The Pleiades. Optical image of the Pleiades or Seven Sisters open star cluster (M45) which is situated in the constellation of Taurus at about 400 light years from Earth. The Pleiades cluster is relatively young, being about 50 million years old. It is still embedded in a cloud of cold gas and interstellar dust, seen here as a blue nebulosity, which is the material left over from the cluster formation. The cloud forms the finest and nearest example of a reflection nebula and appears blue because blue light is reflected more efficiently by the interstellar particles. Despite its common name, up to 12 of this star cluster's stars are visible to the naked eye.