When an international team of scientists, led by Monash University, working with the global network of gravitational-wave detectors measuring tiny ripples in spacetime, recently examined the masses of merging black holes, they noticed something strange – a gap where black holes should exist, but don’t.

The absence points to one of the most extreme phenomena in astrophysics – stars so massive that they don’t collapse into black holes at all, but instead completely destroy themselves in cataclysmic explosions.

These missing black holes raise a simple question: What happened to the stars that should have formed them?

The team’s new research is now published in Nature. The project lead is Monash’s Hui Tong, a PhD candidate from the School of Physics and Astronomy and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav). Major collaborators on the team of 16  are Professor Maya Fishbach, from the University of Toronto and the Canadian Institute for Theoretical Astrophysics  (CITA), and Monash’s Professor Eric Thrane, Chief Investigator at OzGrav.

“It’s a cool result,” says Professor Thrane, “because we’re using black holes to learn about the nuclear reactions inside stars.”

Listening to the ‘graveyard’ of stars

The discovery comes from observations made by the LIGO–Virgo–KAGRA Collaboration, which detects gravitational waves – tiny ripples in spacetime produced when massive objects such as black holes collide.

These signals allow astronomers to measure the masses of black holes. Over time, hundreds of detections have built up a kind of “cosmic census” of black holes, revealing patterns in how they form.

And in that census, a curious feature has emerged.

Black holes more than 45 times the mass of the sun are unexpectedly rare. This “forbidden range” suggests that something is preventing stars in this mass range from leaving behind black holes.

When stars become bombs

The explanation may lie in a delicate balance deep inside massive stars.

Throughout their lives, stars are held up by nuclear reactions pushing outward against the force of gravity pulling in. For a large fraction of massive stars, gravity eventually wins – the core collapses,  leaving behind a black hole.

But for some very massive stars, the ending is different.

At extreme temperatures present in those massive stars, light itself begins to transform into matter – pairs of electrons and anti-electrons (called “positrons”). This process robs the star of pressure, causing it to collapse. But instead of forming a black hole, the collapse dramatically heats up the star, triggering a runaway thermonuclear explosion.

The result is a pair-instability supernova, a blast so powerful that the entire star is obliterated.

Crucially, nothing is left behind. No remnant. No black hole.

A gap that tells a story

This is why the missing black holes matter.

If stars in a certain mass range are completely destroyed, then black holes of corresponding masses should be absent. The newly-observed gap fits with this prediction, offering indirect but compelling evidence that pair-instability supernovae are occurring in nature.

It is, in effect, a signature written not in what we see, but in what we don’t.

Black holes that shouldn’t exist

Yet the story doesn’t  end there.

Despite the gap, a small number of black holes are still found within this “forbidden” mass range. Their existence poses another puzzle: If they didn’t form directly from stars, where did they come from?

One leading idea is that they’re built through mergers. Smaller black holes collide and combine, gradually growing into larger ones that would otherwise be impossible to form in a single step.

In this picture, black holes have a kind of afterlife, continuing to grow long after their parent stars have vanished.

The puzzle is just beginning

The existence of a forbidden mass range, along with black holes that appear to violate it, presents a new challenge for astrophysics.

The questions we and others are asking are: Are current models of stellar evolution complete? How common are these extreme explosions? And how efficiently do black holes grow through mergers?

Future gravitational-wave observations will help answer these questions, as detectors become more sensitive and the catalogue of black hole mergers continues to expand.

For now, one thing is clear – some of the universe’s most massive stars do not quietly collapse into darkness.

They explode, and in doing so, they leave behind a mystery written in the missing black holes they never became.