The Pleiades (also known as Seven Sisters or M45) is an open star cluster with nebulosity. It is one of the closest star clusters to Earth, lying 444 light years away. It contains many young (<100 million years old) hot blue stars. The Pleiades is also a reflection nebula caused by dust reflecting blue light from adjacent young, very luminous stars. Although it was previously thought the dust was left over from the cluster's formation, it is now believed the cluster is simply passing through a particularly dusty region of space.