Imagine a bridge with its middle ripped out: cars on the other side would be a surprise. That's how astronomers long viewed black holes with masses from 50 to 130 solar masses: the stars that could produce them explode as pair-instability supernovae. The mechanism is this: the star's core heats up so much that energetic photons start creating particle-antiparticle pairs; radiation pressure drops catastrophically, and the star is completely destroyed, leaving no compact remnant. This gap in the black hole mass spectrum, known as the 'mass gap,' was long considered a forbidden zone.

However, the latest catalog of gravitational waves, ripples in spacetime detected by LIGO and Virgo during mergers of massive objects, brought a surprise. It contains black holes with masses of 50–70 solar masses, right inside the gap. Moreover, these objects spin suspiciously slowly for those born from the merger of two smaller holes. The question arises: how did they form?

The answer lies in the rate of the nuclear reaction between helium and carbon in the interiors of massive stars. Back in the last century, Fred Hoyle and Hans Bethe showed that this microscopic rate literally dictates the fate of stellar giants: it determines at what mass the star's core becomes hot enough to trigger pair production. New laboratory measurements of this reaction show it proceeds slightly faster than previously assumed. This shifts the mass gap boundary downward, to about 68.5 solar masses. As a result, some stars that were thought to be completely destroyed can now survive the helium burning phase and quietly collapse into black holes with masses in the previously forbidden range.

This refinement explains not only the appearance of 'wrong' black holes, but also another puzzle: the rarity of bright hydrogen-poor supernovae. If a star doesn't explode completely but collapses directly into a black abyss, it simply doesn't produce a bright flash. The bridge analogy still holds, but now we know: the nuclear engineers have slightly moved the supports.