This is an artist’s impression of runaway stars.
Amanda Smith

Space still holds so many mysteries that keep astronomers searching the skies at night.  The Large Magellanic Cloud (LMC), is a dwarf galaxy that orbits around our Milky Way, that is apparently missing a few of its’ stars.  Galaxies orbiting other galaxies is actually pretty mind blowing, but then you have the possibility of rogue stars that escape from one galaxy and fly through the other at hyper velocity speeds.

Astronomers, from the University of Cambridge, have been studying this mystery using data from the Sloan Digital Sky Survey and computer simulations.  Astronomers have known of these hypervelocity large blue stars for some time, and first theorized they were disintegrating dwarf galaxies or chaotic star clusters or may have been expelled from the center of the Milky Way by a supermassive black hole. But while all three theories helped explain the hyper-speed of these stars, it failed to account for why they only found these ‘shot out of a cannon’ stars in a certain part of the sky.

In the paper published in the Monthly Notices of the Royal Astronomical Society, the Cambridge astronomers state that the hypervelocity stars, are able to escape their original home galaxy when the explosion of one star as it went supernova in a binary pair caused the other to fly off with such speed that it was able to escape the gravity of the LMC and shoot off into the Milky Way.  Escaping stars in this scenario are known as a runaway.  A distinction or categorization of runaway stars versus runaway hypervelocity stars, is that those originating in the Milky Way are not fast enough to be hypervelocity, because these blue stars can’t orbit close enough without the two stars merging. But a fast-moving galaxy like LMC, could give rise to these speedy stars.  The LMC, which only has 10% of the mass of the Milky Way, is the largest and fastest of the dozens of dwarf galaxies in orbit around the Milky Way.

Currently there are approximately 20 hypervelocity stars that have been observed, mostly in the northern hemisphere, but there may be more that can only be observed in the southern hemisphere. Cambridge scientists used computers to simulate the birth and death of stars in the LMC over the past two billion years, and then applied a second simulation that included the gravity of the LMC and the Milky Way. These simulations allow the researchers to project how many and where in the sky to expect to find these hypervelocity stars from the LMC.  Surprisingly the simulations predict there are 10,000 runaways from the LMC scattered about, with perhaps 5,000 of these stars fast enough to also escape the gravity of the Milky Way.