The clearest evidence yet of a missing band of black-hole masses has emerged, with the gap beginning near 45 times the Sun’s mass. The research sharpens a long-running mystery about how the biggest stellar black holes form and why some of them seem not to form at all.
In the latest gravitational-wave catalog, one feature stood out: the smaller members of black-hole pairs stopped appearing above about 45 solar masses. Working through those merger signals, Hui Tong at Monash University showed that this missing population marks a real boundary rather than a statistical fluke. The break did not appear on both sides of the binary system, which means the heavier black holes can still enter the forbidden range through earlier mergers. That uneven pattern sets up the next question the paper had to answer, why stars erase one side of the mass spectrum while collisions refill the other.
Stellar models had long predicted a forbidden gap because some giant stars lose pressure and ignite runaway oxygen burning. That collapse can trigger a pair-instability supernova, an explosion that destroys the star completely and leaves no black hole behind. Instead of leaving a black-hole remnant, the star disappears, which creates the missing band researchers expected to see. For decades, that explanation circulated, but until now the sky had not offered a clean population-wide signature.
Because merger pairs are ordered by size, the lighter member offers the cleaner test of ordinary stellar birth. Any black hole recycled from an earlier crash usually shows up as the heavier object, masking the gap on that side. Smaller companions are less likely to be recycled, so their missing masses expose what stars still fail to make. “The only black holes in this mass range are made from merging smaller black holes, rather than directly from stars,” said Tong.
Another clue came from how quickly the black holes were spinning before the mergers finished. Above roughly the same mass where lighter companions disappear, the heavier objects tended to spin faster. That match fit hierarchical mergers, repeat collisions that build a black hole from earlier black-hole unions, better than direct collapse alone. Under that reading, the missing band is not truly empty, it is being partly filled by recycled black holes.
When the team revisited individual mergers, four events stood out as especially likely products of earlier black-hole mergers. Those systems pair a heavier black hole inside the forbidden band with a smaller companion below it. One famous black hole merger event, called GW190521, did not land cleanly in that group because one object may sit beyond the gap instead. That caution keeps the lower edge firm while leaving the far side of the gap much less settled.
This missing range of black hole masses also points back to what happens inside massive stars as they near the end of their lives. By pinning the lower edge near 44 solar masses, the team narrowed the allowed strength of one important nuclear reaction. That reaction helps decide how much oxygen a star builds before catastrophe, which affects whether collapse ends in explosion or remnant. Astronomy rarely measures nuclear physics this way, so the absent black holes now serve as data from inside dead stars.
Early hints of a cutoff vanished when later detections uncovered heavier black holes, so this claim had to survive a harder test. Now, the lower boundary persists even after excluding the catalog’s single most extreme merger, which mostly affects the upper edge. The team ruled out a narrow or missing gap with 99.9 percent confidence, a much stronger statement than past suggestions. Even so, the top of the forbidden zone still leans heavily on one extraordinary event and remains provisional.
Public comments after publication echoed the paper’s central claim and turned a statistical result into plain speech. “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,” Tong said. Behind that simple line is the study’s broader claim: the gap is shaped more by stars being destroyed than by black holes being formed.
Because the gap sits at a known mass scale, future catalogs could use it to help estimate cosmic expansion. The same pattern may also reveal where repeated mergers happen most often, especially in crowded stellar environments. More detections should also show whether spin directions stay randomly arranged, one sign of black holes meeting through chance encounters. If those tests fail, astronomers will need a different explanation for why lighter companions vanish where theory said they should. A missing band, faster spins, and a handful of unusual mergers now point toward the same story about dying giant stars. Another observing run should reveal whether this forbidden zone remains sharp, or whether the pattern softens with more data. The study is published in the journal Nature. Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.
