Questioning a galactic star-forming model.

Like the two halves of a watermelon, concentrations of stars and gas sit above and below the disk of our galaxy. Astronomers have long believed that this dense central bulge, which makes up about onefifth of the Milky Way's visible mass, contains many of the oldest stars in the galaxy, But a new study suggests that a significant number of stars in the bulge are not elderly, just middle-aged.

This finding, along with observations of three neighboring galaxies, flies in the face of conventional wisdom, which holds that the densest parts of a galaxy make most of their stars before other regions begin the process. The finding may force astronomers to revise the standard model of how and when galactic bulges make stars, says I Jon A. Holtzman of Lowell Observatory in Flagstaff, Ariz.

Holtzman and his colleagues used the Hubble Space Telescope's wide-field camera to peer through a relatively dust-free pathway, known as Baade's Window, into the galactic bulge. The camera recorded stars 10 times fainter than those previously seen in the bulge. Relying on the principle that massive stars shine more brightly but die out more quickly than less massive ones, the researchers inferred an age range for the bulge stars by determining the luminosity at which the number of stars abruptly decreases.

The team estimates that a substantial number of stars in the bulge are 6 to 10 billion years old, rather than the 10 to 15 billion years previously suggested by researchers. Astronomers had thought that all bulge stars were related to and roughly the same age as those in the Milky Way's globular clusters, ancient star-packed groupings surrounding the disk of the galaxy, Holtzman and his coworkers describe their study in the November ASTRONOMICAL JOURNAL.

Holtzman cites three difficulties that make the results of the Hubhie study somewhat tentative: the telescope's flawed optics, incomplete mapping of the amount of light-obscuring dust in Baade's Window, and uncertainties about the distance to the bulge.

However, several recent ground-based studies reveal that bulge stars in nearby galaxies are also younger than once thought. For example, in Andromeda, the nearest galaxy similar to our own, stars in the bulge appear to be younger than those in that galaxy's oldest globular clusters. R. Michael Rich of Columbia University in New York City, Jeremy R. Mould of the California Institute of Technology in Pasadena, and James R. Graham of the University of California, Berkeley, will report the Andromeda study in the December ASTRONOMICAL JOURNAL. Other astronomers have reported similar age estimates for the bulge of M32, a satellite of Andromeda, and for the galaxy M33.

Astronomers have proposed that a galactic merger or collision may explain why many stars in the bulge don't form until well after other parts of a galaxy have had their first glimmers of starbirth. In these models, notes Rich, a violent encounter later in the life of a galaxy would drive gas into the core, where it would trigger a burst of star formation and eventually thicken into a bulge.