Globular clusters are compact crowds of hundreds of thousands of stars bound together by gravity. For many years it was believed that all the stars within must be very similar, having formed close together from the same dusty cloud.
But in 2005, the Hubble Space Telescope measured the brightness and colours of stars inside the NGC 2808 globular cluster. Only one generation of stars was expected, but three were found.
Dr Giampaolo Piotto was the leader of the team that observed NGC 2808. “With an age up to 13.5 billion years – only 300 million years less than the age of the Universe – globular clusters are a benchmark for cosmology, and represent an ideal laboratory to understand star formation and chemical evolution in the Universe,” he explains.
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What defines the different generations of stars, also known as ‘stellar populations’, are characteristics such as their chemical composition, age, and their location in the cluster. Hubble’s high-resolution images allowed Dr Piotto and his team to look into the densely packed core of NGC 2808 and measure many stars – something that is difficult for ground-based telescopes to do.
Hubble’s power to observe in both visible and ultraviolet light also made it easier to spot multiple populations of stars and track their evolutionary paths.
“We have now used Hubble to observe more than 60 globular clusters – almost half the known globular clusters in the Milky Way. Preliminary results show that all have multiple stellar populations,” says Dr Piotto.
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