Strip show in M15: naked blue stars.

Globular clusters, the densely packed regions surrounding the disk of the Milky Way, contain some of our galaxy's oldest stars. But peering into the cluster M15, the Hubble Space Telescope has spied 15 seeming youngsters among the elderly residents. Astronomers say these stars- which are unusually blue and hot, and thus appear youthful -- belong to a new stellar class. And if speculation about their origin proves correct, these objects are laying bare the evolution of stars like our sun.

Francesco Paresce and Guido De Marchi of the Space Telescope Science Institute in Baltimore suggest the stars were once red giants, aging stellar objects known for their bloated atmospheres. The researchers speculate that close encounters with other stars in the crowded cluster ripped away the atmospheres of these stars, exposing their naked, blue-hot cores.

"This is the first time we have seen inside [a star]," says De Marchi, who is also affiliated with the University of Florence in Italy. He notes that the stripped atmosphere won't alter the fate of these stars, which will evolve into tiny, burned-out relics called white dwarfs. About 5 billion years from now, our sun will become a red giant, and eventually it too will end its life as a white dwarf. The ability to view even the outer cores of stars older than the sun but similar in mass offers an opportunity to predict the sun's future, he says.

Finding fainter, very blue stars in the cluster may reveal the cores of stars slightly older than the 15 already imaged, says De Marchi. He notes that if Hubble's flawed optics are repaired as planned this December, spectroscopy may be able to determine whether the stars are truly naked.