Earthly dust may seem insignificant and trivial but the cosmic kind is an all important constituent of matter in the universe and is essential to the star making process. The famous Horsehead Nebula represents a dark cloud of dust and non-luminous gas which obscures and silhouettes the emitted light of IC 434 behind it. IC 434 has in turn received all its energy from the bright star Sigma Orionis. Protruding from its parental cloud, the horsehead is really a dynamic structure and a fascinating laboratory of complex physics. As it expands into the surrounding environment areas of the cloud sustain stresses which trigger the formation of low mass stars. One infant star is visible as a partly shrouded glow in horse's brow. Small reddish objects glowing through the dust represent Herbig-Haro objects, light emission of material ejected from invisible protostars.
The surrounding region also contains a multitude of different objects all unique in their own right. The bright emission nebula in the lower left is NGC 2024 (the flame nebula). Infrared studies have revealed a huge cluster of infant stars hidden behind the dust and gas of NGC 2024. The bright blue reflection nebula to the lower left of the Horsehead is NGC 2023. Interstellar dust reveals its presence by blocking light emitted from stars or nebulae behind it. Dust is composed mostly of carbon, silicon, oxygen and some heavier elements. Even organic compounds have been detected.
NGC 2023 is one of the brightest reflection nebulae in the sky and is located just east of the Horsehead nebula forming a thin blister at the edge of the molecular cloud L1630. The B type star HD37903, with a surface temperature of 22,000 degrees, is responsible for most of the excitation of gas and dust within NGC 2023 and lies in front of the molecular cloud. A unique feature of NGC 2023 is the presence of a shell of neutral hydrogen (H2) surrounding HD37903 out to a radius of about 0.65 light years. The shell amazingly emits light not by photoionization of hydrogen but by a unique process called vibrational fluorescence. It is the first reflection nebula known to exhibit this type of phenomenon. In addition to the reflection component, a number of Herbig-Haro objects (HH) which are associated with pre-main-sequence stars exist in the surrounding dust clouds. Two HH objects, HH4 and HH5 in the southeastern part of the nebula are illuminated by a star designated star C. Star C is thought to be a T-Tauri star. NGC 2023 is evidently an active region of star formation.