Origin of name and discoverer
NGC 2264, OCL 495, or LBN 911, is a region of the sky that officially refers to two distinct astronomical objects: the Cone Nebula and the Christmas Tree Cluster. These objects are in the vicinity of the Snowflake Cluster and the Fox Fur Nebula, unofficial components of NGC 2264.
This region of the sky was discovered by William Herschell on the night of January 18, 1784. It is almost about two years later, more exactly on December 26, 1785, that he discovered the Cone Nebula. He designated it as such because of its apparent shape. The other objects also took their names from their shapes and colors: the Christmas Tree Cluster looks like a reversed tree, while the Snowflake Cluster looks like a pinwheel or a snowflake. As for the Fox Fur Nebula, it looks like a red stole made of fox fur.
Position and dimensions
We can locate NGC 2264 at the Unicorn (Monoceros) Constellation, more exactly at 6h 41m right ascension and +9° 53′ declination. We are about 2500 light-years away from NGC 2264. The entire object is over 30 light-years across. But the Cone Nebula is only 2.5 light-years large. With an apparent magnitude of 3.9, it is not typically visible to the naked eye.
NGC 2264 is a region that holds the Cone Nebula, a dark nebula and a star-forming region in the Northern region of the Unicorn constellation, and the Christmas Tree open cluster, a group of very young stars that lies “above” the Cone Nebula, towards the South of the constellation. The Christmas Tree cluster is an active star-forming region. With these two main objects, we can also see the Snowflake Cluster in the middle of NGC 2264. It is also a star-forming region full of very young stars. The other (unofficial but very welcomed) member of the squad is the Fox Fur Nebula. It is a diffuse and dark nebula that lies in the Southern part of the group.
The whole region is considered a diffusive emission nebula, which means that it emits and diffuses light that it receives from neighbouring stars.
This image is a wide-field view of NGC 2264 and all its members. We can see the Cone Nebula on the left, followed by the Christmas Tree Cluster, the Snowflake Cluster, and the Fox Fur Nebula. The dark spots indicate the presence of thick and dense clouds of gas and dust. The Cone Nebula has bright edges as we can see in these images as well. This brightness is due to the Christmas Tree Cluster’s young and bright stars whose radiation has been slowly eroding the cloud for millions of years. The heat from these stars ionizes the hydrogen, causing it to emit what is known as the HII radiation. The use of H-alpha filters (like in images 1 and 2) highlights the ionized hydrogen regions in the nebula, which typically appear in red in these images. But the red color is also due to dust reflection of stellar light.
Although relatively simple, the shape of the Cone Nebula is a mystery. Scientists think that an energetic source of wind particles that lies on the top of the cone.
The closer we look (image 2), the more details and shapes appear in NGC 2264. These features were sculpted by the radiation of young stars that were born in the stellar nurseries nearby.
Using RGB filters (image 3) allows us to appreciate NGC 2264’s real colors.
The brightest star on all of these images is S Monocerotis (or in short S Mon), a massive double variable star. With apparent magnitudes of 4.66 and 7.83, it is the brightest stellar system in the Christmas Tree Cluster. We can see a blue halo around S Mon, which is due to the reflection of the starlight on surrounding dust particles.
The stars forming the Snowflake Cluster are slowly drifting away from the center. With time, the snowflake design will change. Then, we might think of another name for the Cluster.
The NGC 2264 is a rich and fascinating region of the sky that holds star clusters and nebulae. The turbulent nature of the clouds and the interactions with their surroundings gave it its special characteristics that are expected to change someday, but not soon.