The Most Massive Black Hole Merger Ever Seen Was So Rare, It Seemed Impossible. Now, Astrophysicists May Finally Have an Explanation

Past research about black hole births rarely included magnetic fields or the spins of the precursor stars. But considering those factors could explain the origins of two unusual objects that collided

Margherita Bassi

– Daily Correspondent

November 21, 2025 4:28 p.m.

Earlier this year, researchers reported the most massive black hole merger ever detected. But the event was so unusual that questions around its origin sowed confusion within the astrophysics community.

In November 2023, the Laser Interferometer Gravitational-wave Observatory’s (LIGO) sensors in Washington and Louisiana picked up on a brief signal, dubbed GW231123, that indicated two black holes had combined into a single black hole with the gargantuan mass of 225 suns.

But the two initial objects spun surprisingly fast and had unexpected masses. Black holes with those parameters challenge ideas about the objects’ formation—and according to certain assumptions, they shouldn’t exist.

A study published earlier this month in Astrophysical Journal Letters addresses the quandary by including elements that are often overlooked when studying black hole births: magnetic fields and the spins of stars.

When a massive star runs out of fuel, it might explode in a fiery death called a supernova, which can result in a black hole. Stars within a specific mass range, however, are thought to go out with particularly violent supernovas, called pair-instability supernovas, that leave nothing in their wake, instead of dispersing their matter into space.

Because of this, researchers don’t expect black holes with masses of around 70 to 140 suns, called the pair-instability mass gap. But GW231123’s parent black holes, estimated at 103 and 137 solar masses, sat within that puzzling range.

While a merger of smaller black holes might produce a new, larger one within the mass gap, some researchers think that’s an unlikely origin for the detected pair. Had they each formed from another collision, these black holes would be the “most extreme example” of this sort of event, Mark Hannam, a physicist at Cardiff University in Wales who did not participate in either study, told the Guardian’s Ian Sample when the huge merger was initially reported.

So, researchers looked for a different answer. First, they ran computer simulations of the life of a star of about 250 solar masses. By the time it went supernova, it had used up enough fuel to drop down to 150 solar masses—just outside of the mass gap and capable of leaving behind a black hole.

The second set of simulations, which included magnetic fields, modeled the events that followed the supernova, starting with its remnants—a cloud of leftover star material and magnetic fields that surrounds the black hole. Researchers previously thought that the cloud’s entire mass would collapse into the new black hole, meaning the black hole’s mass would match that of the bygone star. But that’s not what happened in the simulations.

If the star had been spinning rapidly before its death, the post-supernova cloud would turn into a spinning disk. That motion makes the black hole also spin faster as the star guts fall into it. But any magnetic fields present would rip some material away from the disk before it’s swallowed by the black hole, reducing the mass that ultimately falls in.

As a result, the final black hole can land “within the mass gap, a range previously thought to be inaccessible,” Ore Gottlieb, a study co-author and astrophysicist at the Center for Computational Astrophysics at the Flatiron Institute, tells Live Science’s Andrey Feldman.

The researchers hope to eventually observe real-world evidence of their newly proposed black hole formation process, per a statement. Their simulations suggest that when the peculiar black holes are born, the process should emit bursts of gamma rays. Searching for those flashes could help validate the newly proposed mechanism.

“No one has considered these systems the way we did; previously, astronomers just took a shortcut and neglected the magnetic fields,” Gottlieb says in the statement. “But once you consider magnetic fields, you can actually explain the origins of this unique event.”