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Starbirth may guide galactic chemistry.

Starbirth may guide galactic chemistry

Stars in dwarf galaxies and the Milky Way apparently started out with the same chemical compositions and a similar range of masses. Yet the chemistry of these galaxies today shows a striking contrast: Compared with dwarf galaxies such as the Large Magellanic Cloud, our own galaxy features a much higher ratio of oxygen to iron.

Astronomers have proposed a variety of complex theories to account for this puzzling disparity. In the Feb. 1 ASTRO-PHYSICAL JOURNAL, two researchers suggest a simple solution, based on widely accepted scenarios for starbirth in the two types of galaxies.

Dwarf galaxies seem to form most of their stars in well-separated bursts, whereas the Milky Way churns out new stars more continuously, notes Rosemary F.G. Wyse of the Johns Hopkins University in Baltimore. That difference alone can account for the chemical parting of ways, according to Wyse and Gerard Gilmore of the University of Cambridge, England.

The chemical makeup of new stars depends on the life cycles of older stars, Wyse explains. Stars with masses more than eight times that of the sun last only a few million years before exploding as Type II supernovas. The gas ejected by a Type II supernova is rich in oxygen but poor in heavier elements such as iron, says Wyse.

In contrast, stars with less than eight solar masses persist for at least a billion years and then exit in the form of Type I supernovas. These explosions, distinguishable from Type II supernovas by their pattern of light emission, eject a more iron-rich gas, Wyse says.

She and Gilmore used this information to model how intermittent star formation would affect a dwarf galaxy's oxygen-to-iron ratio. In that model, they say, sporadic starbirth could not replenish the rapid loss of massive stars that exploded as Type II supernovas. As a result, Type I supernovas -- which suffuse the interstellar medium with iron-rich gas -- dictated the chemical composition of new stars, the researchers report.

Steady star formation, in contrast, constantly replenishes the supply of both types of supernovas. According to Wyse and Gilmore, this ensures a higher ratio of oxygen to iron in galaxies such as the Milky Way.
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Author:Cowen, Ron
Publication:Science News
Date:Feb 2, 1991
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