S Monocerotis, also known as 15 Monocerotis, is a massive multiple and

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...

system located in the constellation

Monoceros Monoceros (Greek: Μονόκερως, "unicorn") is a faint constellation on the celestial equator. Its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, ...

. It is the brightest star in the Christmas Tree

open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, an ...

in the area catalogued as

NGC 2264 NGC 2264 is the designation number of the New General Catalogue that identifies two astronomical objects as a single object: the Cone Nebula, and the Christmas Tree Cluster. Two other objects are within this designation but not officially incl ...

. S Monocerotis is found within an open cluster and the

Washington Double Star Catalog The Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory. The catalog contains positions, magnitudes, proper motions and spectral types and has entries for (as of June 2017) 141, ...

lists many companion stars. The closest and brightest is S Mon B, magnitude 7.8 and 3 arcseconds away. It is classified as B2 main sequence star with a mass of . Designated component C is an 11th-magnitude B8V star. The cluster contains another dozen or so 9th and 10th magnitude stars and many fainter stars. S Monocerotis A is a spectroscopic binary system with an eccentric orbit of about 112 years. Since 1943, the

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

of this star has served as the MK standard for O7 by which other stars are classified. It is also an irregular variable star with a range of less than a tenth of a magnitude. The orbital parameters can be used to derive the masses of the two stars, giving and . 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.

Gaia Early Data Release 3 The ''Gaia'' catalogues are star catalogues created using the results obtained by '' Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, espec ...

contains parallaxes for the companions components B and C of and respectively, consistent with the expected distance to the cluster.

References

External links

S Monocerotis by James B. Kaler
{{DEFAULTSORT:S Monocerotis Monoceros (constellation) Monocerotis, 15 Irregular variables Monocerotis, S BD+10 1220 2456 047839 031978 O-type main-sequence stars B-type main-sequence stars Emission-line stars ?