Caught in the act: Astronomers watch a vanishing star turn into a black hole

(Press-News.org) Astronomers have watched a dying star fail to explode as a supernova, instead collapsing into a black hole. The remarkable sighting is the most complete observational record ever made of a star’s transformation into a black hole, allowing astronomers to construct a comprehensive physical picture of the process.

Combining recent observations of the star with over a decade of archival data, the astronomers confirmed and refined theoretical models of how such massive stars turn into black holes. The team found that the star failed to explode as a supernova at the end of its life; instead, the star’s core collapsed into a black hole, slowly expelling its turbulent outer layers in the process.

The results, published February 12 in Science, are already generating excitement as a rare glimpse into the mysterious origins of black holes. The discovery will help explain why some massive stars turn into black holes when they die, while others don’t.

“This is just the beginning of the story,” says Kishalay De, an associate research scientist at the Simons Foundation’s Flatiron Institute and lead author on the new study. Light from dusty debris surrounding the newborn black hole, he says, “is going to be visible for decades at the sensitivity level of telescopes like the James Webb Space Telescope, because it’s going to continue to fade very slowly. And this may end up being a benchmark for understanding how stellar black holes form in the universe.”

The now-deceased star, called M31-2014-DS1, is located around 2.5 million light-years away from Earth in the neighboring Andromeda Galaxy. De and his collaborators analyzed measurements of the star from NASA’s NEOWISE project and other ground- and space-based telescopes for a period spanning 2005 to 2023. They found that M31-2014-DS1’s infrared light began brightening in 2014. Then in 2016, the star swiftly dimmed far below its original luminosity in barely a year.

Observations in 2022 and 2023 showed that the star essentially vanished in visible and near-infrared light, becoming one ten-thousandth as bright in these wavelengths. Its remnant is now only detectable in mid-infrared light, where it shines at a mere one-tenth as bright as before.

De says, “This star used to be one of the most luminous stars in the Andromeda Galaxy, and now it was nowhere to be seen. Imagine if the star Betelgeuse suddenly disappeared. Everybody would lose their minds! The same kind of thing [was] happening with this star in the Andromeda Galaxy.”

Comparing these observations with theoretical predictions, the researchers concluded that the star’s dramatic fading to such a small fraction of its original total brightness provides strong evidence that its core collapsed and became a black hole.

Stars fuse hydrogen into helium in their cores, and that process generates outward pressure to balance the incessant inward pull of gravity. When a massive star roughly 10 or more times heavier than our sun begins to run out of fuel, the balance between inward and outward forces is disrupted. Gravity begins to collapse the star, and its core succumbs first to form a dense neutron star at the center.

Often, the emission of neutrinos in this process generates a powerful shock wave that is explosive enough to rip apart most of the core and outer layers in a supernova. However, if the neutrino-powered shock wave fails to push the stellar material out, theory has long suggested that most of the stellar material would instead fall back into the neutron star, forming a black hole.

“We’ve known for almost 50 years now that black holes exist,” says De, “yet we are barely scratching the surface of understanding which stars turn into black holes and how they do it.”

The observations and analysis of M31-2014-DS1 enabled the team to reinterpret observations of a similar star, NGC 6946-BH1. This led to an important breakthrough in understanding what had happened to the outer layers that had enveloped the star after it failed to go supernova and collapsed to a black hole. The overlooked element? Convection.

Convection is a byproduct of the vast temperature differences inside the star. Material near the star’s center is extremely hot, while the outer regions are much cooler. This differential causes gases within the star to move from hotter to cooler regions.

When the star’s core collapses, the gas in its outer layers is still moving rapidly due to this convection. Theoretical models developed by astronomers at the Flatiron Institute have shown that this prevents most of the outer layers from falling directly in; instead, the innermost layers orbit outside of the black hole and drive the ejection of the outermost layers of the convective region.

The ejected material cools as it moves farther from the hot material around the black hole. This cool material readily forms dust as atoms and molecules combine. The dust obscures the hot gas orbiting the black hole, warming the dust and producing an observable brightening in infrared wavelengths. This lingering red glow is visible for decades after the star itself disappears.

Co-author and Flatiron Research Fellow Andrea Antoni previously developed the theoretical predictions for these convection models. With the striking observational evidence from M31-2014-DS1, she says, “the accretion rate — the rate of material falling in — is much slower than if the star imploded directly in. This convective material has angular momentum, so it circularizes around the black hole. Instead of taking months or a year to fall in, it’s taking decades. And because of all this, it becomes a brighter source than it would be otherwise, and we observe a long delay in the dimming of the original star.”

Similar to water swirling around a bathtub drain rather than flowing straight down, the gas in motion around this newly formed black hole continues in its chaotic orbit even as it’s slowly pulled inward. Thus, the halted infall generated by convection prevents the entire star from collapsing directly into the newborn black hole. Instead, the researchers propose that even after the core promptly implodes, part of the outflowing material slowly falls back over many decades.

Only about one percent of the original stellar envelope gas falls into the black hole, powering the light that emanates from it today, the researchers estimate.

While parsing the observations of M31-2014-DS1, De and his team also reevaluated a similar star, NGC 6946-BH1, categorized 10 years ago. In the new paper, they present striking evidence explaining why this star followed a similar pattern. M31-2014-DS1 initially stood out as an “oddball,” De says, yet it now appears to be just one member in a class of objects — including NGC 6946-BH1.

“It’s only with these individual jewels of discovery that we start putting together a picture like this,” De says.

About the Flatiron Institute

The Flatiron Institute is the research division of the Simons Foundation. The institute's mission is to advance scientific research through computational methods, including data analysis, theory, modeling and simulation. The institute's Center for Computational Astrophysics creates new computational frameworks that allow scientists to analyze big astronomical datasets and to understand complex, multi-scale physics in a cosmological context.

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