggests that the black hole formed long before it captured the companion star in its powerful gravitational field.
The next batch of Gaia data will be released in late 2025 at the earliest, but the importance of the discovery has led the international team to release details of BH3 early so that astronomers can study it immediately.
“Once it comes out, people will rush to observe it to see if there are any emissions from the black hole,” Panuzzo said. “It will tell us about the wind coming from stars like the one orbiting the black hole, and also about the physics of the black hole and how matter falls into it.”
