‘Forbidden’ Black Holes: How the Biggest Stars Die Without a Trace

EARTH, Laniakea Supercluster—The universe, for all its chaos, appears to have a hard rule: some black holes are not allowed to exist.

A new study published in Nature finds that gravitational‑wave data—faint ripples in the fabric of spacetime kicked off when massive objects collide—reveal a “forbidden zone” in black hole masses. It is a range where dying stars simply refuse to leave black holes behind, instead detonating so completely that they scatter themselves into oblivion.

An international team led by researchers at Australia’s Monash University analyzed data from a global network of gravitational‑wave observatories known as LIGO–Virgo–KAGRA (LVK). In that data, they identified a gap in black hole masses that begins around 44 times the mass of our sun.

Below that threshold, black holes form the usual way: a massive star runs out of fuel, its core collapses, and what remains is a gravitational monster. Above it, something stranger happens.

The culprit is a phenomenon called pair-instability supernova, first theorized in the 1960s. In the most massive stars, roughly 140 to 260 times the mass of the sun, extreme internal temperatures cause photons to spontaneously convert into pairs of electrons and positrons. This bleeds away the radiation pressure holding the star up, and gravity briefly takes over, compressing the core until it detonates in an explosion so violent that nothing is left behind.

No remnant. No black hole. Just cosmic shrapnel scattered across space.

"The observation is well explained by pair instability; there are no stellar-origin black holes in the forbidden zone because stars are undergoing pair-instability supernovae," said Hui Tong, the study's lead author and a Ph.D. candidate at Monash University, in a statement published in Phys.org. "The only black holes in this mass range are made from merging smaller black holes, rather than directly from stars."

That distinction matters enormously for how we think the most massive stars live and die.

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The team combed through 153 binary black hole systems detected by LIGO and Virgo and carefully separated out "first-generation" black holes, i.e., those born directly from stellar collapse, from "second-generation" ones built through earlier mergers. The gap showed up clearly in the secondary masses of binary systems (the smaller partner in each merging pair), with an absence of stellar-origin black holes between roughly 44 and 116 solar masses.

But as with most things in astrophysics, the picture isn’t perfectly clean. In fact, a small number of black holes do appear inside the forbidden range, most likely products of successive mergers rather than single-star collapse. And a blockbuster detection from November 2023, dubbed GW231123, found two black holes with masses near or within the gap merging to form a behemoth 225 times the mass of the sun.

Study co-author Maya Fishbach, an astrophysicist at the University of Toronto, put it more bluntly.

“We are seeing indirect evidence of one of the most titanic blasts in the cosmos: pair‑instability supernovae. At the same time, we are finding that once they are born, black holes can grow via repeated mergers.”

Taken together, the missing black holes and the few that sneak into the gap tell the same story: some of the universe’s biggest stars die so violently they erase themselves, and the only way to break their rule is to build new black holes out of the wreckage.

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