Milky Way's Hypervelocity Stars Are Moving so Fast They Managed to Escape Their Home Galaxy

runaway star
Artist impression of a runaway star. Amanda Smith

Scientists have discovered that the fastest stars in the Milky Way—which travel at over two million miles per hour—originally came from other galaxies, having escaped their homes after stellar explosions pushed them away at almost the speed of light.

The discovery solves a longstanding mystery about how and why these hypervelocity stars travel at such great speeds.

Researchers from the University of Cambridge published the findings in the Monthly Notices of the Royal Astronomical Society and presented the results Wednesday at the National Astronomy Meeting in Hull, U.K.

"Earlier explanations for the origin of hypervelocity stars did not satisfy me," Douglas Boubert, lead author of the study, said in a statement.

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Astronomers had initially thought hypervelocity stars were spewed out of the center of the Milky Way by a supermassive black hole. But a number of anomalies meant this hypothesis did not add up. For example, in 2016 scientists discovered a binary system (where two stars orbit one another) within which stars moved at incredibly fast speeds despite being nowhere near a black hole.

Other explanations, including disintegrating dwarf galaxies and chaotic star clusters, also fall short as they fail to explain why hypervelocity stars appear to come from the same place in the sky.

In the latest study, researchers used data from the Sloan Digital Sky Survey and computer simulations to work out exactly where most of these stars came from. The findings showed they appeared to originate from the Large Magellanic Cloud (LMC), a dwarf galaxy that orbits Milky Way.

"The hypervelocity stars are mostly found in the Leo and Sextans constellations—we wondered why that is the case," Boubert said.

The LMC is the biggest and fastest dwarf galaxy that orbits the Milky Way, moving around it at a speed of about 250 miles per second (900,000 miles per hour). Researchers showed that binary stars in this system whizz around one another at incredible speeds. When this system breaks up—for example, if one of the stars explodes as a supernovae—it can send the other flying off into space as a hypervelocity star. At this point, it becomes known as a runaway star.

The extreme speed at which the LMC is travelling in the first place is what allows the stars to become superfast—they travel at the velocity at which they were ejected, plus the velocity of the LMC. "These stars have just jumped from an express train—no wonder they're fast," said co-author Rob Izzard. "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."

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Simulations of the birth and death of stars in the LMC matched the idea of hypervelocity stars coming from binary systems. It also fits with the gravitational pulls of both the LMC and the Milky Way and explains why hypervelocity stars end up in the part of the sky where astronomers find them. The researchers now hope to confirm their findings with data from planned satellite missions.

"We'll know soon enough whether we're right," Boubert said. "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."