While most stars are content with their place in the galaxy, others seem to have jumped ship and shot off in search of a better life. Hypervelocity stars are zipping through the universe fast enough to leave their home galaxy behind, but it's a mystery how they reached those speeds and where they came from. Now, astronomers from the University of Cambridge believe they've found the answers.
There are a few theories on what may have caused these hypervelocity stars to up and run away. It was previously thought that they may have been dislodged and sent packing by the supermassive black hole at the center of the galaxy, or dragged out of a neighboring dwarf galaxy that passed a little too close to the Milky Way. However, the most likely explanation is that these runaways were once part of a binary pair, shooting off when their partner went supernova.
But these theories still leave some questions unanswered, such as why hypervelocity stars are mostly found in one specific part of the sky. And runaways from broken-up binaries originating in the Milky Way can't actually launch fast enough to hit those super-high speeds – which led the Cambridge team to investigate the idea that they weren't locals.
"Earlier explanations for the origin of hypervelocity stars did not satisfy me," says Douglas Boubert, lead author of the study. "The hypervelocity stars are mostly found in the Leo and Sextans constellations – we wondered why that is the case."
To study the strange stars, the team combined data from the Sloan Digital Sky Survey with computer simulations of the most likely culprit: the Large Magellanic Cloud (LMC), a dwarf galaxy that orbits the Milky Way. Simulating the life cycles of the stars in the LMC over the last two billion years or so, the team took note of every star that became a runaway. Then, they ran those through a second simulation that tracked how the gravity of both galaxies would affect the stars' orbits as they leave the LMC and enter the Milky Way.
Since the LMC zips around the Milky Way at 400 km (248.5 miles) per second, stars ejected from the cloud are given an extra speed boost. About half of them were traveling fast enough to be classed as hypervelocity stars, and sure enough, their simulated position in the sky lined up with observations.
"These stars have just jumped from an express train – no wonder they're fast," says Rob Izzard, co-author of the study. "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."
The team estimates there are some 10,000 runaway stars passing through the Milky Way – and because they still live and die as normal, there could be as many as a million neutron stars and black holes shooting through our neck of the woods like "cosmic bullets."
"We'll know soon enough whether we're right," says Boubert. "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."
The research was published in the Monthly Notices of the Royal Astronomical Society.
Source: University of Cambridge
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