The Christmas Tree Cluster, an open star cluster in Monoceros

The Christmas Tree Cluster, an open star cluster in Monoceros

Image Credit: SIRTF/NASA/ESA

The Christmas Tree Cluster (NGC 2264) is a young triangular-shaped open cluster of stars embedded in a star-forming cloud, located around 2600 light-years away in the north-eastern corner of the constellation of Monoceros (the Unicorn), while it is moving away from us at about 17.7 kilometers per second.

In fact, the designation of NGC 2264 in the New General Catalogue (NGC) refers to both the Christmas Tree Cluster and the Cone Nebula, and not the cluster alone. Two other objects are within this designation but not officially included: the Snowflake Cluster and the Fox Fur Nebula (Sh2-273).

The Christmas Tree Cluster contains about 40 stars. At the base of the cluster is its brightest member, S Mon (also known as 15 Monocerotis), a bright blue-white variable star of magnitude 4.6 that is at least 8000 times as luminous as the Sun, and can be seen by the naked eye. The furry texture of the area next to this star is called the Fox Fur Nebula.

At the top is the 6th magnitude star HD 47887. Close to this star lies the Cone nebula, which is very hard to detect visually. Several stars of magnitude 8 to 9 mark the outline of the cluster.

While most of the visible-light stars that give the Christmas Tree Cluster its name and triangular shape do not shine brightly in this image, all of the stars forming from the dusty nebula are considered part of the cluster.

But, the newly revealed infant stars appear as pink and red specks toward the center of the image. The stars appear to have formed in regularly spaced intervals along linear structures in a configuration that resembles the spokes of a wheel or the pattern of a snowflake. Hence, astronomers have nicknamed this the “Snowflake Cluster.”

The NGC 2264 region contains clouds of interstellar hydrogen gas, mixed with small grains of dust. The majority of the hydrogen is neutral, which means that the nucleus of the hydrogen atom, a single proton, has a single electron attached. This is called an HI cloud (or region). In regions near hot, bright stars such as S Mon, the ultraviolet radiation from the hot star strips away the electrons from the majority of the hydrogen atoms. This process is called ionization and these regions are called HII regions. The free electrons frequently recombine with the nuclei for brief periods of time. When they recombine they must give up energy in the form of particles of light. Thus, the ultraviolet radiation from a nearby star is converted into a hydrogen glow. The result is a glowing nebula.

Intermixed with these glowing HII regions are the cooler, more dense HI clouds. Stars are forming in the most dense parts of these clouds (called cores). The stars of the Christmas Tree Cluster were formed very recently from the surrounding cloud complex in which new stars are forming today.

Dark clouds form where the gas isn’t glowing and the density is high enough that the dust particles intermixed with the gas can absorb significant amounts of starlight. If you look at a star through one of these so called dark nebulae, it will appear much dimmer (perhaps it will even be invisible). When you mix regions of glowing gas with dark nebulae, you can get interesting patterns such as the one which defines the Cone nebula

This image is a section of the Christmas Tree Cluster from NASA’s Spitzer Space Telescope, created in joint effort between Spitzer’s Infrared Array Camera (IRAC) and Multiband Imaging Photometer (MIPS) instruments.

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