Reflection Nebula M78 in Orion

M78 is the large blue object in this image. It is a reflection nebula 1,600 light years away in the Orion constellation, above the left-hand star (Alnitak) in the three-star “belt of Orion.” The blue nebulosity is caused by light from one or more background stars passing through a cloud of dust along our line of sight. The light is reflected or "scattered" by microscopic dust particles.

British physicist Lord Raleigh first described this effect in the late nineteenth century. It is called "Raleigh scattering" in his honor. Raleigh discovered that light is scattered by particles that are smaller than the wavelength of the light. Light having a longer wavelength than the size of the intervening particles will pass through undisturbed. This means that small atoms and molecules scatter short-wavelength light (blue and violet) and let longer wavelengths (red, orange, yellow, green) pass through undisturbed. In this image, the intervening dust cloud consists mostly of carbon atoms whose small size only scatters blue light.

It should also be noted that larger molecules will scatter all wavelengths of light equally, resulting in white light. This effect is observed whenever you drive a car at night into dense fog. The large water molecules effectively scatter all wavelengths of visible light from your car's headlights, resulting in the whitish illumination of the fog.

Reflection Nebula M78 in Orion

November 2017

New Mexico

Takahashi FSQ-106 apochromatic refractor, Paramount ME mount, SBIG STL11000 CCD camera, Astrodon LRGB filters.

LRGB 12 hours, processed in Pixinsight and Affinity Photo

1600 light years

Orion