Milky Way's oldest star clusters are stolen fakes

 作者:解感     |      日期:2019-03-05 04:15:02
By Robin Orwant A set of star clusters previously thought to be the most ancient native systems in our galaxy, were in fact stolen by the Milky Way from a puny neighbour, Korean astronomers believe. Their conclusion supports results from other groups that are shaking up long-held ideas about how the Milky Way evolved. “It’s telling us a lot about how our galaxy came together and formed,” says astronomer Robert Kennicutt from the University of Arizona. The fact that these pirated star systems are younger than thought could also force astronomers to recalculate the distances between galaxies. “We’re going to have to reassess a lot of things,” says astronomer Christine Clement of the University of Toronto. She believes that intergalactic distance estimates based on these stars could be wrong by as much as seven per cent. Previous research had suggested that in the Milky Way, nearly two dozen of the 150 compact assemblies of stars called globular clusters are actually stolen from other galaxies. But no one had ever shown this for group II-b clusters – the metal-poor star systems thought to be the oldest in our galaxy. “They were considered representative globular clusters that formed during the early collapse of the Milky Way,” says lead author Suk-Jin Yoon from Yonsei University in Seoul. “Now, our results show that this is not true. The clusters originated from a satellite system which appears about one billion years younger than the oldest genuine globular clusters.” That age difference had been too small to detect by standard methods, so Yoon and colleague Young-Wook Lee used a newer approach that relies on the colour of particular stars within the clusters. The colour varies with mass, temperature and age and other factors. Yoon and Lee used computer models to create the colour distributions of specific hypothetical stars, and then matched these to the Milky Way’s globular clusters. This revealed the group II-b clusters as young imposters, distinct from the other clusters in our galaxy, and suggested they originated elsewhere. Yoon and Lee noticed that they were in the same plane as a smaller galaxy nearby called the Large Magellenic Cloud, suggesting that was the source of the stolen stars. “I was surprised to see how these clusters are lined up in the sky,” Kennicutt says. “It’s a very tantalising result.” Clement agrees, but adds: “I think it’s a starting point. We have to build other evidence.” Nonetheless, she adds that if the Korean astronomers are right, it could have important implications for distance measurements. This is because a younger age for the stars in the clusters is likely to mean that they are more massive than thought. This in turn affects their brightness, a measurement that is part of some estimates of distance. Journal reference: