Astronomers have, for the first time, observed a massive star collapsing directly into a black hole without undergoing a supernova explosion – a phenomenon theorized to occur but rarely witnessed. The discovery, made in the Andromeda galaxy, offers a unique glimpse into the final moments of a star and challenges existing understandings of stellar evolution.
The star, designated M31-2014-DS1, was initially identified in 2014 by NASA’s Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) as a source of increasing infrared light. A team of astronomers, while sifting through NEOWISE data for variable sources, uncovered the unusual behavior of this supergiant star. Over a two-year period, the star’s mid-infrared flux increased by 50%, followed by a year of fading, and continued to diminish in brightness through 2022.
The findings, published in the journal Science, are detailed in a research paper titled “Disappearance of a massive star in the Andromeda Galaxy due to formation of a black hole.” Kishalay De, an astronomy professor at Columbia University and lead author of the study, described the discovery as “probably the most surprising” of his career. “The evidence of the disappearance of the star was lying in public archival data and nobody noticed for years until we picked it out,” he stated in a press release.
The team analyzed sequential images of Andromeda, taken between and , every six months, searching for luminous transient sources. The observed fading of M31-2014-DS1 was dramatic, with its optical light decreasing by a factor of approximately 100 between and . By , the star was undetectable in ground-based optical observations.
Follow-up observations with the Hubble Space Telescope in confirmed the lack of optical light, revealing only a faint source in the near-infrared. Subsequent spectroscopic analysis with the Keck Observatory in further confirmed the presence of a faint near-infrared source, supporting the conclusion that the star had collapsed into a black hole.
This direct-collapse scenario differs significantly from the more commonly observed fate of massive stars – a supernova explosion. Typically, as a massive star exhausts its nuclear fuel, its core collapses, releasing a flood of neutrinos. These neutrinos drive a shock wave through the star’s outer layers, resulting in a spectacular explosion. However, in the case of M31-2014-DS1, this shock wave appears to have failed, leading to a quiet implosion.
The researchers suggest that the outcome – supernova or direct collapse – depends on the behavior of neutrinos within the star. If the neutrinos are unable to generate a sufficiently powerful shock wave, the star’s outer layers fall back onto the collapsing core, forming a black hole without a dramatic explosion. The original star is estimated to have had approximately 13 times the mass of our sun, shrinking to around 5 solar masses after the collapse, having shed much of its mass through stellar winds.
“Stars with this mass have long been assumed to always explode as supernovae,” explained De. “The fact that it didn’t suggests that stars with the same mass may or may not successfully explode, possibly due to how gravity, gas pressure, and powerful shock waves interact in chaotic ways with each other inside the dying star.”
While This represents only the second direct-collapse black hole candidate observed, it is significantly closer than the first, N6946-BH1, located in the galaxy NGC 6946, approximately 25 million light-years away. N6946-BH1, discovered in , also exhibited a similar pattern of brightening followed by fading without a supernova. However, the greater distance and lower data quality made observations of N6946-BH1 less detailed.
The discovery highlights the challenges in understanding the diverse ways massive stars end their lives. While supernovae are relatively easy to detect due to their intense luminosity, direct-collapse black holes are far more subtle and can easily go unnoticed. The researchers emphasize the importance of large-scale surveys and archival data analysis in uncovering these rare events.
The upcoming Vera C. Rubin Observatory, with its Legacy Survey of Space and Time, is expected to significantly increase the detection rate of these direct-collapse black holes, providing a larger sample for study and furthering our understanding of stellar evolution. “It comes as a shock to know that a massive star basically disappeared (and died) without an explosion and nobody noticed it for more than five years,” De said. “It really impacts our understanding of the inventory of massive stellar deaths in the universe. It says that these things may be quietly happening out there and easily going unnoticed.”
