Massive Star Collapses Directly into Black Hole in Andromeda, Confirming "Failed Supernova"
Astronomers have observed a massive star collapsing directly into a black hole without undergoing a supernova explosion. This event, located in the Andromeda Galaxy, provides detailed observational evidence for a "failed supernova" scenario, contributing to a more comprehensive understanding of stellar black hole formation. The findings were published in the journal Science.
Discovery and Initial Observations
The observed star, designated M31-2014-DS1, is located approximately 2.5 million light-years away in the Andromeda Galaxy. Researchers, led by Kishalay De of the Simons Foundation's Flatiron Institute, analyzed over a decade of archived observations combined with new telescope data.
Data collected between 2005 and 2023 from instruments including NASA's NEOWISE mission, the Hubble Space Telescope, and the James Webb Space Telescope were utilized. The star, previously visible with small backyard telescopes and once among the brightest in its galaxy, began to brighten in infrared light around 2014. By 2016, its brightness sharply decreased.
By 2022 and 2023, its brightness in visible and near-infrared wavelengths had decreased to one ten-thousandth of its previous intensity. It is now primarily detectable in mid-infrared light, glowing at approximately one-tenth of its original intensity and is undetectable by most sensitive telescopes in optical light.
This significant and prolonged dimming suggests a core collapse.
Stellar Collapse Mechanism
Stars maintain luminosity through nuclear fusion, which generates outward pressure counteracting gravitational forces. In stars at least 10 times more massive than the Sun, this balance destabilizes when nuclear fuel is depleted. Gravity then overwhelms outward pressure, causing the core to collapse.
Typically, this collapse leads to the formation of a neutron star, and a flood of neutrinos can generate a powerful shock wave that expels the star's outer layers in a supernova explosion. However, in the case of M31-2014-DS1, the shock wave from the core collapse was insufficient to detonate the star, a phenomenon termed a "failed supernova." Theoretical models propose that if the shock wave is too weak, much of the star's material falls back inward, transforming the neutron star into a black hole.
Convection, driven by large temperature differences within the star, played a role in the aftermath. This churning motion is believed to have prevented most of the outer material from directly plunging into the black hole, instead pushing the outermost layers outward while some inner layers orbited the nascent black hole.
As the expelled material cooled, atoms and molecules formed dust, which blocks light from hotter gas near the black hole, absorbs energy, and reemits it in infrared wavelengths, contributing to a lingering glow.
Characteristics of M31-2014-DS1 and its Remnant
Before collapse, M31-2014-DS1 had an estimated mass of at least 13 times that of the Sun. The star is estimated to have lost approximately 60% of its mass over 15 million years through stellar winds prior to the collapse. The resulting black hole is estimated to have a mass five times that of the Sun.
Andrea Antoni, a co-author, noted that the accretion rate of material falling into the black hole is slower than a direct implosion. This is attributed to the angular momentum of the convective material, causing it to circularize around the black hole over decades rather than months or a year. This delayed infall contributes to the observed prolonged dimming.
Researchers estimate that only about one percent of the star's original outer envelope ultimately accretes onto the black hole. The faint infrared glow from residual stellar material is expected to continue fading over decades.
Broader Implications and Ongoing Research
This observation provides evidence for black hole formation without a supernova explosion and suggests that stars with masses as low as 13 times that of the Sun can form black holes through this process. The findings may help clarify why some massive stars explode at the end of their lives while others collapse without a significant outward explosion.
Researchers re-examined a similar object, NGC 6946-BH1, identified a decade prior, and the new study suggests both stars followed a similar evolutionary path, indicating a broader category of "failed supernovae." Kishalay De stated that light from dusty debris surrounding the newborn black hole is expected to remain visible for decades, potentially serving as a benchmark for understanding stellar black hole formation.
While observational evidence of stellar transformation into black holes has been limited, scientists are investigating the frequency of this quiet black hole formation, having identified another potential instance. Theoretical uncertainties currently exist regarding the proportion of massive star core collapses that result in black hole formation, according to Andrea Antoni.
Some astronomers have proposed an alternative explanation for vanishing stars, suggesting they could be merging binary stars whose combined light then becomes obscured by a dust disk. Further long-term observations are anticipated to provide clarity, with the distinguishing factor for a black hole formation scenario being the star's eventual complete fading into darkness.