The S Normae Cluster, C89 // Fernando Oliveira de Menezes
This open cluster is named after the brightest of its member stars, S Normae, a white-yellow giant star about 6 times more massive and 65 times larger than the Sun.
Importantly for astronomers, S Normae is a class of star called a Cepheid variable. These stars change their brightness at a regular and predictable rate. There is also a relationship between a Cepheid's pulsation rate and its intrinsic brightness, a fact discovered by Henrietta Swan Leavitt (1868-1921). That means that a Cepheid's pulsation rate tells us how truly bright it is, so how bright it appear to us tells us how far away it is! This makes Cepheids important standard candles for measuring distances in the universe.
