When a dead star meets a giant black hole, something weird can happen. The astronomical meeting can create a zombie.

Upcoming research in the Astrophysical Journal outlines what might happen if a white dwarf encounters an intermediate-mass black hole. Its conclusion: The violent pull of the black hole could, in theory, reignite fusion inside the dead star.

The intermediate-mass black hole in this study contains between 1,000 and 1 million times the mass of the Sun, but this type is little understood because until now they've eluded detection. But like supermassive black holes—the types of black holes at the centers of galaxies—they may be able to pull in vast amounts of material from an unlucky passing star. When this happens, the star gets ripped apart, in what's called a tidal disruption event.

In this case, College of Charleston professor Chris Fragile hypothesized what might happen if a type of dead star called a white dwarf were in the crosshairs. While super-large stars become dense objects like neutron stars or black holes when they die and collapse, ordinary Sun-like stars don't. Instead, they blow out their gas layers, exposing a dense, Earth-sized core made out of degenerate electron matter instead of helium and hydrogen. That leftover is called a white dwarf.

Under most circumstances, there's no way to make a white dwarf a "normal" helium-fusing star again. But if the dwarf encounters an intermediate-mass black hole, weird physics come into play. The white dwarf is stretched and compressed in such a way that nuclear fusion resumes again—before the stellar remnant is jostled back to the land of the dead seconds later.

There are some caveats here. The white dwarf needs to pass close, and an intermediate- mass black hole more massive than 10,000 suns would probably just devour the white dwarf. With a black hole of less mass than 1,000 suns, nothing so extreme will happen.

Such an event has never been witnessed, but future sky surveys could capture such an event in action, so the paper—which relied on computer simulations—serves as a sort of guideline to what to look for. The weird physics afoot show that some dead stars don't necessarily stay that way forever.

College of Charleston

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