The Most Complete Portrait Yet of a Star Becoming a Black Hole

For most of its existence, M31-2014-DS1 was one of the Andromeda Galaxy's more conspicuous residents - a massive hydrogen-depleted supergiant radiating at luminosities that placed it among the brightest stars in its neighborhood. Then, between 2014 and 2023, it essentially ceased to exist as a visible object. What replaced it is a faint infrared glow expected to persist for decades, powered by material slowly falling into a newly formed black hole.

Connecting those two endpoints - the luminous star and the dim remnant - required assembling observations spanning 18 years from multiple instruments. A team led by Kishalay De at the Simons Foundation's Flatiron Institute did exactly that, and the result, published February 12 in Science, is the most physically detailed reconstruction of a direct stellar collapse ever made.

The observational record, point by point

De and colleagues analyzed data from NASA's NEOWISE infrared mission alongside ground- and space-based optical and infrared telescopes, covering 2005 through 2023. The timeline that emerged was precise. M31-2014-DS1's infrared brightness began rising in 2014. In 2016, the star dimmed sharply in visible light - dropping far below its original luminosity in less than a year. By 2022 and 2023, observations showed the star had faded to roughly one ten-thousandth of its original brightness in visible and near-infrared wavelengths. In mid-infrared, it persisted at about one-tenth its former output - the lingering heat signature of orbiting dust.

"This star used to be one of the most luminous stars in the Andromeda Galaxy, and now it was nowhere to be seen," De said. The comparison to Betelgeuse - a red supergiant visible to the naked eye from Earth - is apt for conveying the scale: if Betelgeuse did the same thing, it would disappear from the night sky within a human lifetime.

Convection: the overlooked piece of the puzzle

What distinguishes this study from previous work is not just the completeness of the observational record but what it enabled theoretically. Parsing M31-2014-DS1's behavior in detail allowed the team to revisit and resolve a long-standing question: what happens to a star's outer layers after the core collapses but a supernova fails to materialize?

The answer involves convection - the large-scale circulation of hot gas within massive stars driven by the temperature difference between the intensely hot core and the cooler outer regions. When the core collapses, convective motions in the outer envelope do not simply stop. Gas is still moving. Rather than falling straight into the nascent black hole, the innermost convective layers orbit around it, and their angular momentum drives the ejection of the outermost convective layers outward.

Flatiron Research Fellow Andrea Antoni previously developed the theoretical predictions for this convection-driven ejection. The M31-2014-DS1 observations provided a direct test. "The accretion rate - the rate of material falling in - is much slower than if the star imploded directly in," Antoni said. "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."

The gas that orbits and slowly falls inward powers the infrared glow. The gas that is ejected cools as it moves away from the black hole and forms dust - the same dust that obscures the system in visible light and radiates in mid-infrared. The researchers estimate that only about 1% of the original stellar envelope actually accretes onto the black hole; the rest is ejected or remains in orbit. That small fraction is nonetheless enough to sustain an observable signal for the foreseeable future.

What this means for understanding black hole formation

M31-2014-DS1 and the earlier candidate NGC 6946-BH1 now form the nucleus of a small but growing category of directly observed stellar black hole births. The implication is that direct collapse may occur more often than the near-absence of prior detections suggested. Massive stars in the 5-15 solar mass range at time of death - particularly hydrogen-depleted supergiants - may represent a class prone to failed supernovae, though the conditions that determine success or failure of the explosion remain poorly understood.

The James Webb Space Telescope's sensitivity in the mid-infrared makes it well positioned to monitor M31-2014-DS1's ongoing fade and potentially detect similar events in more distant galaxies. As De noted, the dust-enshrouded remnant "is going to be visible for decades at the sensitivity level of telescopes like the James Webb Space Telescope." Each additional measurement narrows the theoretical uncertainties around how stellar collapse produces black holes - and how often it does so quietly.

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