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The Milky Way's youngest globular.

SITUATED HIGH ABOVE the galactic plane, globular clusters are as old as the Milky Way itself. But one astronomer has identified a possible globular cluster whose age is closer to that of the Sun. If he's right, it may be the youngest globular in our galaxy.

Globular clusters are concentrated stellar groups that are thought to have originated from the primordial gas cloud that collapsed to form the Milky Way's disk more than 13 billion years ago. A survey of the southern sky in 2002 identified a faint, compact cluster of blue stars far from the spiral arms in the constellation Cetus. It was named Whiting 1 after the project's lead scientist, Alan Whiting (Cerro Tololo Inter-American Observatory), and later classified as an open cluster.

Prompted by its unusual location and uncharacteristic faintness, Giovanni Carraro (University of Chile) set out to study the cluster more closely. He used Cerro Tololo's 1-meter telescope to measure the brightnesses and colors of nearly 1,800 cluster members. Then he plotted the stars' apparent magnitudes against their colors. The resulting color-magnitude diagrams enabled Carraro to classify the stars into low-mass dwarfs still on the hydrogen-burning main sequence and higher-mass stars that had evolved to the red-giant stage or beyond.

By comparing the mix of stellar types with evolutionary models, Carraro deduced that the stars in Whiting 1 have barely 1/20 the proportion of "metals" (elements heavier than hydrogen and helium) as our Sun and that the cluster is only 5 billion years old. Its observed properties are consistent with a globular cluster about 150,000 light-years out in our galaxy's halo. As he notes in the March 1st Astrophysical Journal, this would make it the Milky Way's youngest globular cluster.

"I think Carraro did a nice and careful job," says Francois Schweizer (Carnegie Observatories, Pasadena), who studies ages and metallicities of young globulars. "The age estimate of 5 billion years would appear reasonable and secure to perhaps within a range of 4 to 7 billion years. But the metallicity is always hard to estimate from a color-magnitude diagram, especially one as sparsely populated by stars as this one."

Carraro plans to follow up his paper with high-resolution spectroscopy of the cluster's element abundances to get a better measurement of metallicity, which will help constrain its age. "If spectroscopy shows that Whiting 1 has low metal abundances, then it must be a globular cluster," says Carraro.

A potential problem is Whiting 1's small size--it contains fewer than 2,000 stars. The cluster seems out of its league when compared to the Milky Way's other 150 known globulars, most of which contain hundreds of thousands of stars.

"Two thousand stars is a very low number for a globular cluster, in the traditional sense. However, other youngish globular clusters in the Milky Way halo also have low masses and similar brightnesses to Whiting 1," says Schweizer. Several faint globulars found in Palomar Sky Survey plates also have low masses. Palomar 1, for example, contains fewer than 400 stars and is estimated to be only 6.3 to 8 billion years old. "Whiting 1 may well be another case of a youngish halo globular in an advanced stage of breakdown," says Schweizer, referring to Palomar 1's stripped-down size resulting from gravitational perturbations by the Milky Way and smaller star systems.

But the youngest globular cluster in the galaxy may not be from the galaxy at all. "An appealing interpretation might be that this cluster has been captured by the Milky Way and is not a genuine product of our galaxy," says Carraro. If Whiting 1 is in fact an extragalactic cluster that became tangled within our halo, Carraro points out that "upcoming radial velocity measurements will provide hints of the kind of orbit the cluster has, and that will help clarify this issue."
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Author:Johnston, Lisa R.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Aug 1, 2005
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