鈥榊oung鈥� stars that seem to have formed impossibly close to our galaxy鈥檚 supermassive black hole could in fact be ancient interlopers merely masquerading as youngsters, a new study claims.
Several clusters of what appear to be massive young stars have been found just a few dozen light years from the black hole at the centre of the galaxy. Watch a .
But that is puzzling, since astronomers think the black hole鈥檚 intense gravity should rip apart gas clouds before they have a chance to condense and form stars (although some recent work has disputed this). At the same time, such massive stars are too short-lived to have survived a journey from much farther out.
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Now, Douglas Lin of the University of California in Santa Cruz and Stephen Murray of the Lawrence Livermore National Laboratory in Livermore, California, both in the US, have proposed a third possibility.
They say the clusters are actually old and look young only because they are collecting a lot of fresh gas in the region.
Inner core
They believe the clusters migrated to the galactic centre from elsewhere, gradually spiralling inwards as they lost energy due to the drag exerted by stars and gas they encountered in their orbits. Although most of the stars in each cluster would be ejected in the migration process, the cluster鈥檚 inner core could survive all the way to the black hole鈥檚 neighbourhood.
Once there, compact stellar corpses called white dwarfs and neutron stars, which are known to collect in the centres of massive star clusters, could absorb material from nearby gas clouds. The corpses would heat up and glow where the gas fell on them, making them appear as bright as young, massive stars.
鈥淵ou don鈥檛 need to accrete very much before the individual stars start to become quite luminous,鈥� Lin told 快猫短视频.
Lin says the scenario avoids a problem associated with the idea that the clusters formed where they are now. Unless such star formation began only recently, successive generations of stars over the galaxy鈥檚 lifetime would have left behind a lot of heavy elements around the galactic centre 鈥� something that is not seen there.
Torn apart
But Mark Morris of the University of California in Los Angeles, US, who studies the galactic centre, is sceptical of this scenario. Aside from white dwarfs and neutron stars, an old cluster should contain some bloated, dying stars called red giants.
But no red giants have been found there. 鈥淪o far there鈥檚 been no evidence for that and believe me, we鈥檝e looked,鈥� he told 快猫短视频.
Warren Brown of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, is also unconvinced. He says migrating clusters would likely be torn apart before reaching the galactic centre. 鈥淚 tend to buy into this idea that maybe these clusters formed where they are now,鈥� he told 快猫短视频.
Lin says the red giants could escape detection because they would be relatively faint, a problem compounded by the fact that this whole area is obscured by dust. And although clusters would tend to get torn apart on their way to the galactic centre, only a small percentage would need to survive to explain the massive clusters seen there, Lin鈥檚 co-author Stephen Murray says.
