NASA / ESA / STSCI / H.RICHER,UBC / SCIENCE PHOTO LIBRARY NASA / ESA / STSCI / H.RICHER,UBC / SCIENCE PHOTO LIBRARY
Stars in M4 globular cluster, optical Hubble Space Telescope (HST) image. Globular star clusters are balls of hundreds of thousands of stars. M4 is one of the nearest globular clusters, lying about 7000 light years from Earth in the constellation Scorpius. Its closeness allows the HST to observe (in more magnified views) the white dwarf stars in the cluster. These white dwarf stars are remnants of stars, and no longer shine by nuclear fusion. Instead, they slowly cool over time. This allows them to be used to date the age of the universe.
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