A white dwarf is one of the last stages of life for stars of a certain size, and it's all downhill from there – with no new source of energy, the object will gradually fade away into obscurity. But supercomputer simulations run by astronomers at Lawrence Livermore National Laboratory (LLNL) suggest that these dying stars could be reignited by a close encounter with an enigmatic class of black hole.

While the largest stars die dramatic deaths, exploding as a supernova and collapsing into a black hole, lower mass stars are a bit more discreet. They throw off their outer layers as a less explosive planetary nebula, leaving behind a white dwarf core. Too small and cool to continue producing fusion reactions, white dwarfs can shine for trillions of years before eventually fading away into a hypothetical black dwarf – a fate that our own Sun faces, in about a quadrillion years.

But perhaps that end isn't inevitable. The LLNL team had a theory that white dwarfs could be kickstarted into producing new fusion reactions (nucleosynthesis) by the extreme forces around intermediate-mass black holes. To put the idea to the test, the researchers used a supercomputer to run relativistic simulations of dozens of variations of the scenario.

The study found that a close encounter with this kind of black hole could indeed fire a white dwarf back up. The tidal forces exerted by the black hole would stretch the star out, creating enough pressure to kickstart the fusion of stellar material into calcium and iron once again. The closer the star passes to the black hole, the more efficient this process is – but, of course, too close and the star won't survive the encounter. The simulations suggested that the optimal distance would be within about two or three times the radius of the black hole, which would fuse up to 60 percent of the material into iron.

"The stretching phenomena can be very complicated," says Rob Hoffman, co-author on the paper. "Imagine a spherical star approaching a black hole. As it approaches the black hole, tidal forces begin to compress the star in a direction perpendicular to the orbital plane, reigniting it. But within the orbital plane, these gravitational forces stretch the star and tear it apart. It's a competing effect."

While these simulations show that the scenario is hypothetically possible, the events should be relatively easy to spot using current technology. So much energy is involved that the encounter would throw off significant waves of electromagnetic signals, such as light, radio waves and gamma rays, as well as gravitational waves.

That's important not just to validate the theory, but as a way to detect intermediate-mass black holes. So far it's only been confirmed that black holes exist as either stellar mass (a few times the mass of the Sun) or supermassive (millions or billions of solar masses), but a middleweight category has long been suspected too. The search is ongoing, and recently one may have given itself away by devouring a star, but generally they're hard to spot. Reigniting a white dwarf could be just the light-show we need to confirm their existence.

"It was exciting to see that the zombie star reignited in each of the close encounter scenarios we looked at," says Peter Anninos, lead author of the study. "But what really captured my imagination was the idea that these energetic events could be visible. If the stars align, so to speak, a zombie star could serve as a homing beacon for a never-before-detected class of black holes."

The research was published in The Astrophysical Journal.

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