Rogue stars ejected from the galaxy are found in intergalactic space

Apr 30, 2012
Vanderbilt astronomers have identified nearly 700 rogue stars that appear to have been ejected from the Milky Way galaxy. When these stars received the powerful kick that knocked them out of the galaxy, they were small, yellow stars like the sun. But in the multi-million-year journey they evolved into red giant stars. Credit: Michael Smelzer, Vanderbilt University

It's very difficult to kick a star out of the galaxy.

In fact, the primary mechanism that astronomers have come up with that can give a star the two-million-plus mile-per-hour kick it takes requires a close encounter with the at the galaxy's core.

So far astronomers have found 16 of these "hypervelocity" stars. Although they are traveling fast enough to eventually escape the galaxy's gravitational grasp, they have been discovered while they are still inside the galaxy.

Now, Vanderbilt astronomers report in the May issue of the 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 . They selected these stars based on their location in intergalactic space between the Milky Way and the nearby and by their peculiar red coloration.

"These stars really stand out. They are red with high metallicity which gives them an unusual color," says Assistant Professor Kelly Holley-Bockelmann, who conducted the study with graduate student Lauren Palladino.

In astronomy and cosmology, "metallicity" is a measure of the proportion of chemical elements other than hydrogen and helium that a star contains. In this case, high metallicity is a signature that indicates an inner galactic origin: Older stars and stars from the galactic fringes tend to have lower metallicities.

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 .

Astronomers have now found evidence for giant 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.

The typical scenario involves a binary pair of stars that get caught in the black hole's grip. As one of the stars spirals in towards the black hole, its companion is flung outward at a tremendous velocity.

A second scenario takes place during periods when the central black hole is in the process of ingesting a smaller black hole. Any star that ventures too close to the circling pair can also get a kick.

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. 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.

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User comments : 15

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Tuxford
1.5 / 5 (8) Apr 30, 2012
In LaViolette's model, these rogue stars can eventually grow from within large enough to become unstable and eject new matter into surrounding space forming the basis for a new cluster.

http://phys.org/n...lky.html

And then grow large enough to form the nucleus of a satellite galaxy.

http://phys.org/n...axy.html

Still, it is not clear that these stars must all be hypervelocity and ejected from the core. Perhaps some are simply very old, and grown into red giants. Afterall, nearly all stars have been shown to be moving toward the galactic anti-center.

http://phys.org/n...ays.html

XQuantumKnightX
1 / 5 (8) Apr 30, 2012
Can anyone say 2012 Dec 21? This may happen to us!
Shabs42
5 / 5 (8) May 01, 2012
Can anyone say 2012 Dec 21? This may happen to us!


No it mayn't.
Graeme
not rated yet May 01, 2012
Brown dwarfs or even red dwarfs at that distance are likely too dim to observe. However for XQuantumKnightX's benfit someone should observe in the direction of the galactic core to see if there are any stars on the way here. If it was a blackhole we may not see it coming until we see asteroids going berserk as it passes through the belt.
alfie_null
5 / 5 (3) May 01, 2012
should have been small stars like the Sun when they tangled with the central black hole. ... it would take a star about 10 million years to travel from the central hub to the spiral's edge

Our sun is a fair bit older but it's (hopefully) nowhere near becoming a red giant. Has to be some more to this explanation.
Husky
not rated yet May 01, 2012
interesting point! 10 million years is a small timeframe in the main sequence of stars, so, the "like our suns" should already be more evolved (several billion years) towards red giant when they got kicked out
Husky
not rated yet May 01, 2012
just imagine the snowball change in hell that two of these roque stars , kicked out of different galaxie have a head on collision at 2million mph, that bang would probably dwarf a hypernova, could have happened at least once, and kicked out stars colliding with other, stationary stars must have happened more often, should display an impressive signature too.
HannesAlfven
1 / 5 (1) May 01, 2012
When it's necessary to divide by zero in order to explain an observation, it's also worth questioning the logic which got you there.
Lurker2358
not rated yet May 02, 2012
Older stars... ...tend to have lower metallicities.


How does that make sense?

Older stars have burned of their hydrogen and Helium and started the CNO chain, how can they have lower metallicity?
typicalguy
not rated yet May 03, 2012
Older stars... ...tend to have lower metallicities.


How does that make sense?

Older stars have burned of their hydrogen and Helium and started the CNO chain, how can they have lower metallicity?

I think they mean that more hydrogen has fused so there is less hydrogen but they didn't really state it in a way that made sense.

As far as 10 million years aging the star, perhaps the interaction with the black hole caused some time dilation to occur?
Terriva
1 / 5 (1) May 03, 2012
Can anyone say 2012 Dec 21? This may happen to us!
The Mayan prediction must be based on some periodic, i.e. predictable effect: excentric comet, planet or red dwarf from Oort cloud, Great Rift shielding, passage of solar system through galactic plane, etc.
holtonsys
1 / 5 (1) May 06, 2012
HannesAlfin: Good point, there wasn't even a conceptual zero until after the dark ages. Show me somewhere in the real world where there is a zero anything. Even so called empty space is inhabited by quantum foam. Zero and infinity are mathematical conveniences, not a real entity. I learned early on from Trig that the Tangent function has a positive and a negative infinity. Now that is weird and I wouldn't bet my house on it. Math is fun but is only tangental to reality in rare cases. How many negative roots have you thrown out in finding quadratic roots? Lots I'll bet. Come on Mathematicians, take me on just don't bash my head with a hammer like a math major did to someone who challenged him.
holtonsys
not rated yet May 06, 2012
Terriva: Mayan math was supposedly based on radix 20, but after the first iteration they changed the base to radix 18 for some reason, no clear reason why that I can find. So the whole Mayan calendar thing is a weird application of convoluted math. I wouldn't put any faith in it. Look what happened to the Mayans!
rwinners
not rated yet May 07, 2012
Brown dwarfs or even red dwarfs at that distance are likely too dim to observe. However for XQuantumKnightX's benfit someone should observe in the direction of the galactic core to see if there are any stars on the way here. If it was a blackhole we may not see it coming until we see asteroids going berserk as it passes through the belt.


Odds are very good that anything coming our 'way' will miss by far enough so as to cause no commotion among the followers.
Eoprime
not rated yet May 07, 2012
... stars colliding with other, stationary stars must have happened more often, should display an impressive signature too.


Dont think so, even in the case of a complete merger of two galaxys its unlikely.

Quote from the wiki "Andromeda-Milkyway Collision"

"Stars are much denser near the centres of each galaxy, the average separation is only 1.6x1011 km. But that is still a density which represents one ping-pong ball every 3.2 km."

http://en.wikiped...ollision