Speeding star sighted from here (Image: Andrew Cooper/Keck Observatory)

If a thermonuclear explosion went off next door, you’d run away too. The fastest known free-flying star in the galaxy is hightailing it out of here at 1200 kilometres a second after surviving its sibling star’s death as a massive supernova. Clocking the star’s speed could pin down the nature of this stellar explosion and shed light on the cosmic ruler by which we measure the universe.

Known as US 708, the star was first discovered around 10 years ago and appears to be the remnant of a red giant that has been stripped of all its hydrogen, leaving behind a dense, hot core of helium that is around a third of the mass of the sun.

Previous observations of the star revealed its radial velocity, the speed at which it travels across the 2D plane of the sky. Now Stephan Geier of the European Southern Observatory in Garching, Germany, and his colleagues have measured its full 3D speed, relative to the centre of the galaxy.


The team used the Keck Observatory on Mauna Kea in Hawaii to analyse the light from US 708, and found its full speed to be a little under 1200 kilometres a second. They also discovered it was spinning quickly, at over 100 kilometres a second, suggesting it received an energetic boost in its past.

Broken pair

All previously discovered hypervelocity stars – those going fast enough to escape the gravitational clutches of the Milky Way – are slower than US 708, and more similar in mass and temperature to our sun. These stars are thought to have originally been part of pairs that got too close to the supermassive black hole at the centre of our galaxy. One star got stuck orbiting the black hole, while the other was flung out at high speed.

But that explanation doesn’t work for US 708: something must have stolen its hydrogen before it was sped up. This could only happen if it was in a very close pair with another star – so close that even a black hole couldn’t tear them apart.

Now Geier thinks he knows what happened. “The only way to get rid of the companion is a thermonuclear supernova,” he says. The idea is that US 708 was once in a bound pair with a white dwarf star that might have stolen all its hydrogen, leaving a helium core. The white dwarf, still not satisfied, then began sucking up helium until it became destabilised to the point of ignition, causing a massive supernova.

It was actually the breaking of the pair that propelled the helium star, not the explosion itself – a bit like when you hold hands with someone, spin in a circle and let go. “It’s not a shockwave, it’s the mere fact that this star is unleashed from this very tight binary,” says Geier. US 708’s fast spin also supports this explanation, he says.

If the theory is correct, finding more fast stars like US 708 could provide a way to study this kind of supernova. The scenario is a variant of an important process called a type Ia supernova. These are thought to always occur in one of a few ways, meaning the resulting explosions all have the same brightness.

Astronomers can use these “standard candles” to measure distances in the universe, but any variations in their origins could change the cosmic ruler, so it needs checking out. “If we can really prove this scenario, then we have a means to study those explosions in a new and very interesting way,” says Geier.

Journal reference: Science, DOI: 10.1126/science.1259063