S Monocerotis

It is the brightest star in the Christmas Tree open cluster in the area catalogued as NGC 2264.

S Monocerotis A is a spectroscopic binary system with an eccentric orbit of about 112 years.

[19] It is also an irregular variable star with a range of less than a tenth of a magnitude.

[6] The distance to S Monocerotis and NGC 2264 has been derived in various ways, including dynamical parallax and isochrone fitting.

These consistently give estimates of 700 - 900 parsecs, although this is double the likely distance derived from the Hipparcos parallax measurements.

NGC 2264 including the Cone Nebula and the Christmas Tree Cluster (upside down in this image) with S Monocerotis at the very top of the image (and the base of the Christmas tree)