Tags: Star.
S Monocerotis also known as 15 Monocerotis is a massive variable star system located in the constellation Monoceros. It is a spectroscopic binary system with an orbital period of 74 years. Since 1943 the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It is a type O main sequence dwarf varying in between magnitude 4.2 and 4.6. The star lies at the base of the Christmas Tree Cluster in NGC 2264.Physical data for the system is disputed.