Massive Star Vanishes, Forming Black Hole in Rare Observation | NASA NEOWISE Discovery

Astronomers have, for the first time, directly observed a massive star transitioning into a black hole without a supernova explosion – a phenomenon theorized for decades but rarely witnessed. The observation, made possible by combining archival data from NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission with observations from Hubble and ground-based telescopes, provides crucial insights into the lifecycle of massive stars and the formation of black holes.

The star, designated M31-2014-DS1, resides in the Andromeda galaxy, approximately 2.5 million light-years from Earth. Prior to its disappearance, M31-2014-DS1 was a hydrogen-depleted supergiant. In 2014, it exhibited a brightening in mid-infrared wavelengths. However, between 2017 and 2022, the star underwent a dramatic and sustained dimming, becoming undetectable in optical light and significantly fainter in total light – a factor of 10,000 in optical and 10 overall. This fading signaled a fundamental change in the star’s state.

Traditionally, massive stars, those exceeding eight times the mass of our Sun, are expected to end their lives in spectacular supernova explosions. These explosions occur when the star’s core collapses under its own gravity, releasing an immense amount of energy. The resulting supernova can briefly outshine entire galaxies. However, theoretical models have long predicted that some massive stars might bypass the supernova stage, collapsing directly into a black hole. The challenge has been observing this process directly.

“Stars with this mass have long been assumed to always explode as supernovae,” explained Kishalay De, an associate research scientist at the Simons Foundation’s Flatiron Institute and lead author of the study published in Science. “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.”

The team’s analysis of the data revealed a faint, red remnant detectable in the near-infrared, heavily shrouded in dust. This suggests that the star’s outer layers didn’t disperse in a violent explosion, but rather slowly collapsed inward, forming a dust-filled shell around the newly formed black hole. The infrared signature is key, as it indicates the presence of heated dust – a byproduct of the collapse.

A crucial element in understanding this process was identifying the fate of the star’s outer layers. The researchers found that convection, the process of heat transfer through the movement of fluids, played a significant role. Inside massive stars, a large temperature difference exists between the hot core and the cooler outer layers, driving convective currents. When the core collapses, these currents continue to move material, preventing a direct implosion. Instead, the innermost layers orbit the black hole, expelling the outermost layers of the convective region.

“The accretion rate is much slower than if the star imploded directly in,” said Andrea Antoni of the Flatiron Institute. “This convective material has angular momentum, so it circularizes around the black hole.” This slower accretion rate explains the prolonged infrared brightening. Instead of fading quickly after the collapse, the dust heated by the orbiting material continues to glow for decades, providing a detectable signal.

“Instead of taking months or a year to fall in, it’s taking decades,” Antoni added. “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.”

The researchers also identified another massive star, NGC 6946-BH1, that may have undergone a similar fate, further strengthening their findings. The observations of both stars provide a more complete picture of the conditions that lead to direct black hole formation.

This discovery has significant implications for our understanding of black hole populations. If a substantial number of massive stars collapse directly into black holes without supernovae, it would explain why astronomers observe fewer supernovae than predicted based on the number of massive stars in galaxies. It also suggests that stellar-mass black holes may form through a more diverse range of mechanisms than previously thought.

The team anticipates that the lingering infrared glow from M31-2014-DS1 will remain visible for decades, offering a unique opportunity to study the formation of a black hole in real-time. Future observations with telescopes like the James Webb Space Telescope will be crucial for monitoring the evolution of the dust shell and gaining further insights into the physics of this remarkable event. “Light from dusty debris surrounding the newborn black hole…is going to be visible for decades at the sensitivity level of telescopes like the James Webb Space Telescope,” De stated. “And this may end up being a benchmark for understanding how stellar black holes form in the universe.”