Astronomers get rare look at how black holes form

(CN) — One of the brightest stars in the Andromeda galaxy has disappeared without the explosion astronomers expected. Instead, they say, it collapsed into a black hole.

The event, described Thursday in the journal Science, offers the most complete observational record yet of a star’s transformation into a black hole.

By combining nearly two decades of data, researchers were able to reconstruct what happens when a massive star fails to explode at the end of its life.

Instead of going out in a blaze of glory, researchers say the star’s core collapsed inward while its outer layers were slowly expelled, leaving behind a newborn black hole cloaked in dust.

“This is just the beginning of the story,” said Kishalay De, an associate research scientist at the Simons Foundation’s Flatiron Institute and lead author of the study, in a press release. “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, 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 star, known as M31-2014-DS1, sat about 2.5 million light-years away in the Andromeda Galaxy. De and his colleagues analyzed archival observations from NASA’s NEOWISE project along with other ground- and space-based telescopes, tracking the star from 2005 to 2023.

In 2014, the star’s infrared light began to brighten. Two years later, it dimmed rapidly, dropping far below its original brightness in less than a year. By 2022 and 2023, it had effectively vanished in visible and near-infrared light, shining at just one ten-thousandth of its former brightness in those wavelengths.

Today, it can only be detected in mid-infrared light and is just one-tenth as bright as before.

“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,” De said.

Massive stars normally end their lives in spectacular supernova explosions. As they run out of fuel, gravity overwhelms the outward pressure generated by nuclear fusion, and the core collapses. In many cases, neutrinos emitted during this process power a shock wave that blasts apart the star’s outer layers.

But if that shock wave fails, theory predicts that most of the star’s material falls back inward, forming a black hole.

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

By comparing observations of M31-2014-DS1 with theoretical models, the team concluded that the star’s extreme fading is strong evidence that its core collapsed directly into a black hole.

The new data also helped researchers reinterpret a similar object discovered a decade ago, NGC 6946-BH1. In both cases, convection inside the dying star appears to have played a crucial role.

As a massive star nears the end of its life, hot material near its center rises while cooler material sinks, setting up turbulent motion. When the core collapses, this motion prevents much of the outer material from falling straight in. Instead, gas swirls around the black hole, gradually feeding it over decades. As the ejected material cools, it forms dust that glows in infrared light, explaining the lingering red signal seen long after the star disappeared.

Co-author Andrea Antoni, a Flatiron Research Fellow who developed key theoretical models, said the process slows the collapse dramatically.

“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," Antoni said. "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.”

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