Milky Way’s Fastest-Moving Stars Were Absorbed From Another Galaxy That Orbits Our Own

The Milky Way is home to some extremely fast-moving stars that are traveling at speeds fast enough to escape the galaxy, and they are observed only in a specific part of the northern hemisphere sky. A new study suggests these stars were actually a part of another galaxy, and were later absorbed by ours.

Known as hypervelocity stars, they were once a part of the Large Magellanic Cloud — a dwarf galaxy that orbits the Milky Way. Researchers from the University of Cambridge in the United Kingdom applied computer simulations to data from the Sloan Digital Sky Survey, and showed in a paper that those stars escaped “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.”

Read: Hubble Spots Possible Remains Of Supernova Explosion Survivor In Large Magellanic Cloud

About 20 of these hypervelocity stars — all of them large blue stars — have been observed so far, mostly in the northern hemisphere, but it is possible there are others that can be seen only from the southern hemisphere. These massive blue stars collapse into neutron stars or black holes once they run out of fuel.

There have however, been other explanations for the presence of these stars in the Milky Way. Astronomers thought they could have been kicked out of the galactic center by the supermassive black hole that sits there. Others also suggested disintegration of star clusters or dwarf galaxies as possible sources, but none of the explanations could account for the specific location of these stars.

Douglas Boubert, a doctoral student at Cambridge’s Institute of Astronomy and the paper’s lead author, said in a statement Tuesday: “Earlier explanations for the origin of hypervelocity stars did not satisfy me. The hypervelocity stars are mostly found in the Leo and Sextans constellations — we wondered why that is the case.”

The theory put forth by Boubert and his team is based on an explanation of the origin of hypervelocity stars that involves binary star systems. This explanation suggests that if two stars are orbiting each other at really fast speeds (as would happen if the stars were close to each other), and if one of them explodes as a supernova, the other star could leave the system and fly off at the speed it had been orbiting earlier. This escaping star is called a runaway, and this phenomenon can’t happen in a slow-moving galaxy like the Milky Way.

Being the largest and the fastest of the dozens of galaxies that orbit the Milky Way, LMC is a prime candidate for being the source of hypervelocity stars. It has a mass of only 10 percent that of our galaxy, and orbits it at about 250 miles a second. Therefore, its fastest runaways can easily escape its gravitational pull.

Using the analogy of a bullet fired from a moving train (which would add the speed of the train to the bullet’s), Rob Izzard, a Rutherford fellow at the Institute of Astronomy and co-author of the paper, said: “These stars have just jumped from an express train — no wonder they’re fast. 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.”

Read: Most Stars In Small Magellanic Cloud Formed Relatively Recently, ESO’s VISTA Survey Finds

The researchers’ simulation showed about 10,000 runaway stars that were kicked out of LMC. It also showed another million or so stars that turned into neutron stars or black holes, which the researchers predict are moving across the Milky Way.

“We’ll know soon enough whether we’re right. The European Space Agency’s Gaia satellite will report data on billions of stars next year, and there should be a trail of hypervelocity stars across the sky between the Leo and Sextans constellations in the North and the LMC in the South,” Boubert said.

Titled “Hypervelocity runaways from the Large Magellanic Cloud,” the paper will appear in the journal Monthly Notices of Royal Astronomical Society. It will be presented Wednesday at the ongoing National Astronomy Meeting in Hull, U.K.