Australian scientists have spotted a star moving at unbelievable speeds – so fast it generates a glowing blue "bow shock" as it races on its journey.

It is moving nearly 2000 kilometres per second, making it the fastest star in our galaxy.

That's despite it being twice the size of our sun, and weighing about 4 octillion tonnes – a four followed by 27 zeroes.

The only thing scientists know of that would be capable of accelerating a star to that speed is the supermassive black hole lurking in the centre of our galaxy.

The star is moving so quickly it is generating a "bow shock" - a bubble of plasma that glows blue. James Josephides / Swinburne

“To accelerate it to that speed, it would be the same amount of energy to accelerate the Earth to nearly the speed of light – it’s a huge amount of energy,” says Macquarie University’s Associate Professor Daniel Zucker, one of the leaders of the team that made the discovery.

“The black hole is the only mechanism we can think of.”

A missing twin

At the centre of the Milky Way lies a black hole. Orbiting around the hole is a vast maelstrom of gas and dust, the spat-out remnants of stars crushed by the dark monster.

These stellar ashes are the perfect fertiliser for new stars. It was there, a few-hundred-million years ago, a pair of stars was born.

An artist impression of the ejection mechanism by the supermassive black hole.

The siblings fell into a tight orbit. They may have even attracted a small brood of planets, summoned by gravity from the dust field.

And then they strayed too close to the black hole. One star was captured, while the other was kicked out.

The physics of how this happens are difficult to describe.

This map shows the projected path of the star out of the Milky Way. Sergey Koposov

“I’m trying to think of a good analogy – this is not something we encounter very often in day-to-day life,” says Dr Dougal Mackey, an Australian National University astronomer who was part of the research team.

Essentially, the black hole's gravity grabs one star in the pair. This transfers a great deal of energy to the other star, kicking it away at extreme speeds.

A chance sighting

The super-fast star was seen, entirely by accident, by the 3.9-metre telescope housed at Siding Spring Observatory in NSW.

The team was tracking the remnants of other, smaller galaxies the Milky Way has consumed in the past.

The best way to do this is to track as many stars as possible and look for clusters of stars moving at the same speed – they are likely to be the remnants of swallowed galaxies.

That is also the best way, as it happens, to look for super-fast stars.

"This popped out to us because it was 10 times as fast as anything else. You see this one strange measurement, and you automatically think it’s a mistake," says Dr Mackey.

The discovery was documented in the Monthly Notices of the Royal Astronomical Society, published on Wednesday.

As part of the find, scientists also plotted the star's eventual course, and found its journey will soon take it past the edge of the Milky Way, where it will roam in the vast space between galaxies.