White dwarf stars, like the ones glittering in this image, can violently explode. The question is, what triggers the explosion?
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A New Way to Explode a Star?

Among the most contentious unsolved mysteries in astronomy is the question of how, exactly, a white dwarf star explodes.  Now, as described at the American Astronomical Society’s winter meeting, a team of scientists has come up with an idea that just might solve part of the problem.

Nearly two decades ago, scientists used these exploding stars to measure cosmic distances and came up with a surprising result: The rate at which the universe is flying apart is increasing. Calling that discovery profound is no overstatement – it transformed our understanding of the cosmos and pointed toward the existence of an enigmatic force called dark energy.

But scientists still don’t thoroughly understand how to blow up a white dwarf. It’s kind of like understanding the point of a story without being fluent in the language. The plot is discernible, but important subtleties are lost. Learning how those subtleties shape the final story is crucial if scientists want to understand how dark energy behaves over the lifetime of the universe.

“Say, for example, an exploding white dwarf happens differently in the early universe than it does now. Or it happens differently in different types of galaxies,” says Rosanne Di Stefano, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. “If we don’t understand the differences, then we can’t correct for them and make a more precise ruler.”

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The remnant of a type 1a supernova that exploded in the year 1006. (NASA/CXC/et al. Click for full credit.

The mile-marking explosions are called type 1a supernovas. They’re the result of a runaway thermonuclear reaction that rips through a white dwarf star and blows its guts into space, transforming the dwarf from a boring point of light into a beacon that shines brighter than entire galaxies. Because those beacons blaze with a predictable brightness that diminishes with distance, scientists can figure out how far away they are.

But it’s the part before the explosion that has everyone so perplexed. A lonely white dwarf won’t spontaneously combust. However, when a dwarf lives with a companion star, its enormous gravity means that it sometimes steals material from its companion. Like an out-of-control shoplifter, the dwarf star will continue stealing until its mass exceeds a threshold beyond which physics can’t keep the star in one piece. Then, boom.

What scientists have been arguing about is the identity of the dwarf’s stellar companion. Opinions have shifted dramatically over the last few decades, with some favoring a large, gassy companion star like a red giant, and others favoring something smaller and denser, perhaps another white dwarf. In the last five years, observational evidence has made it clear that both of those scenarios can happen.

The trouble is, the two recipes don’t account for the frequency of type 1a supernovas. “For some reason, we’re underpredicting them,” Di Stefano says.

So, she and her colleagues began to look for other ways to cook a type 1a.

What if, Di Stefano asks, some white dwarfs don’t need starry companions? Could a passing rocky body – like a large asteroid or a planet – smash into and detonate a dwarf?

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Artist’s impression of an exo-asteroid falling into a white dwarf. (NASA/ESA/G. Bacon)

The answer, she says, is potentially yes. There’s already a pile of observational evidence suggesting that collisions between white dwarfs and rocky bodies do happen. While it might sound strange, those observations aren’t necessarily unexpected.

White dwarfs are the corpses of collapsed stars that were once very much like the sun. Like the sun, many of them probably have asteroids and other rocky debris lying around that never quite made it to planethood. In our solar system, we see this debris in the asteroid and Kuiper belts, and suspect it’s also in the Oort Cloud (though it hasn’t been directly observed). Many of these bits and pieces are far enough out that they will survive the collapse of their stars unscathed, unlike any planets that happen to be close to their stars.

So, Di Stefano reasons, more than a few white dwarfs are probably flying through the galaxy surrounded by clouds of rocky debris. She calls these clouds “balls of planetoidal gas,” and notes that they can extend up to 100,000 times farther from a star than Earth is from the sun. Every now and then, and especially in crowded places like the galactic bulge, a dwarf will come close enough to another star for its gravity to perturb the rocky bits and pieces.

“These balls, every once in a while, they’ll graze each other. They’ll overlap with each other, and that leads to consequences,” Di Stefano explains.Some of the planetoids, particularly the asteroids and comets, are thrown out into interstellar space, and some of them are placed on orbits where they’re going to end up coming close to the white dwarf.”

Simulations suggest that maybe 1 percent of those disruptions will lead to collisions with one of the stars.

“The idea that occasionally, not necessarily a planet but a large asteroid will impact white dwarfs? That has to happen,” says astronomer Ryan Foley of the University of Illinois, Urbana-Champaign, noting observations of rocky signatures polluting the otherwise pristine atmospheres of white dwarfs. “The real question is, what happens after?”

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Type 1a supernova 1994D exploded near the outskirts of galaxy NGC 4526. (NASA/ESA/Hubble/High-Z Supernova Search Team)

If the conditions are just right – the collisions energetic enough, the rocky hammer big enough, the dwarf’s history of similar encounters optimal – it’s possible the impact could explode the star. Calculations suggest you’d need something at least as big as Earth to trigger an explosion, but this doesn’t have to happen particularly often to help solve the type 1a supernova frequency problem.

“Even if there’s an explosion one of out every 10 million times, it would actually be a significant contribution to the rate of type 1a supernovas,” Di Stefano says. “It’s worth exploring. And even if they don’t make the white dwarf explode, they’ll do something interesting that we should be able to see.”

Foley likes the idea of exploding a dwarf with a rocky body, and calls it a creative way to address an abiding problem. “The first part seems to be true — it should happen,” he says. “And then the second part just needs to be nailed down. It’s not crazy.”

For more from the AAS Meeting, see this meeting report.