Family Feud: White Dwarf Stars Battle It Out

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Let’s get ready to rumble! Researchers at the University of Warwick, UK, might have heard famed ring announcer Mike Buffer’s signature catchphrase ringing in their ears when they discovered an unusual double-white dwarf star system in which each star appears to be battling for supremacy, stripping each other down to just helium.

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White dwarfs are kind of like the corpses of burnt-out stars — smaller ones, anyway, like our own sun. As the star runs out of nuclear fuel, it expels most of its outer material, forming a nebula. All that’s left is a hot core. Really hot. We’re talking on the order of 100,000 degrees Kelvin.

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So, if there’s no internal pressure from nuclear fusion, what keeps a white dwarf from completely collapsing in on itself in response to gravity? Degenerate matter, that’s what! This is what forms when the electrons in the hot gas of the star are dense enough — pressed very close together to create very high pressure — to fill all the energy levels. When all the energy levels are filled up, gravity can’t compress the degenerate matter anymore, because there’s no more available space. This is what physicists call the Pauli exclusion principle.

The result is an outward pressure that counters the inward pull of gravity. So a white dwarf survives not by nuclear fusion, but by a weird quirk of quantum mechanics. And the more massive the white dwarf, the smaller it will be, because those electrons have to squeeze together that much more densely to keep that extra mass from collapsing inward.

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A white dwarf’s surface gravity is roughly 100,000 times that of Earth, which means that a white dwarf’s atmosphere is usually pure hydrogen or helium, because the heavier atoms in its atmosphere sink, and the lighter ones stay near the surface. The cores, though, are mostly carbon and oxygen — unless you’re talking about a helium white dwarf, in which case it’s an indication that at some point the star lost a lot of mass. Think of it as the stellar equivalent of a crash diet.

If you find two helium white dwarfs locked into a binary system, that becomes very interesting to astronomers. That’s what the Warwick team found lurking in the constellation Leo some 1,140 light-years away: a double white dwarf star system they dubbed CSS 41177, in which both stars are mostly helium.

The cause: a cosmic sibling rivalry. You know how family dynamics work. The larger (more massive) sibling is trying to expand into a red giant star, but then the smaller (less massive) sibling gets jealous and rips off the other star’s outer hydrogen envelope, ruining its chances at greatness — because without that outer hydrogen envelope, the larger star doesn’t have the ability to start fusing helium and become the red giant it always dreamed of being. It becomes a helium white dwarf instead.

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And then the smaller star tries to start the helium-fusing process that would make it a red giant star, leaving its sibling rival in the shadows. Now it’s payback time! The once-larger sibling rips off the outer hydrogen envelope of the smaller star in revenge — even though it has no use for the hydrogen, which just goes to waste.

Now both siblings are helium white dwarfs, doomed to circle each other for, oh, a good 1 billion years or more in an endless cycle of pointless recriminations. But there’s a happy ending — sort of. Eventually, they’ll spiral together and merge, and this will finally start the helium fusion process. They become a hot sub-dwarf, and get a second life, so to speak, for another 100 million years.

That’s right: they’re stronger working together than ripping each other apart. Some siblings pay a fortune in family therapy bills to learn that lesson. In the cosmos, it just works itself out.

Image: University of Warwick