Hubble snaps best-ever picture of ancient, mysterious star cluster

Meet the globular cluster known as Messier 15.

Visible as a bright mote in the constellation Pegasus, floating just beneath the galactic disk of the Milky Way, Messier 15 is around 35,000 light years away from us here on Earth. Even at that distance, it's still bright enough to be seen with just a pair of binoculars, and although it might be mistaken for one very bright star, it's actually a collection of over 100,000 stars, roughly 175 light years wide.

NASA and the European Space Agency used the Hubble Space Telescope to snap the above picture on November 14th, combining visible light, infrared light and ultraviolet light images to show off the stars of this cluster in amazing detail. Exceptionally hot stars shine blue in the image, while cooler stars are more golden/red, and the image captures one of the features that sets Messier 15 apart from other globular clusters. The bright blue 'splotch', about halfway between the cluster's core and the left-hand side of the image, is a planetary nebula — a ball of expanding gas blown off from a dying star — called Pease 1, and Messier is the first globular cluster that astronomers found one of these nebula in.

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Messier 15 is ancient — one of the oldest globular clusters we know about — forming around 12 billion years, when our galaxy was still in its infancy. Although it has been imaged and studied extensively, astronomers are still finding new surprises there. The bright, densely-packed stars at the core of the cluster apparently hide a dark secret. It might be a cluster of dark, dense neutron stars, or it could be a rare type of black hole — larger than the black holes created during the collapse of a massive star, but smaller than the supermassive black holes that exist at the cores of galaxies like the Milky Way.

Studies of these clusters not only give us insights into the workings of ancient stars, but if we can confirm that Messier 15 does have one of these 'intermediate-mass' black holes at its core, it could lead to a better understanding of how black holes form and grow.

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