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The story I learned about how a star forms a black hole goes something like this: A star containing about 10 times the mass of the Sun or greater forms, and contracts. Under the intense gravitational pressure, the hydrogen is fused into helium, releasing energy, which creates a back-pressure against gravitational collapse. Once the hydrogen runs out, the star starts fusing helium into heavier elements, and this progression to heavier elements continues until the star has made sufficient iron. Fusion of iron does not release any energy, and therefore, the back-pressure from the process of fusion no longer resists gravitational collapse. Under the pressure, the core is converted into a neutron star, a process which produces a huge number of neutrinos, which in turn produces a powerful shock wave that blows off the outer layers of the star. This explosion is what a supernova consists of. If the core is sufficiently massive, the core will continue to collapse into a black hole. The impression I had was that the process of black hole formation requires a supernova.
Researchers investigating a star in the Andromeda Galaxy that seemed to just wink out report that the star in question collapsed into a black hole without going through a supernova. Because the process of this collapse was not shrouded by the brilliant radiation produced by a supernova, it’s the clearest picture that astronomers have had of black hole formation anyone has yet observed.
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 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.
The former star, catalogued as M31-2014-DS1, had been observed using both ground-based and space-based telescopes between 2005 and 2023. In 2014, the star brightened in the infrared range, but then in 2016, its intensity quickly dimmed. Starting in 2022, visible and near-infrared light essentially disappeared. What remains of the star can only be observed in the mid-infrared.
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."
So why was there no supernova when M31-2014-DS1 became a black hole? The key is the process of convection of heat due to temperature differences between the star’s core and its outer layers. The core, under great pressure, is extraordinarily hot, while the outer layers are much cooler (though we would, of course, regard them to still be very hot). Heat has a natural tendency to migrate to cooler regions, and so hot gases near the core, which, due to their high temperatures, are moving at high velocity, migrate to the outer layers and beyond, where they cool and congeal into dust.
Furher, the outer layers of the star have angular momentum, that is, they are revolving around the core, not unlike water in a sink circling a drain. This circular motion prevents the matter in the outer layers from falling into the nascent black hole at the core. In such a situation, the rate of accretion of the the matter in the outer layers of the dead star into the black hole could take decades rather than the sudden collapse that is seen in a supernova. This newly discovered process is being used to reinterpret other stellar disappearances as being black hole formations without supernovas.
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