Renegade stars in sun's neighborhood.

Like travelers that come together from different parts of the world, the sun and many of its stellar neighbors have vastly different origins. Although the sun probably formed where it now resides--about halfway between the Milky Way's core and its outer edge--a substantial fraction of the sun's neighboring stars may be immigrants from the core, a new study suggests.

To track the motion of nearby stars as they move across the sky, Benoit Famaey of the Free University of Brussels in Belgium and his collaborators used data from the European Space Agency's Hipparcos satellite, which ended its mission in 1993. The team then used a ground-based telescope to determine the bodies' motion toward or away form Earth.

Models had already indicated that although the sun and most of its neighbors follow nearly circular orbits about the galaxy's center, some buck the trend. The new study reveals that 20 percent of stars within 1,000 light-years of the sun are renegades from elsewhere in the galaxy. Most of these have nearly straight-line orbits, heading toward or away from the galactic center, like spokes of a wheel, the team reports in a mid-October Astronomy & Astrophysics.

Stars that have a high abundance of elements heavier than hydrogen are common at the center of the Milky Way, but some show up in the region around the sun. Stars with this composition are the most likely to form planets, previous studies have shown. Famaey and his collaborators suggest that stars surrounded by planets were forced onto their current paths because they got a kick from one or more of our galaxy's spiral arms, regions of dense gas and stars that sweep across space. The vicinity of the sun appears to be a crossroads of several such streams.--R.C.