The many processes of the Rho Ophiuchi ‘dark nebula’ highlighted by the colour of its clouds. To wit:
The blue regions shine primarily by reflected light. Blue light from the Rho Ophiuchi star system and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth’s daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula’s atomic and molecular gas. Light from nearby blue stars – more energetic than the bright star Antares – knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark brown regions are caused by dust grains – born in young stellar atmospheres – which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible here on the upper right, are even more colorful than humans can see – the clouds emits light in every wavelength band from the radio to the gamma-ray.
(Image: Amir H. Abolfath)
How come the sky near the star Antares and the Rho Ophiuchi cloud complex is so dusty and colourful? Who hasn’t wondered that? No one hasn’t. To wit:
The colours result from a mixture of objects and processes. Fine dust illuminated from the front by starlight produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the lower left of the featured image. Rho Ophiuchi lies at the center of the blue nebula near the top. The distant globular cluster M4 is visible to the right of Antares. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.
(Image: David McGarvey)
Behold: the colourful cloud complex of one of our nearest star forming regions.
And they call it a ‘dark nebula’. Honestly. To wit:
Rho Ophiuchi itself is a binary star system visible in the blue reflection nebula just to the left of the image center. The star system, located only 400 light years away, is distinguished by its multi-colored surroundings, which include a red emission nebula and numerous light and dark brown dust lanes. Near the lower left of the Rho Ophiuchi molecular cloud system is the yellow star Antares, while a distant but coincidently-superposed globular cluster of stars, M4, is visible just to the right of Antares. Near the image top lies IC 4592, the Blue Horsehead nebula. The blue glow that surrounds the Blue Horsehead’s eye — and other stars around the image — is a reflection nebula composed of fine dust. On the featured image right is a geometrically angled reflection nebula cataloged as Sharpless 1. Here, the bright star near the dust vortex creates the light of surrounding reflection nebula. Although most of these features are visible through a small telescope pointed toward the constellations of Ophiuchus, Scorpius, and Sagittarius, the only way to see the intricate details of the dust swirls, as featured above, is to use a long exposure camera.
(Image: Mario Cogo (Galax Lux))