Hipparcos finds hint of star streams.

Observations over the past few decades have revealed that two key components of the Milky Way galaxy appear to have assembled separately. Stars within the galaxy's disk are, on average, considerably younger than those in its halo, the vast, tenuous sphere of material that envelops the disk.

Recent studies suggest that the halo may have acquired its population of stars by snaring material from tiny neighboring galaxies several billion years ago. Streams of stars stolen from these galaxies merged into the halo, while gas clouds ripped from these neighbors crashed together, settling into a flattened pinwheel that became the Milky Way's disk.

If this model is correct, then instead of possessing an evenly distributed hodgepodge of stars, the halo should contain huge streams. The tug of gravity would stretch these stellar streams around the galaxy like strands of spaghetti.

Evidence that the halo contains such streams has been tenuous. In a new study analyzing highly accurate measurements of the position and velocity of 41 halo stars near the sun, B. Chen of the University of Barcelona finds additional hints of these structures. Chen describes the results in the March 1 Astrophysical Journal Letters.

The study relies on data gathered by the Hipparcos satellite. From 1989 to 1993, the spacecraft measured the distances to stars by recording tiny shifts in their positions caused by Earth's motion around the sun (SN: 2/15/97, p. 101). Blurring caused by Earth's atmosphere makes such measurements difficult to take with ground-based telescopes.

In examining the Hipparcos measurements, Chen finds three large-scale clumps that could represent distinct streams of halo stars. "The results ... further suggest that accretion of stellar [material] plays an important part in the formation of the galaxy," says Chen.

Scott D. Tremaine of Princeton University agrees that the Hipparcos data provide a powerful new tool for astronomers seeking streams. He notes, however, that "the disappointing aspect of this paper is the sample size. Chen's `halo' sample contains only 41 stars, which is much too small to make strong statements about the number of distinct streams in the sample."

Rosemary F.G. Wyse of Johns Hopkins University in Baltimore generally agrees with Tremaine's assessment. She and her colleagues recently examined individual stars in a nearby dwarf galaxy captured by the Milky Way (SN: 4/9/94, p. 228). They find that the tiny galaxy makes a complete orbit around the Milky Way in a little less than 1 billion years and that it has managed to evade the larger galaxy's gravitational appetite for at least 10 orbits.

Unseen material, known as dark matter, may help the dwarf galaxy hang onto its stars, Wyse says. She reviewed the findings early this month in Philadelphia at the annual meeting of the American Association for the Advancement of Science.