A star is racing out of the Milky Way at a blistering 2.6 million miles an hour (4.2 million kilometers an hour), astronomers report Thursday inScience, making it the fastest moving star ever found. Most other hypervelocity stars, as these speedsters are known, have been flung outward by the enormous gravity of the Milky Way's central black hole.

This one, by contrast, was probably launched on its high-speed trajectory by a kind of exploding star known as a Type Ia supernova, one of the most powerful and brightest bursts of energy in the universe. Nobody knows for sure what triggers a Type Ia, however—and the fast-moving star, known as US 708, might provide valuable clues.

Why It Matters

Astronomers would love to know what makes a star explode so violently that the flash can be seen halfway across the universe. Theorists think the process probably starts with a white dwarf, the leftover husk of a star after it swells into a red giant (as our own sun will in about five billion years) and then puffs off its outer layers.

If enough extra matter falls onto the white dwarf—most likely from an ordinary companion star—it can reach critical mass and detonate in a thermonuclear explosion. Or a Type Ia might result when two white dwarfs collide. And thanks to a discovery in 2013, there's a third option. Astronomers found a white dwarf in tight orbit around a hot subdwarf that was rich in helium. And when helium falls onto a white dwarf, a thermonuclear explosion can happen especially easily.

Looking for such a star, Stefan Geier of the European Southern Observatory and colleagues trained one of the powerful Keck telescopes on US 708, a fast-moving, hot subdwarf and found out how astonishingly fast it was moving. They also calculated its trajectory, which was clearly in the wrong direction if it had come from the black hole at the core of the Milky Way—a hint that it could have come instead from a Type Ia explosion. Another hint, says Geier: US 708 rotates very fast, just what you'd expect of a star formerly in a tight gravitational dance with a companion.

The Big Picture

Type Ia supernovae are so bright that astronomers use them to measure the distances to faraway galaxies and to calculate speeds at which those galaxies are flying away from us. Astronomers learned to their astonishment in the late 1990s that this speed has changed: The cosmos is flying apart faster today than it was billions of years ago, pointing to a still unexplained antigravity force known as dark energy.

To understand dark energy, astronomers need to know exactly how the cosmic expansion is changing, which depends on a full understanding of Type Ia's-including exactly what sets a Type Ia supernova off and how much energy comes out of it.

What's Next

Armed with this new information, Geier and his colleagues are headed back to their telescopes to find more fast-fleeing stars and stellar pairs that might be the raw material for future explosions. If they find enough examples, they might get a handle on the step-by-step process that triggers these mammoth explosions.

It could turn out that a fast-moving star helps solve one of the thorniest problems in astrophysics.