Fastest star in galaxy got an unusual start

Hurtling through the galaxy at about 1,200 kilometres per second is a compact helium star with an unusual origin. Unlike most high-velocity stars, scientists say this one – thought to be the fastest unbound star in the galaxy – was set in motion not by the black hole at the centre of our galaxy, but by a former companion.

By measuring the velocity, trajectory and rotation of the star, known as US 708, researchers at the European Southern Observatory determined that it started life as half of a close binary pair – two stars that closely orbited one another. Originally, US 708 was a red giant star, and its partner was a white dwarf.

Once the orbit of these two stars began to tighten, helium from the red giant was transferred to the white dwarf, according to a study published in Science.

Eventually, the helium ignited, causing the white dwarf to explode. Both the force of the explosion and the loss of its tether sent US 708 – or what remained of it – hurtling into space.

“Imagine you go to a fair and ride a carousel, and the carousel goes round and round, and then the carousel explodes,” said Stephan Geier, an astronomer at the observatory and lead author of the study. “What happens to you? You go flying away because your seat has such high velocity.”

The researchers decided to investigate US 708 because its trajectory suggested the black hole at the centre of our galaxy, which frequently acts as a slingshot for fast-moving stars, was not its origin point.


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