Astronomers have witnessed a rare event in which a dying star failed to explode as a supernova and instead collapsed directly into a black hole. This remarkable observation becomes the most complete observational record to date of a star's transformation into a black hole, allowing astronomers to build a comprehensive physical picture of the process.
The "dead" star, named M31-2014-DS1, is about 2.5 million light-years away from Earth and is located in the nearby Andromeda Galaxy. A research team led by Keesarai De, an associate researcher at the Fratyron Institute of the Simons Foundation, combined recent observational data with more than ten years of archival data to confirm and improve the theoretical model of the transformation of this type of massive star into a black hole. The findings, published in the journal Science, are attracting widespread attention and provide a rare glimpse into the mysterious origins of black holes.
The research team analyzed measurements of the star from NASA's NEOWISE project and other ground-based and space-based telescopes from 2005 to 2023. They found that M31-2014-DS1's infrared light began to brighten in 2014, then quickly dimmed in 2016, falling far below its original luminosity in just one year. Observations in 2022 and 2023 showed that the star basically disappeared in the visible and near-infrared bands, and its brightness dropped to one ten thousandth of its original value. Its remnants are currently detectable only in the mid-infrared, and are only one-tenth as bright as before.
"This star was once one of the brightest stars in the Andromeda Galaxy, but now it is nowhere to be found. Imagine if Betelgeuse suddenly disappeared, everyone would go crazy! The same thing happened to this star in the Andromeda Galaxy," De said. After comparing these observational data with theoretical predictions, the researchers concluded that the star's dramatic dimming to a very small fraction of its original total brightness provided strong evidence that its core collapsed and formed a black hole.
Stars fuse hydrogen into helium in their cores, a process that creates outward pressure to balance gravity's constant inward pull. When massive stars, about 10 times the mass of the Sun or more, begin to run out of fuel, the balance between inward and outward forces is disrupted. Gravity begins to compress the star, and its core first yields to form the dense neutron star at the center. Normally, the release of neutrinos during this process creates a shock wave powerful enough to tear apart much of the core and outer layers in a supernova explosion. However, if the neutrino-driven shock wave fails to push the star's material out, theory has long held that most of the star's material will fall back into the neutron star, forming a black hole.
Observations and analysis of M31-2014-DS1 allowed the team to reinterpret observations of the similar star NGC 6946-BH1, which led to an important breakthrough in understanding what happens to the star's outer layers after it fails to explode as a supernova and collapses into a black hole. The key element that is overlooked is convection. Convection is a byproduct of huge temperature differences within a star. Material near the star's center is extremely hot, while the outer regions are much cooler. This temperature difference causes gas within the star to move from hotter to cooler areas.
As the star's core collapses, the gas in its outer layers is still moving rapidly due to convection. Theoretical models developed by astronomers at the Fratiron Institute show that this prevents most of the outer layers from falling directly into the black hole; instead, the innermost layers orbit around the black hole's periphery and drive the ejection of the outermost layers in the convection zone. Co-author Andrea Antoni, a Fratilone researcher, previously developed the theoretical predictions for these convection models. "The accretion rate - the rate at which matter falls in - is much slower than the direct implosion of the star," she said. "This convective material has angular momentum, so it orbits the black hole. Instead of falling in in months or a year, it takes decades. Because of this, it becomes a much brighter source than it would otherwise be, and we observe long delays in the dimming of the original star."
Similar to water swirling around a bathtub drain instead of flowing straight down, the gas moving around this newly formed black hole continues its chaotic orbit even as it is slowly pulled inward. Therefore, the stagnant infall caused by convection prevents the entire star from collapsing directly into the nascent black hole. Researchers estimate that only about one percent of the original star's envelope gas fell into the black hole, powering the light it emits today.
When analyzing the observational data of M31-2014-DS1, De and his team also re-evaluated the similar star NGC 6946-BH1, which was classified 10 years ago. In the new paper, they present strong evidence for why this star follows a similar pattern. M31-2014-DS1 initially seemed like a "freak," but now it appears to be just one of a class of objects that includes NGC 6946-BH1, De said. "Only through these individual discovery gems are we starting to piece together the picture," De said. "The light from the dust debris surrounding a newborn black hole will be visible for decades to come at the sensitivity levels of telescopes like the James Webb Space Telescope as it continues to decay very slowly. This could ultimately become a benchmark for understanding how stellar black holes form in the universe."