Streams of 鈥渞ebel鈥� stars are blazing new trails to and from our galaxy鈥檚 centre instead of travelling in circles around it, according to a new study of 6000 nearby stars. The stars appear to be nudged into their new grooves by the Milky Way鈥檚 spiral arms.
About a trillion stars trace out a pinwheel that makes up the Milky Way. Most of the stars in the five main spiral arms take more or less circular paths around the galactic centre. The Sun, located midway down one of the arms, takes about 100 million years to complete one loop.
Now, astronomers have caught a fifth of the stars within 1000 light-years of the Sun taking detours. The stars are still moving around the Milky Way鈥檚 centre, but they are taking more elongated routes, trooping more directly toward and away from the heart of the galaxy.
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But though they are flowing in orderly streams, the stars themselves are a motley crew. They vary greatly in age, which suggests they did not form at the same time and place.
鈥淭hey resemble casual travel companions more than family members,鈥� says Benoit Famaey, an astronomer at the Free University of Brussels, Belgium. He is lead author of the study, which was published in Astronomy & Astrophysics.
Star funnel
The stars, which were initially on different tracks, probably got funnelled into coherent streams when a spiral arm 鈥� dense with stars and gas 鈥� approached and deflected the stars in its path, say the researchers.
鈥淚f these stars are kicked by a spiral arm, they can be displaced thousands of light-years away from their birthplace,鈥� says Famaey.
鈥淭hat鈥檚 a new idea 鈥� I don鈥檛 know whether it鈥檚 right or wrong,鈥� says Antony Stark, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US. 鈥淭he dynamics of stars in the galaxy is pretty complicated.鈥�
Even though most stars move generally in circles, they do not necessarily stay in the same orbits. The new paper suggests 鈥渟ome of that shuffling can lead to increased order鈥�, Stark told New 杏吧原创.
Side to side
Famaey and his colleagues studied the stars鈥� motion with data collected from the European Hipparcos satellite 鈥� which completed its four-year mission in 1993 鈥� and a 1-metre telescope in France.
Hipparcos captured the stars鈥� movement from side to side in the sky, but could not measure their movement toward or away from Earth. That information was captured by the telescope in France.
鈥淲e now have a comprehensive, three-dimensional view of how nearby stars move about us,鈥� says Famaey.
Another European mission called Gaia, scheduled to launch in 2011, will measure the 3-D motion of more than a billion stars. It will cover a wider region of the galaxy, so it should reveal how common these rebel stars are.
Journal reference: Astronomy & Astrophysics (DOI:10.1051/0004-6361:20041272)