The Cone Nebula

The Cone Nebula is a famous H II region located in the constellation Monoceros, the Unicorn. It lies at an approximate distance of 2,700 light years from Earth and is located in the Orion Arm of the Milky Way. The nebula is part of the larger star forming region around the Christmas Tree Cluster. The two objects share the designation NGC 2264 in the New General Catalogue.

The Cone Nebula has an apparent length of 10 arcminutes, corresponding to a spatial extension of about 7 light years. It is faint when observed visually, but makes a stunning sight in long exposure photographs.

The nebula forms the southern part of NGC 2264, while the Christmas Tree Cluster forms the northern.  The entire region of NGC 2264 occupies an area of 20 arcminutes and has a visual magnitude of 3.9.

The Cone Nebula is relatively easy to find because it lies in a region that contains many bright stars. It is located in the northern part of Monoceros, north of the line drawn from Procyon in Canis Minor to Betelgeuse in Orion, and not far from the brighter Rosette Nebula. The Cone Nebula can be seen in 10-inch and larger telescopes under dark skies.

The giant pillar of gas and dust was named for its conical shape, which is a result of a dark nebula absorbing the light of an emission nebula that lies behind it. The faint emission nebula is illuminated by the star S Monocerotis, the brightest member of the NGC 2264 cluster. S Monocerotis (15 Monocerotis) is a massive variable star system that lies at the base of the Christmas Tree Cluster, believed to consist of up to eight components.

The Cone Nebula was likely formed by the wind from the massive star designated as NGC 2264 IRS. The radiation of the hot nearby stars illuminates and erodes the giant pillar of gas, while ultraviolet radiation is responsible for the pillar’s reddish glow.
The infrared source NGC 2264 IRS is embedded within the dust and gas of the nebula and cannot be seen by optical telescopes. Hubble images taken in infrared light have revealed that the massive young star is surrounded by six very young, pre-main sequence stars with a mass almost that of the Sun, all found within less than a tenth of a light year of NGC 2264 IRS. NGC 2264 IRS is also known as Allen’s Source, after the astronomer D.A. Allen, who pointed out in 1972 that the object is the likely source of the radiation pressure responsible for creating the shape of the Cone Nebula.

The Cone Nebula resembles the better known Pillars of Creation, a star forming region made famous by the Hubble image in 1995, located in the Eagle Nebula (M16) in the constellation Serpens. Pillar structures like these, formed of cold gas, are common in large star forming regions and believed to be incubators for very young stars.

The Cone Nebula was discovered by the German-British astronomer William Herschel on December 26, 1785, almost two years after he had discovered the Christmas Tree Cluster. Herschel catalogued the nebula as H V.27.

There are several notable objects located in the vicinity of the Cone Nebula. The reflection nebula IC 2169, also known as Dreyer’s Nebula, is detached from the Cone Nebula, but believed to be part of the same large molecular cloud. The nearby NGC 2261, or Hubble’s Variable Nebula, probably lies behind the Cone. It is a bipolar nebula about 3 years across, illuminated by the variable star R Monocerotis. Its name comes from the fact that, like the central star, the nebula varies in brightness.

Cone Nebula

Constellation: Monoceros
Right ascension: 06h 41m 15s
Declination: +09°21′
Apparent magnitude: –
Distance: 2,700 light years
Apparent size: 10 arcminutes
Radius: 4 light years
Designations: Cone Nebula,  NGC 2264 (portion)