page_title: S Monocerotis
hierarchical_section_title: S Monocerotis
caption_attribution_description: English: This colour image of the region known as NGC 2264 — an area of sky that includes the sparkling blue baubles of the Christmas Tree star cluster and the Cone Nebula — was created from data taken through four different filters (B, V, R and H-alpha) with the Wide Field Imager at ESO's La Silla Observatory, 2400 m high in the Atacama Desert of Chile in the foothills of the Andes. The image shows a region of space about 30 light-years across.
context_page_description: S Monocerotis, also known as 15 Monocerotis, is a massive multiple and variable star system located in the constellation Monoceros. It is the brightest star in the Christmas Tree open cluster in the area catalogued as NGC 2264.
  S Monocerotis is found within an open cluster and the Washington Double Star Catalog lists many companion stars. The closest and brightest is S Mon B, magnitude 7.8 and 3 arc seconds away. It is classified as B7 main sequence star with a mass of 5.12 M☉. 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 orbital period of 74 years. Since 1943, the spectrum 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 29 M☉ and 21 M☉.
  The distance to S Monocerotis and NGC 2264 has been derived in various ways, including dynamical parallax and isochrone fitting.
context_section_description: S Monocerotis, also known as 15 Monocerotis, is a massive multiple and variable star system located in the constellation Monoceros.  It is the brightest star in the Christmas Tree open cluster in the area catalogued as NGC 2264.
  S Monocerotis is found within an open cluster and the Washington Double Star Catalog lists many companion stars.  The closest and brightest is S Mon B, magnitude 7.8 and 3 arc seconds away.  It is classified as  B7 main sequence star with a mass of 5.12 M☉.  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 orbital period of 74 years. Since 1943, the spectrum 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 29 M☉ and 21 M☉.
  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.
caption: 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)
exif: {"Image ImageDescription": "This colour image of the region known as NGC 2264 \u2014 an area of sky that includes the sparkling blue baubles of the Christmas Tree star cluster and the Cone Nebula \u2014 was created from data taken through four different filters (B, V, R and H-alpha) with the Wide Field Imager at ESO's La Silla Observatory, 2400 m high in the Atacama Desert of Chile in the foothills of the Andes. The image shows a region of space about 30 light-years across. This image is available as a mounted image in the ESOshop. #L"}