Milky Way's tug robs stellar cluster.

There are hundreds of tails in the Milky Way. This is just one of them.

In this drawing the globular cluster NGC 6712 is seen at two different times--before (A) and after (B) the swarm of stars passes through the plane of our massive, pinwheel-shaped galaxy. The cluster's repeated passage may have stretched NGC 6712 like a comet's tail.

That scenario could explain a new observation: None of the several-hundred thousand stars in NGC 6712 are less massive than the sun. That's a surprise, because clusters usually contain many more lightweight stars than heavyweights. The tug of the Milky Way's dense center has robbed NGC 6712 of its lightest members, says Francesco Paresce of the European Southern Observatory in Garching Germany.

"NGC 6712 is the first real example of `evaporation' of stars, allowing us to watch the process unfold in front of our eyes, "he notes. Other clusters don't show the same pattern because they don't come as close to the Milky Way's center. NGC 6712 may have ventured within 1,000 light-years of the core just a few million years ago. The lightest stars are more easily detached because they tend to lie at the periphery of a cluster, says Lars Hernquist of Harvard University.

Like the rest of the universe, as much as 99 percent of the Milky Way's mass is thought to be made of invisible material, or dark matter. By studying the extent to which clusters, as well as tiny satellite galaxies, are distorted or torn apart by our galaxy's gravity, astronomers hope to shed light on the distribution and the amount of dark matter in the Milky Way.

Paresce and his collaborators made their observations with the first component of what will be a quartet of 8.2-meter telescopes, known as the Very Large Telescope, on Cerro Paranal in Chile. The team describes its findings in the March 1 Astronomy and Astrophysics.