
An ancient star cluster has three different groups of stars whose ages appear out-of-sync, as seen by NASA’s Hubble Space Telescope.
Astronomers used Hubble to find the dimmest stars in the NGC 6791 cluster. The cluster's location appears in the Digital Sky Survey image (left image), with a closeup courtesy of Hubble on the stars in question (upper right image). Two of the star populations appear as burned-out white dwarfs (lower right image), with one group (red circles) appearing to be six billion years old and another group (blue circles) appearing just four billion years old. The rest of the cluster’s normal stars are eight billion years old.
The finding puzzles the research team because they assume that stars in the same cluster should have formed roughly at the same time. However, they soon realized that the younger-looking white dwarfs might be paired off in binary star systems, so that the paired stars appear as a brighter, younger single star. That would explain why the different white dwarf groups appear to have different ages, which just leaves the mystery of why the white dwarfs aged slower than the cluster’s normal stars.
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