Small, 2026. DOI: 10.1002/smll.202507934 (About DOIs).
Vanishing star became a black hole
In 2014, NASA’s NEOWISE project picked up a gradual brightening of infrared light coming from a massive star in the Andromeda galaxy, an observation that was confirmed by several other ground- and space-based telescopes. Astronomers kept monitoring the star, so they also noticed when it quickly dimmed in 2016. Once one of the brightest stars in that galaxy, it effectively “vanished” from sight; it would be like Betelgeuse suddenly disappearing. It’s now only detectable in the mid-infrared range.
The obvious explanation was that the star was dying and had collapsed into a black hole, but if so, it didn’t go through the supernova phase that usually occurs with stars of this size. That makes it an intriguing object for further study. After analyzing archival data from NEOWISE, a team of astronomers concluded that this was indeed a case for direct collapse, according to a paper published in the journal Science.
Theoretical work from the 1970s provided a possible explanation. As gravity begins to collapse the star, and the core first forms a dense neutron star, the accompanying burst of neutrinos typically creates a powerful shock wave strong enough to rip apart the core and outer layers, leading to a supernova. But some theorists suggested that the shock wave might not always be powerful enough to expel all that stellar material, which instead falls inward, and the baby neutron star directly collapses into a black hole without ever going supernova.
Convection, it seems, is key. It occurs because the matter near the star’s center is hotter than the outer regions, so the gases move from hotter to cooler regions. The authors of this latest paper suggest that as the core collapses, gas in the outer layers is moving rapidly, which prevents them from falling into the core. The inner layers orbit outside the new black hole and eject the outer layers, which cool and form dust to hide the hot gas still orbiting the black hole. The dust warms in response into mid-infrared wavelengths, giving the object a slight glow that should last for decades.
