Astronomers have discovered a new class of 'hypervelocity stars' - solitary stars moving fast enough to escape the gravitational grasp of the Milky Way galaxy.
"These new hypervelocity stars are very different from the ones that have been discovered previously," said Lauren Palladino of Vanderbilt University located in Tennessee, US.
The original hypervelocity stars are large blue stars and appear to have originated from the galactic centre.
"Our new stars are relatively small - about the size of the sun - and the surprising part is that none of them appear to come from the galactic core," Palladino said in a study published in the Astrophysical Journal.
The discovery came as astrophysicists were mapping the Milky Way by calculating the orbits of sun-like stars.
"It's very hard to kick a star out of the galaxy. That means when you trace the star back to its birthplace, it comes from the centre of our galaxy. None of these hypervelocity stars come from the centre, means there is an unexpected new class of hypervelocity star," said Kelly Holley-Bockelmann, assistant professor of astronomy at Vanderbilt.
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Astrophysicists calculate that a star must get a million-plus mile-per-hour kick relative to the motion of the galaxy to reach escape velocity.
They also estimate that the Milky Way's central black hole has a mass equivalent to four million suns, large enough to produce a gravitational force strong enough to accelerate stars to hyper velocities.
"The big question is - what boosted these stars up to such extreme velocities? We are working on that now," added Holley-Bockelmann.