PREVIEW

Astronomers have discovered a vast black hole that formed following the explosion of a star just 2,000 light years from Earth.

BH3 is the most massive stellar black hole yet identified in the Milky Way and revealed its presence to scientists through the strong gravitational pull it exerts on a companion star that orbits the object in the constellation of Aquila, the Eagle.

The fortuitous discovery was deemed so significant that scientists released details of the object earlier than scheduled to enable other astronomers to carry out further observations as soon as possible.

"It's a complete 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 nearest discovered so far."

Stellar black holes are formed when massive stars collapse at the end of their lifecycle. Dozens have been identified in the Milky Way, most weighing around 10 times the mass of the sun.

The most remarkable black hole in the Milky Way, Sagittarius A, possesses the combined mass of several million suns. It resides at the heart of the galaxy and originated not from an exploding star but from the collapse of immense clouds of dust and gas.

Researchers detected BH3 in the latest batch of data collected by the European Space Agency’s Gaia mission. The space telescope, launched in 2013, aims to create a 3D map of a billion stars.

While reviewing the Gaia observations, researchers noticed a distinct wobble in one of the stars in Aquila, a constellation visible in the summer sky in the northern hemisphere. The motion indicated the star was being pulled by a black hole 33 times more massive 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 remarked.

While BH3 is more massive than other stellar black holes in the Milky Way, it is akin to some revealed by gravitational waves, or ripples in spacetime, which occur when black holes collide in distant galaxies.

"We have only seen black holes of this mass with gravitational waves in faraway galaxies," Panuzzo noted. "This makes the link between the stellar black holes we observe in our galaxy and those gravitational wave discoveries." Details are published in Astronomy and Astrophysics.

There could be 100 million stellar black holes in the Milky Way, but despite their immense mass and the powerful forces they generate, they can be extremely difficult to detect. "Most of them don’t have a star orbiting around them, so they are almost invisible to us," Panuzzo explained.

Measurements of BH3’s companion star showed no signs that it was contaminated with material blasted out from the stellar explosion that formed the black hole. The finding suggests the black hole formed long before it captured the companion star in its powerful gravitational field.

The next set of Gaia data is expected to be released in late 2025 at the earliest, but the importance of the discovery led the international team to release details of BH3 early so astronomers can study it immediately.

"As soon as this comes out, people will rush to observe it to see if there are any emissions from the black hole," Panuzzo said. "That will tell us about the wind that comes from stars like the one orbiting the black hole, and also about the physics of the black hole and how matter falls into it."