No Supernova Needed: Star in Andromeda Collapsed Directly Into Black Hole, Astronomers Discover

[United States/North America] - [Ekhbary News Agency]

No Supernova Needed: Star in Andromeda Collapsed Directly Into Black Hole, Astronomers Discover

In a remarkable celestial event that challenges our understanding of stellar evolution, astronomers have identified compelling evidence of a massive star in the neighboring Andromeda Galaxy that collapsed directly into a black hole, omitting the violent supernova explosion typically associated with the death of such stars. This rare phenomenon, long predicted by theory but scarce in observational proof, was found hidden within archival data from NASA's Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) mission.

The star, designated M31-2014-DS1, was first observed by NEOWISE in 2014 exhibiting an increase in its infrared luminosity. However, these crucial observations remained unnoticed in the vast dataset until a team of researchers recently sifted through NEOWISE's data, specifically looking for variable sources. Their meticulous work led to the identification of M31-2014-DS1, a supergiant star whose peculiar behavior suggests a direct collapse into a black hole.

The findings, published in the prestigious journal *Science* under the title "Disappearance of a massive star in the Andromeda Galaxy due to formation of a black hole," are the result of a study led by Kishalay De, an astronomy professor at Columbia University. The research team examined sequential images of the Andromeda Galaxy (M31), taken every six months between 2009 and 2022, searching for transient events. They discovered that M31-2014-DS1 showed a significant increase in its mid-infrared flux—a 50% rise over a two-year period starting in 2014. Intriguingly, after this period of brightening, the star began to fade dramatically, dimming below its initial brightness within a year and continuing its decline until 2022.

"This has probably been the most surprising discovery of my life," lead author De stated in a press release. "The evidence of the disappearance of the star was lying in public archival data and nobody noticed for years until we picked it out."

Further analysis involved leveraging data from other ground-based and space telescopes, including the Hubble Space Telescope. Optical light curves retrieved for the object revealed a drastic dimming by a factor of approximately 100 between 2016 and 2019. By 2023, the object was undetectable in ground-based optical observations. Hubble's imaging in 2022 showed no optical source, only a faint near-infrared (NIR) signal. Follow-up observations with the Keck Observatory in 2023 confirmed this faint NIR source.

This unusual and sustained fading strongly suggests that a supernova failed to occur. According to the research paper, the fate of massive stars hinges on a delicate balance involving neutrino interactions during core collapse. When a massive star exhausts its nuclear fuel, its core implodes under gravity. This collapse triggers the release of a massive burst of neutrinos, which are expected to drive a shockwave outward, causing the star's outer layers to be ejected in a supernova explosion.

However, if the outward-propagating shockwave is insufficiently strong, the star's outer envelope fails to be ejected and instead falls back onto the collapsing core. This scenario is predicted to result in the formation of a stellar-mass black hole and the star's complete disappearance from view, a process termed "direct collapse."

The progenitor star of M31-2014-DS1 initially had a mass of about 13 solar masses. By the time of its demise, its mass had reduced to approximately 5 solar masses, with most of the lost mass likely expelled through powerful stellar winds over its lifetime. The assumption that stars of this mass invariably explode as supernovae is now being questioned, as De noted, "Stars with this mass have long been assumed to always explode as supernovae. 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.”

Another candidate for a direct-collapse black hole, N6946-BH1, was observed in 2010 in the spiral galaxy NGC 6946, approximately 25 million light-years away. While similar in its behavior—a period of brightening followed by fading—N6946-BH1 is significantly more distant, making its observational data less detailed than that of M31-2014-DS1. This new discovery revitalizes interest in N6946-BH1 and strengthens the case for direct collapse events.

"We've known that black holes must come from stars. With these two new events, we're getting to watch it happen, and are learning a huge amount about how that process works along the way," commented Morgan MacLeod, a lecturer on astronomy at Harvard and a co-author of the paper.

The discovery of M31-2014-DS1 represents a significant achievement, stemming from the largest study ever conducted on variable infrared sources. The rarity of these events, contrasted with the conspicuous nature of supernovae, makes their detection exceedingly difficult. "Unlike finding supernovae which is easy because the supernova outshines its entire galaxy for a few weeks, finding individual stars that disappear without producing an explosion is remarkably difficult," De explained. The stealthy nature of these stellar disappearances means many more could be lurking undetected in astronomical archives. Future observations, particularly from facilities like the Vera C. Rubin Observatory, are expected to uncover more such events, significantly advancing our understanding of massive stellar deaths and black hole formation.