'Vanishing' stars may be turning into black holes without going supernova, new study hints

Scientists have discovered strong evidence that some massive stars end their existence with a whimper, not a bang, and sink into a black hole of their own making without the light and fury of a supernova.

To understand why this is important, we must begin with a crash course on stellar evolution. Stars generate energy through nuclear fusion processes in their cores by which they turn hydrogen into helium. When stars with at least eight times the mass of our sun run out of that hydrogen supply, they start fusion reactions involving other elements instead — helium, carbon, oxygen, and so on, until they end up with an inert core of iron that requires more energy be put into the fusion reaction than what it can produce. At this stage, the fusion reactions cease, and the production of energy that holds the star up evaporates. Suddenly, gravity has free reign and causes the core to collapse, while the outer layers of the star rebound off the contracting core and explode outwards — sparking a supernova that, for a few weeks, can sometimes shine brighter than an entire galaxy.

Meanwhile, the collapsing core forms a compact object. This object is often a spinning neutron star called a pulsar — but, under certain conditions, it could be a stellar-mass black hole. This is the standard story of stellar timelines. However, astronomers are now beginning to come around to the idea that some stars that produce black holes may do so without a supernova explosion.

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Researchers have occasionally noticed occurrences of failed supernovas — stars that begin to brighten as though they are about to explode, but which then falter and die away. Elsewhere, studies of old photographic plates as part of the Vanishing And Appearing Objects During a Century of Observations (VASCO) project, led by Beatriz Villarroel, have found dozens of stars on those old plates that simply aren't seen anymore; it's like they have vanished without a trace. 

Could these failed supernovas and vanishing stars be evidence for stars being almost entirely pulled into the black hole they form before they get a chance to explode? Well, maybe, some scientists believe.

"Were one to stand gazing up at a visible star going through a total collapse, it might, just at the right time, be like watching a star suddenly extinguish and disappear from the heavens," said Alejandro Vigna-Gómez of the Max Planck Institute for Astrophysics in Germany in a statement. "Astronomers have actually observed the sudden disappearance of brightly shining stars in recent times."