To their surprise, astronomers using the VISTA telescope at the Paranal Observatory (Chile) have discovered a thin disk of very young stars surrounding the core of the Milky Way.
The discovery comes from the Vista Variables in the Vía Láctea Survey (VVV). The survey aimed to map the bulge of the Milky Way using the infrared telescope as infrared light can travel through the dust present in the plane of the galaxy that can obscure views.
The survey looked at variable stars called cepheids. A cepheid star pulsates radially with a well-defined period. The period is linked to a cepheid's intrinsic luminosity – the longer the period, the brighter the star is. This makes cepheids ideal “standard candles”: by observing how bright they appear and knowing exactly how bright they are, we can work out their distance from observatories. The relationship between period and brightness was discovered by Henrietta Swan Leavitt, in 1908.
There are two types of variable cepheids. Classical ones are young, large and very bright stars while the other, called Type II, are significantly older and smaller stars. The latter type is found mostly in the galactic bulge, as the center of the Milky Way has an older population of stars compared to the disk. The disk is gas-rich so stars form more easily there, thus we see classical cepheids as expected.
The team discovered 655 cepheids, 620 Type II, but also 35 classical ones, which the team did not expect to observe so close to the core. The objects are all surprisingly young, at least in astronomical terms. All 35 stars are less than 100 million years old, with the youngest being only 25 million years old. This is not the only surprise in the study, though. The 35 objects are spread in a thin disk around the central bulge, a feature that had not been seen before.
This diagram shows the locations of the newly discovered Cepheids in an artist's rendering of the Milky Way. The yellow star indicates the position of the Sun. By ESO/Microsoft Worldwide Telescope.
The new disk has remained undiscovered until now because it is hidden by thick dust clouds. "This study is a powerful demonstration of the unmatched capabilities of the VISTA telescope for probing extremely obscured galactic regions that cannot be reached by any other current or planned surveys," said Istvan Dékány, lead author of the new study, in a statement.
Future investigation on the thin disk of cepheids will have to assess if the stars were born there or they simply moved towards the center. The detailed past of the Milky Way is still a very mysterious subject but discoveries such as this provide us with important clues on how our galaxy evolved.
Top Image Credit: Milky-way seen through Mobius Arch, Alabama Hills, by Kartik Ramanathan via Flickr. CC BY-NC-ND 2.0
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