"There are vast empty distances between stars, and so a close stellar encounter is very unlikely as a hypervelocity star travels out of the galaxy," said lead researcher Warren Brown, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Brown told SpaceDaily.com that tidal disruption also is unlikely, unless the encounter occurred at extremely close range, "because a hypervelocity star travels so quickly there would be very little time for the gravitational pull to disrupt any planets."

Using the Multiple-Mirror Telescope in Arizona, Brown and colleagues have spotted two such stars � each about four times more massive than the Sun � moving fast enough so the galaxy's gravitation cannot contain them. The first, in the direction of the constellation Ursa Major, is traveling out of the Milky Way at about 1.25 million miles per hour. It is located about 240,000 light-years from Earth. The second, in the direction of Cancer, is moving outward at 1.43 million miles per hour and is about 180,000 light-years away.

The HSCA team discovered the first stellar exile last year. A European group has identified two more, one of which might have originated in the Large Magellanic Cloud, which is adjacent to the Milky Way. The latest discovery brings the total number of known exiles to five.

"These stars form a new class of astronomical objects � exiled stars leaving the galaxy," Brown said, adding that there may be 1,000 such stellar rogues speeding through the Milky Way, or about one star in 100 million. Despite the long odds of discovery, the team found their target by pre-selecting candidates spread over a large area of the sky.

Stars become exiles when their path takes them too close to the supermassive black hole that exists at the Milky Way's center. In general, exiled stars start out as part of binary systems. Just as engineers can boost the velocity of spacecraft by maneuvering them close to planets, these stars receive a huge gravitational kick from the black hole, enough to eject them completely from the galaxy, and to break the bond with their binary partner.

One intriguing prospect regarding galactic exiles is whether they are carrying any planets along with them. "It is possible that these stars have planetary systems," Brown said. "If planets do currently orbit the exiled stars, the planets' motion is relative to the stars and is unaffected by the extreme speeds of their parent stars."

He said the odds of planets are low, however. "We believe that � the most likely time for a planet to be either disrupted or stripped from its parent star altogether is during the encounter with the massive black hole."

Regarding the possibility of a close encounter with a rogue star, Brown said although anything massive that passes near or within the orbits of the planets would disrupt our solar system, "lucky for us, a close encounter with even one of the galaxy's 100,000,000,000 stars is very unlikely. Roughly speaking, there is a 0.01 percent chance of finding a hypervelocity star within 1,000 light years of Earth at any given time."

In terms of their ultimate fate, Brown said hypervelocity stars are doomed to travel alone in the blackness of intergalactic space. "They are traveling at speeds well over twice that needed to escape the gravitational pull of our galaxy, and so will never return," he said. "The stars will shine normally until they end their lives as supernova."

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