Disappearance of a massive star in the Andromeda Galaxy due to formation of a black hole.

When a massive star reaches the end of its lifetime, its core collapses and releases neutrinos that drive a shock into the outer layers (the stellar envelope). A sufficiently strong shock ejects the envelope, producing a supernova. If the shock fails to eject it, the envelope is predicted to fall back onto the collapsing core, producing a stellar-mass black hole (BH) and causing the star to disappear. We report observations of M31-2014-DS1, a hydrogen-depleted supergiant in the Andromeda Galaxy. In 2014, it brightened in the mid-infrared, then from 2017 to 2022, it faded by factors of ≳ 10 4 in optical light (becoming undetectable) and ≳ 10 in total light. We interpret these observations, and those of a previous event in NGC 6946, as evidence for failed supernovae forming stellar-mass BHs. Editor's summary: In a dying massive star, the core collapses to form a compact object. Simultaneously, the outer layers can be ejected at high speed, which is observed as a supernova. Theory predicts that sometimes the ejection speed is too low to escape the gravitational field, so the outer layers fall back over several years, producing no supernova and a more massive compact object. De et al. have identified a supergiant star in the nearby Andromeda Galaxy that brightened in the infrared then decreased in optical brightness over the next few years until it became undetectable. They attribute this behavior to a failed supernova and the formation of a black hole. —Keith T. Smith [ABSTRACT FROM AUTHOR]