10 Lacertae (10 Lac) is a star in the constellation Lacerta. With an apparent magnitude of 4.9, it is located around 550pc distant in the small Lacerta OB1 association. It is a hot blue main-sequence star of spectral type O9V, a massive star that is currently fusing its core hydrogen. It is a suspected Beta Cephei variable star.
It was one of the first O-type stars (along with S Monocerotis) to be defined as an anchor point for the MKK spectral classification; since the early twentieth century it has served as such a point. Specifically, the star is representative of O9V stars, meaning relatively cool O-type stars on the main-sequence.[1]
It is the star with the smallest angular diameter measured by the CHARA array, at . A study by Kathryn D. Gordon and other five astronomers used this angular diameter to find the physical size of 10 Lacertae. Using a distance of, an average of earlier distances, they calculated a physical size that of the Sun, or million km.[2]
10 Lacertae has an 8th magnitude companion about one arc-minute away.[3]