我们银河系的传记可能需要进行一些修订。这是因为银河系中心的一个条形恒星群似乎比预期的要年轻得多。
这个棒状结构是我们星系(SN: 6/25/21)的一个显著特征。它横跨数千光年，连接着银河系中恒星的螺旋臂，使它们看起来像从旋转的草坪洒水器中流出的水流。在对银河系演化的计算机模拟中，条形结构倾向于在银河系大约130亿年的生命周期早期形成。但研究人员报告说，从富含金属的恒星的年龄和位置来看，这个棒状结构在几十亿年前就已经形成了。这项研究于11月28日提交给arXiv.org，发表在《天文学与天体物理学快报》上。
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“These metal-rich stars are basically like fossil records of ancient stars that are telling the story of our home galaxy,” says Samir Nepal, an astrophysicist at the Leibniz Institute for Astrophysics Potsdam in Germany.
Stars with large proportions of metal elements are built from the remnants of stars that have since exploded, ejecting the metals they forged from lighter elements. Those spewed metals enrich the materials in the core of galaxies like the Milky Way, which is why a new generation of metal-rich stars can form only deep inside galaxies. The spinning bar at the center of the Milky Way then scattered some of those stars throughout our galaxy.
Using data from the European Space Agency’s Gaia space telescope, Nepal and colleagues reconstructed the development of the Milky Way bar through its influence on the distribution of metal-rich stars (SN: 5/9/18). They inferred the bar’s history, just as you might deduce where the batters stand in a baseball game by looking at the flight of the balls they hit, even if you can’t see home plate.
In tracking the ages of the metal-rich stars, the researchers identified a burst of star formation in the central part of the galaxy that petered out about 3 billion years ago. The downturn seems to mark the end of the Milky Way bar’s developmental phase, the researchers report. After that burst, they say, the inflow of new material into the bar probably dropped off substantially. That suggests the bar we see today is a stable feature that’s about 10 billion years younger than the galaxy as a whole.
The new insights about the metal-rich stars “are like the tip of the iceberg” of data coming from the Gaia telescope, says astrophysicist Cristina Chiappini, also with the Leibniz Institute for Astrophysics Potsdam. Assuming the revised age estimate of the bar is ////confirm/i/i/i/ied, future models of the galaxy’s evolution will have to account for why the bar developed so late.
The study has broader implications than correcting the history of our galaxy, says Ortwin Gerhard, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who was not involved in the research. “The possibility of detailed observations of the motions and chemical abundances of stars in the Milky Way, particularly based on [data] from the Gaia satellite,” he says, means we can “expect to learn about the evolution of bars [in other galaxies] generally by studying the bar in the Milky Way.”
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