Rebel Stars

22 October 2004 (All day)

Spinning out. Rebel stars are flung out by the galaxy’s spiral arms into streams stretching 1000 or more light-years across the Milky Way.

Almost all the stars in the Milky Way's disk were thought to orbit in orderly, nearly circular paths around the galaxy's core, but now astronomers find that many of the sun's neighbors have strayed from this course. Experts say these wandering stars could play a critical role in how planets are distributed in the galaxy.

For decades, astronomers have known that a tiny percentage of stars, dubbed "superclusters," go against the flow. These clumps of stars appeared young, as they mainly included short-lived blue stars. Scientists had developed a theory of a common origin for these wayward youths, saying they arose from a long-evaporated cluster. But in 1998, investigators discovered evidence of far older supercluster stars that could not have formed at the same time.

Using the European Space Agency's (ESA's) Hipparcos space observatory, astrophysicists Benoit Famaey and Alain Jorissen of the Université Libre de Bruxelles teamed up with astronomers across Europe to measure the distance and motion of more than 100,000 stars within 1000 light-years of the sun. Ground-based measurements of each star's Doppler shift (see link below for explanation) determined whether they were moving toward or away from Earth. Roughly a fifth of the observed stars weren't following the standard orbit around the Milky Way, the researchers report in the October issue of Astronomy & Astrophysics. The observations validate recent simulations by research groups at Princeton, Oxford, and Rutgers universities, which suggest that as the Milky Way's spiral arms and galactic bar travel in space, they can gravitationally deflect stars into streams that run inward or outward from the galactic core, like spokes in a wheel. The new study reveals that three streams of stars of all ages criss-cross near the sun. These streams mean that more metallic stars --the kind more likely to host planets--that were thought to lie near the galactic core could find their way edgeward, suggests Jorissen. "Models of the chemical evolution of disk galaxies will have to be radically revised," says theoretical physicist James Binney of Oxford University, from the traditional view that metallic stars are concentrated at the core of growing galaxies. But astrophysicist Walter Dehnen of the University of Leicester, U.K., says that spiral stirring may not be enough to cause these streams, and he points instead to galaxies merging with the Milky Way.

Related sites
European Space Agency's Hipparcos space observatory
Explanation of the Doppler shift
The Astronomy & Astrophysics paper

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