Ultracool stars take 'wild rides' around, outside the Milky Way
June 9, 2009Astronomers have found that stars of a recently discovered type, dubbed ultracool subdwarfs, take some pretty wild rides as they orbit around the Milky Way, following paths that are very different from those of typical stars. One of them may actually be a visitor that originated in another galaxy.
Adam Burgasser and John Bochanski of MIT will present the findings on Tuesday, June 9, in a press conference at the American Astronomical Society's semi-annual meeting in Pasadena, Calif. The result clarifies the origins of these peculiar, faint stars, and may provide new details on the types of stars the Milky Way has acquired from other galaxies.
Ultracool subdwarfs were first recognized as a unique class of stars in 2003, and are distinguished by their low temperatures ("ultracool") and low concentrations of elements other than hydrogen and helium ("subdwarf"). They sit at the bottom end of the size range for stars, and some are so small that they are closer to the planet-like objects called brown dwarfs. Only a few dozen ultracool subdwarfs are known today, as they are both very faint up to 10,000 times fainter than the Sun and extremely rare.
Burgasser, associate professor of physics at MIT and lead author of the study, was intrigued by the fast motions of ultracool subdwarfs, which zip past the Sun at astonishing speeds. "Most nearby stars travel more or less in tandem with the Sun tracing circular orbits around the center of the Milky Way once every 250 million years," he explains. The ultracool subdwarfs, on the other hand, appear to pass us by at very high speeds, up to 500 km/s, or over a million miles per hour.
"If there are interstellar cops out there, these stars would surely lose their driver's licenses," says Burgasser.
Burgasser's team of astronomers assembled measurements of the positions, distances and motions of roughly two dozen of these rare stars. Robyn Sanderson, co-author and MIT graduate student, then used these measurements to calculate the orbits of the subdwarfs using a numerical code developed to study galaxy collisions. Despite doing similar calculations for other types of low-mass stars, "these orbits were like nothing I'd ever seen before," says Sanderson.
Sanderson's calculations showed an unexpected diversity in the ultracool subdwarf orbits. Some plunge deep into the center of the Milky Way on eccentric, comet‐like tracks; others make slow, swooping loops far beyond the Sun's orbit. Unlike the majority of nearby stars, most of the ultracool subdwarfs spend a great deal of time thousands of light‐years above or below the disk of the Milky Way.
"Someone living on a planet around one of these subdwarfs would have an incredible nighttime view of a beautiful spiral galaxy our Milky Way spread across the sky," Burgasser speculates.
Sanderson's orbit calculations confirm that all of the ultracool subdwarfs are part of the Milky Way's halo, a widely dispersed population of stars that likely formed in the Milky Way's distant past. However, one of the subdwarfs, a star named 2MASS 1227‐0447 in the constellation Virgo, has an orbit indicating that it might have a very different lineage, possibly extragalactic.
"Our calculations show that this subdwarf travels up to 200,000 light years away from the center of the Galaxy, almost 10 times farther than the Sun," says Bochanski, a postdoctoral researcher in Burgasser's group at MIT. This is farther than many of the Milky Way's nearest galactic neighbors, suggesting that this particular subdwarf may have originated somewhere else.
"Based on the size of its one billion‐year orbit and direction of motion, we speculate that 2MASS 1227‐0447 might have come from another, smaller galaxy that at some point got too close to the Milky Way and was ripped apart by gravitational forces," explains Bochanksi.
Astronomers have previously identified streams of stars in the Milky Way originating from neighboring galaxies, but all have been distant, massive, red giant stars. The ultracool subdwarf identified by Burgasser and his team is the first nearby, low‐mass star to be found on such a trajectory. "If we can identify what stream this star is associated with, or which dwarf galaxy it came from, we could learn more about the types of stars that have built up the Milky Way's halo over the past 10 billion years," says Burgasser.
Source: Massachusetts Institute of Technology (news : web)
Feb 13, 2006 |
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It is just the natural part of science that has been neglected.
Any star that has a billion year orbit around our own galaxy has to make you wonder just how old the universe really is. That is only 14 orbits since the beginning of an expanding big bang universe. And our galaxy must have been around considerably less than that right?
If our galaxy has not been here since the beginning of the universe (something that would be impossible) and we have a star that is in a semi stable orbit around the galaxy with a maximum number of possible complete orbits something less than 14 which came from some other galaxy where presumably it formed out of something (old star matter perhaps) and possibly had an orbit there which was perturbed by some near collision or other - plus take into account travel time from that galaxy to ours.
Now if we consider our galaxy has been here for 10 billion years (what was once considered to be the age of the universe by many) and the brown dwarf came from another galaxy that had been around for x years and took y years to get to our galaxy and then had to get into billion year orbit. Well it has to make you wonder yes.
If you consider that perhaps the milky way has been around longer and other galaxies have been around longer then there is much more time for these types of events to occur.
If we look for all the exceptions we are much more likely to find something interesting. But what are the odds of finding a star from another galaxy that is both hard to see and in an orbit that takes it way outside the galaxy and out of sight of observers?
Do the math and it makes a 14 billion year old galaxy and a 10 billion year old Milky way seem rather a lucky coincidence.
Even if the star was born in our galaxy it could for instance just mean that the stuff from which it originated was orbiting about the galaxy very slowly in the first place, or it could mean that at some point its orbit was disturbed by a close encounter with another star.