Bird, wildlife, and landscape photography by Jon Rista
The Blue Horsehead
Tucked away in the constellation Scorpius, near the bright star that mark’s the great arachnid’s upper claw, is the faint and delicate shape of a horses head. Cool blue, shrouded in a cloak of faint red, this nebula is not the famous horse head most people may be familiar with. Also known as IC 4592, this nebula is formed at the tail end of an immense arm of dust that reaches out of a prominent feature of the summer core of the Milky Way, The Dark River. Lit faintly in a cool blue glow of reflected light from nearby hot blue stars, this is a feature of the night sky that can only be seen with high sensitivity stellar imaging equipment.
The Blue Horsehead is actually a fairly complex structure. It is comprised of both dust, and low level emission nebula. Parts of the dust reflect light from a few key prominent stars, most blue, some more yellow or red. The blue colors of the nebula are primarily reflections of starlight from stars in relative proximity to the dust, rather than emissions. The other parts of the nebula that show up a rusty red are dust that has absorbed the aggregate light of nearby and background stars, which is then re-emitted as mostly infrared light, with very low level red light emissions. The background sky surrounding the nebula is a slightly more blood red, as the regions is filled with a diffuse distribution of hydrogen gas, which emits more in the red spectrum rather than infrared.
Structurally, the largest and brightest star in the image represents the “eye” of the horse, making the whispy parts of the nebula behind the mane. The rest of the “horse’s head” shape should become evident once these features are identified.
Above and to the right of this image is a complex structure in Scorpius and his neighbor, Ophiuchus the Serpent-bearer, a somewhat famous region among astrophotographers, called the “Rho Ophiuchus” region. This region is a complex congregation of bright emission nebula, reflection nebula, dark dust nebula, and a variety of open and globular star clusters. A prominent feature of the region is the great star Antares, a brilliant yellow-orange supergiant star that gives the region a unique characteristic of colors, mixing the common blues of reflection and reds of emission with the yellows of brilliant reflected starlight. A knot of dust surrounding a cluster of three stars ultimately glows a slightly green color, as the blue light of reflection from those three stars blends a bit with the yellow light from Antares, creating a color seen with exceptional rarity in space.