Rogue Stars Ejected from the Galaxy
Rogue stars ejected from the galaxy are found in intergalactic space
In fact, the main mechanism that astronomers have come up with that can give a star the two-million-plus mile-per-hour kick it takes involves tangling with the supermassive black hole at the galaxy's core.
So far astronomers have found 16 of these "hypervelocity" stars. Although they are traveling fast enough to eventually escape galaxy's gravitational grasp, they have been discovered while they are still inside the galaxy.
Now, Vanderbilt astronomers report in a recent issue of the Astronomical Journal that they have identified a group of more than 675 stars on the outskirts of the Milky Way that they argue are hypervelocity stars that have been ejected from the galactic core. They selected these stars based on their location in intergalactic space between the Milky Way and the nearby Andromeda galaxy and by their peculiar red coloration.
The researchers identified these candidates by analyzing the millions of stars catalogued in the Sloan Digital Sky Survey.
"We figured that these rogue stars must be there, outside the galaxy, but no one had ever looked for them. So we decided to give it a try," said Holley-Bockelmann, who is studying the behavior of the black hole at the center of the Milky Way galaxy.
Astronomers have now found evidence for giant black holes at the centers of many galaxies. They estimate that the Milky Way's central black hole has a mass of four million solar masses. They calculate that the gravitational field surrounding such a supermassive black hole is strong enough to accelerate stars to hypervelocities.
Red giant stars are the end stage in the evolution of small, yellow stars like the Sun. So, the stars in Holley-Bockelmann's rogues gallery should have been small stars like the Sun when they tangled with the central black hole. As they traveled outward, they continued to age until they reached the red giant stage. Even traveling at hypervelocities, it would take a star about 10 million years to travel from the central hub to the spiral's edge 50,000 light years away.
"Studying these rogue stars can provide us with new insights into the history and evolution of our home galaxy," said Holley-Bockelmann. Information regarding the nature of our galaxy is valuable for astrobiologists who are trying to determine the best locations in which to search for habitable planets. The researchers next step is determine if any of their candidates are unusually red brown dwarfs instead of red giants. Because brown dwarfs produce a lot less light than red giants, they would have to be much closer to appear equally bright.