10 Lacertae

10 Lacertae (10 Lac) is a star in the constellation Lacerta. With an apparent magnitude of 4.9, it is located around 550 pc 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.

It is the star with the smallest angular diameter measured by the CHARA array, at $-1.274$. 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 $-5.605$, an average of earlier distances, they calculated a physical size $552$ that of the Sun, or $1,800$ million km.

10 Lacertae has an 8th magnitude companion about one arc-minute away.