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 systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

system located in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

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

. It is the brightest star in the

Christmas Tree A Christmas tree is a decorated tree, usually an evergreen pinophyta, conifer, such as a spruce, pine or fir, associated with the celebration of Christmas. It may also consist of an artificial tree of similar appearance. The custom was deve ...

open cluster An open cluster is a type of star cluster made of tens 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, and ...

in the area catalogued as NGC 2264. 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 January 2024) 1 ...

lists many companion stars. The closest and brightest is S Mon B, magnitude 7.8 and 3

arcsecond A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

s 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 A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

system with an eccentric orbit of about 112 years. Since 1943, the

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

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 ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

parallax measurements. Gaia Early Data Release 3 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 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 ? NGC 2264