Astronomers have caught a massive star in the Andromeda Galaxy quietly dying, collapsing into a black hole without producing a supernova, leaving behind little more than a fading trace. The findings provide some of the strongest evidence yet that so-called "failed supernovae" can produce stellar-mass black holes. Near the end of their lives, massive stars can become unstable and swell in size, producing noticeable changes in brightness over timescales humans can observe. In many cases, these stars die in brilliant supernovae, which are extremely luminous and easy to detect. However, not all dying stars explode. Theory suggests that some massive stars fail to produce a successful explosion. Instead, when the star's core collapses, its outer material falls back inward, forming a black hole. Yet such failed supernovae are difficult to detect because they emit weak energy signatures and appear mainly as stars that simply vanish from view. Using archival, long-term infrared observations from the NEOWISE space mission, Kishalay De and colleagues searched for variable stars in the nearby Andromeda galaxy and discovered an unusual stellar object that briefly brightened but then steadily faded. According to the authors, this star, known as M31-2014-DS1, increased in infrared brightness over roughly two years starting in 2014, but then dimmed and eventually became nearly invisible in optical light by 2022, effectively disappearing. Follow-up observations with Hubble and large ground-based telescopes revealed only a very faint, red remnant detectible in the near-infrared, suggesting the star is now heavily shrouded in dust – a mere shadow of the luminous supergiant it had been just years before. De et al. interpret these observations as evidence for a failed supernova leading to the birth of a stellar-mass black hole.
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