Astronomers have documented the most comprehensive set of observations yet detailing how a massive star concluded its life without a supernova explosion, instead collapsing directly into a black hole. Rather than a massive burst, its core imploded while the outer layers dispersed gradually.
The subject of this study is the star M31-2014-DS1, found in the neighboring Andromeda galaxy, situated approximately 2.5 million light-years from Earth. A team of astronomers scrutinized data gathered between 2005 and 2023 using both ground-based and space telescopes.
In 2014, the star’s infrared output began to intensify, followed by a dramatic year-long drop in its brightness by 2016—falling significantly below its prior levels. Observations from 2022 and 2023 revealed that the star has almost entirely vanished across the visible and near-infrared spectra, becoming 10,000 times dimmer in these ranges. Its remnant is now detectable only in mid-infrared light, where its luminosity is roughly one-tenth of what it once was.
“This star used to be one of Andromeda’s brightest beacons, and now it’s virtually invisible. Imagine if Betelgeuse suddenly disappeared—it would cause an enormous shock,” explains Kshitij K. Desai, the lead researcher. He notes that an event of comparable magnitude unfolded in the adjacent galaxy.
By aligning these observations with theoretical models, the researchers concluded that such a drastic dimming event is best accounted for by a core collapse leading to black hole formation. In such a scenario, a supernova does not occur, and the majority of the stellar material is not ejected into space. This investigation was instrumental in clarifying what happens to the star’s outer layers under these conditions. The authors highlighted convection—the churning motion of gas driven by temperature differences between the hot center and the cooler envelope—as playing a crucial role. Even after the core’s collapse, the gas continues to mix and rotate vigorously.
The developed models illustrate that this moving gas does not fall directly into the black hole. Instead, it organizes into a rotating disk, which gradually expels some matter outwards. As this material drifts away, it cools, allowing dust to form. This dust absorbs the radiation emitted by the hot gas and then re-radiates that energy in the infrared spectrum.
Co-author Andrea Antonini clarifies: “The rate at which matter falls in is much slower than in a direct collapse scenario. The gas possesses angular momentum, swirls around the black hole, and takes decades, not months, to finally plunge in.” Consequently, the source remains detectable in infrared light for a prolonged period.
The authors estimate that ultimately, only about 1% of the gas from the star’s outer envelope is accreted by the black hole. It is precisely this residual material that sustains the faint infrared glow observable for decades—including observations that could be made with instruments like the James Webb Space Telescope.
The analysis of M31-2014-DS1 has also provided a new framework for interpreting earlier observations of NGC 6946-BH1, an object classified similarly about a decade ago. Both objects are now viewed as belonging to a distinct class of “failed supernovae”—stars that reached the end of their lifecycles without a brilliant explosion.
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