The fastest-moving stars in our galaxy come from a much smaller galaxy in orbit around our own, a group of astronomers has found.
The stars, known as hypervelocity stars, are originally from the Large Magellanic Cloud (LMC), a dwarf galaxy in orbit around the Milky Way. To date, roughly 20 hypervelocity stars have been observed, mostly in the northern hemisphere, though it's possible that there are many more that can only be observed in the southern hemisphere.
These fast-moving stars escaped their original home when the explosion of one star in a binary system caused the other to escape the gravity of the LMC and get absorbed into the Milky Way. 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't orbit close enough without the two stars merging. But 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. With only 10% of the mass of the Milky Way, the fastest runaways born in this dwarf galaxy can easily escape its gravity. The LMC flies around the Milky Way at 400 km per second. Like a bullet fired from a moving train, the speed of these runaway stars equals the velocity they were ejected at plus the velocity of the LMC--fast enough for them to be hypervelocity stars.
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. The researchers 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 in the sky we would expect to find runaway stars from the LMC.
--University of Cambridge (www.eurekalert.org/pub_releases/2017-07/uoc-fsi062817.php)