Milky Way's fastest stars are 'runaways' from another galaxy

The fastest-moving stars in our galaxy - which are travelling so swiftly that they can escape the Milky Way - are in fact 'runaways' from a much smaller galaxy orbiting around our own, scientists say.
The researchers, from the University of Cambridge in the UK, showed that these stellar sprinters originated in the Large Magellanic Cloud (LMC), a dwarf galaxy orbiting around the Milky Way.
These fast-moving stars, known as hypervelocity stars, were able to escape their original home when the explosion of one star in a binary system caused the other to fly off with such speed that it was able to escape the gravity of the LMC and get absorbed into the Milky Way.

Astronomers first thought that the hypervelocity stars, which are large blue stars, may have been expelled from the centre of the Milky Way by a supermassive black hole.
To date, roughly 20 hypervelocity stars have been observed, mostly in the northern hemisphere.
"The hypervelocity stars are mostly found in the Leo and Sextans constellations - we wondered why that is the case," said Douglas Boubert, a PhD student at Cambridge.

An alternative explanation to the origin of hypervelocity stars is that they are runaways from a binary system.
In binary star systems, the closer the two stars are, the faster they orbit one another. If one star explodes as a supernova, it can break up the binary and the remaining star flies off at the speed it was orbiting.

The escaping star is known as a runaway. Runaway stars originating in the Milky Way are not fast enough to be hypervelocity because blue stars can not orbit close enough without the two stars merging. However, a fast-moving galaxy could give rise to these speedy stars.
The LMC is the largest and fastest of the dozens of dwarf galaxies in orbit around the Milky Way. It only has 10 per cent of the mass of the Milky Way, and so the fastest runaways born in this dwarf galaxy can easily escape its gravity.
The LMC flies around the Milky Way at 400 kilometres per second and the speed of these runaway stars is the velocity they were ejected at plus the velocity of the LMC. This is fast enough for them to be the hypervelocity stars.

"This also explains their position in the sky, because the fastest runaways are ejected along the orbit of the LMC towards the constellations of Leo and Sextans," said Rob Izzard, a Rutherford fellow at the Institute of Astronomy.
The researchers used a combination of data from the Sloan Digital Sky Survey and computer simulations to model how hypervelocity stars might escape the LMC and end up in the Milky Way.
They simulated the birth and death of stars in the LMC over the past two billion years and noted every runaway star.
The orbit of the runaway stars after they were kicked out of the LMC was then followed in a second simulation that included the gravity of the LMC and the Milky Way.

These simulations allow the researchers to predict where on the sky we would expect to find runaway stars from the LMC.
"We are the first to simulate the ejection of runaway stars from the LMC - we predict that there are 10,000 runaways spread across the sky," said Boubert.