An international team of astrophysicists has found evidence that the universe recycles black holes. By merging them to form even larger entities, these “second-generation” black holes challenge conventional stellar physics.

The evolutionary theory of stars states that at the end of a massive star’s life, its core compresses until space-time curves infinitely, forming classic black holes with masses 10 to 40 times that of our sun. Supermassive black holes, found in galactic centres, are millions or billions of solar masses and have origins linked to early cosmic processes.

Between these extremes lies a controversial category: black holes with masses between 40 and 100 solar masses. These black holes are too heavy to form from the collapse of a single star but not massive enough for their creation via giant matter clouds, making them “impossible” under conventional models.

Astrophysicists propose that these massive black holes could form through the merging of smaller, ultradense objects. The idea was plausible, but required evidence until recent advances in gravitational wave detectors allowed researchers to identify a statistical signature: heavier black holes, spinning rapidly and chaotically—a sign of previous collisions.

This study shows that the heaviest black holes are built rather than born, arising from previous generations of collisions in dense stellar clusters. So far, these “impossible” black holes remain undetected but their vibrations reveal masses beyond what current physics can explain.