Starlight spotlights galaxy's slow start.

A study of starlight from two of the Milky Way's oldest structures strongly supports the notion that our galaxy took three times longer to evolve than estimated by a widely accepted theoretical model.

During the past decade or so, several researchers have speculated that the young Milky Way may have taken as long as 3 billion years to collapse from a spherical cloud of gas into its present disk shape. This contrasts with a standard theory proposed in 1962, which calculates the collapse time at 1 billion years.

The researchers based their revised timetable on differences in the color and brightness of stars, including some residing in globular clusters--ancient, densely packed stellar regions that surround both the central bulge and periphery of the Milky Way (SN: 4/6/91, p.218). In particular, several teams of astronomers in 1989 and 1990 asserted that differences in the properties of stars from two globular clusters, NGC 288 and NGC 362, could best be explained if the clusters were separated in age by 3 billion years. Since globular clusters rank among the first objects formed in the Milky Way, the proposed age span would require the galaxy to take at least that long to evolve from a gaseous sphere to a disk.

Critics countered that this evolutionary scenario left open a major loophole. The observed differences in stellar color and brightness, they argued, might instead result from a variation in chemical composition among stars in the two clusters. If the two clusters had the same age but different compositions, then the standard theory of formation would still hold for the Milky Way.

An international research team has now gathered data that appear to close the loophole. Roger A. Bell of the University of Maryland in College Park and his colleagues used a high-resolution spectrograph to analyze the visible light emanating from the surfaces of a total of 15 red giant stars in the two clusters. In the May 16 NATURE, they report that NGC 288 and NGC 362 have nearly identical chemical compositions, supporting claims that the clusters indeed differ in age by 3 billion years.

Bell notes that astronomers previously estimated that both clusters had a relatively low ratio of iron to hydrogen. But his team's spectroscopic study, conducted at the Anglo-Australian Telescope in Siding Springs, Australia, used a different strategy to determine and compare chemical composition. For the first time, the researchers measured the abundances of three key elements -- relative to the abundance of hydrogen.

As a stellar core burns hydrogen, nuclear reactions convert one element to another, changing their relative abundances in the star's interior, Bell explains. Eventually, the modified composition may alter the elements' relative abundances on the surface of the star, masking the star's original chemical make-up, he says. This makes it difficult to determine whether one star really began with a composition similar to another.

But a star's total abundance of all three elements remains constant and thus provides a more reliable guide for comparison, Bell says. When he and his collaborators added up the abundances of carbon, nitrogen and oxygen for each of six stars from NGC 288 and nine from NGC 362, they found nearly equal total abundances among all the stars -- clinching the argument that the two globular clusters have a similar chemical make-up.

"The significance of this paper is that it shows chemical composition is not involved [in the observational differences between stars in the two clusters]," comments Leonard Searle of the Carnegie Institution of Washington in Pasadena, Calif. "That certainly increases the presumption that age is what is involved."

Searle, who first suggested in 1978 that our galaxy took 3 billion years to evolve into a disk, speculates that a longer-than-expected time interval for the Milky Way's evolution indicates that it may have formed from the merger of two galaxies.

Bell says his group plans to confirm and expand the new results by analyzing the chemical composition of stars in other Milky Way globular clusters.