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Boom and bust

作者:于一    发布时间:2019-03-07 03:17:00    

By Robert Adler THE Milky Way has brightened and faded like a cloud of fireflies blinking on and off together. A new study of nearby stars hints that the number of new stars being born in the Galaxy changes significantly over time, suggesting the Milky Way’s history is far more dramatic than anyone realised. An international team of astronomers led by John Scalo of the University of Texas at Austin looked at the spectra of 552 nearby stars and from these were able to work out their ages. Although these stars are nearby now, their birthplaces would have been scattered around the Milky Way, so they can be taken to be typical of the population of stars in the Galaxy as a whole. Once the astronomers knew how old the stars were, they could calculate the star formation rate in the Milky Way at different times in the past. And the results gave them a big surprise. It turned out that the rate has varied by as much as a factor of 10 over the Milky Way’s 15-billion year history, with bursts of star formation separated by much quieter periods. The number of stars being born can vary greatly within a few hundred million years, Scalo says. Similar bursts of star formation have been seen in certain other types of galaxies, such as dwarf types and colliding galaxies. But they were not expected to occur in large spiral galaxies such as the Milky Way. “The surprising result is that our Galaxy can do this,” says Scalo, who has submitted the results to Astrophysical Journal Letters. It’s unclear what causes these changes. Sudden increases might be due to small galaxies falling into the Milky Way. Even galaxies that are simply passing close by could be involved. Their gravity could trigger the star formation by disturbing interstellar gas. The results will spark controversy because most models of star formation in large spiral galaxies like our own assume that stars are born at a very steady rate, averaging a few Sun-like stars per year. “Our findings are similar to the punctuated equilibrium model in biological evolution,” says Scalo. “They imply that our models are incorrect, and new ones are required.” Richard Ellis, an astronomer at the University of Cambridge, says the work is thought-provoking. “It opens our eyes to the idea that we really don’t know how smooth star formation has been in the past,” he says. Ellis would like to see if the result can be confirmed in more thorough studies. If it stands up, he thinks it would suggest that astronomers have underestimated how many smaller galaxies have fallen into the clutches of the Milky Way. Joseph Silk, an astrophysicist at Oxford University, says he is not surprised by such variability. “Most models do assume a steady state,” he says. “But that doesn’t bind us to believe that.” Scalo and his colleagues plan to extend their studies to include more stars and better estimates of their ages. In the meantime, he would like theorists to come up with alternative models of star formation. “I’m hoping this work will spur some thinking that’s different from the past,

 

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