Gravitational waves reveal a forbidden zone for stellar-origin black holes

A team of researchers led by Monash University has found compelling observational evidence for what astrophysicists call the pair-instability gap: a range of masses where stellar-origin black holes are essentially absent. The finding, published in Nature, was made possible not by traditional telescopes, but by the ripples in spacetime themselves — gravitational waves.

When stars are too massive to leave anything behind

At the end of their lives, most massive stars collapse under their own gravity and leave behind a black hole. But stars above a certain mass threshold meet a different fate. They become so hot in their cores that gamma-ray photons begin spontaneously converting into electron-positron pairs — a process that reduces the radiation pressure holding the star up. The result is a runaway collapse followed by an explosion so violent that the star is entirely destroyed, leaving no compact remnant at all. This is a pair-instability supernova.

First predicted theoretically in the 1960s, these events have been notoriously difficult to distinguish from more conventional stellar explosions. The new study changes that by using the black hole population itself as evidence.

A gap written in gravitational waves

By analyzing the distribution of black hole masses detected through the LIGO-Virgo-KAGRA observatory network, the team found a conspicuous absence: black holes with masses greater than roughly 45 solar masses formed directly from stars are extremely rare. The gap is not a measurement artifact — it reflects a genuine physical boundary imposed by pair-instability physics.

«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 project lead Hui Tong, a Ph.D. candidate at Monash University’s School of Physics and Astronomy and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav). «The only black holes in this mass range are made from merging smaller black holes, rather than directly from stars.»

This distinction matters enormously. It means that when we observe a black hole in the forbidden mass zone, we can conclude it did not form from a single collapsing star — it must have grown through successive mergers, accumulating mass over cosmic time.

Nuclear physics encoded in black hole populations

The implications of this study extend well beyond black hole demographics. Because pair-instability supernovae are governed by nuclear physics inside stellar cores, the location of the mass gap encodes information about fundamental physical processes that would otherwise be invisible to observers on Earth.

«It’s a cool result because we are using black holes to learn about the nuclear reactions inside stars,» said Professor Eric Thrane, Chief Investigator at OzGrav.

Professor Maya Fishbach of the University of Toronto and the Canadian Institute for Theoretical Astrophysics (CITA) highlighted the broader significance: gravitational waves are proving to be a remarkable tool not just for studying black holes in isolation, but for tracing the entire life cycle of the most massive objects in the universe. «We are seeing indirect evidence of one of the most titanic blasts in the cosmos: pair-instability supernovae,» Fishbach noted, adding that the observation also shows how black holes can grow through repeated mergers once they are born.

A long-standing question, now closer to resolution

Confirming the pair-instability gap resolves a major outstanding question in stellar astrophysics: what actually happens to the most massive stars? The answer, written in the statistical distribution of gravitational wave events, is that they go out not with a black hole left behind, but with a complete and catastrophic self-destruction — an explosion so thorough that nothing remains.

As gravitational wave detectors grow more sensitive and accumulate more events, the boundaries of the forbidden zone will be mapped with increasing precision, turning the black hole mass spectrum into a direct probe of stellar nuclear physics across cosmic history.

The study is authored by Hui Tong et al. and published in Nature (2026). DOI: 10.1038/s41586-026-10359-0. A preprint is available on arXiv: 0.48550/arxiv.2509.04151.

© 2026 SKYCR.ORG — All rights reserved. Reproduction of this content, in whole or in part, without prior written permission from SKYCR.ORG is prohibited.