APOD: IC 4592: The Blue Horsehead Nebula... (2013 Apr 02)

[Ann]

What I see is blue. Is that a big cloud of oxigen?

I agree with you that blue is cool!

But the blue cloud is a reflection nebula, so the cloud doesn't emit its own light, unlike, say, blueish planetary nebulae whose blue-green light is really due to oxygen. Instead, this is a reflection nebula, similar to the blue "mist" surrounding the Pleiades.

The blue "cloud" we see is starlight scattered by surrounding dust. The starlight is intrinsically blue, too, which makes the reflection nebula so obviously blue.

Ann

[neufer]

What I see is blue.

A typical human eye will respond to wavelengths from about (far violet) 390 to (far red) 700 nm.

• 1) Rayleigh λ⁻⁴ scattering law (violet/red) ratio: (390/700)⁴ = 10.4
  
  2) Rayleigh–Jeans λ⁻⁴ black body radiation law (violet/red) ratio: (390/700)⁴ = 10.4
  
  The Blue–Jeans Horsehead Nebula λ⁻⁸ scattered radiation law (violet/red) ratio: (390/700)⁸ = 108

<<Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. [The λ⁻⁴ dependence of Rayleigh scattering is based on classical physical arguments.] Rayleigh scattering results from the electric polarizability of the particles. The oscillating electric field of a light wave acts on the charges within a particle, causing them to move at the same frequency. The particle therefore becomes a small radiating dipole whose radiation we see as scattered light.

The Rayleigh scattering cross-section is given by:

The Rayleigh scattering coefficient for a group of scattering particles is the number of particles per unit volume N times the cross-section. As with all wave effects, for incoherent scattering the scattered powers add arithmetically, while for coherent scattering, such as if the particles are very near each other, the fields add arithmetically and the sum must be squared to obtain the total scattered power.>>

<<In physics, the Rayleigh–Jeans law attempts to describe the spectral radiance of electromagnetic radiation at all wavelengths from a black body at a given temperature through classical arguments.

For wavelength λ, it is:

In 1900, the British physicist Lord Rayleigh derived the λ⁻⁴ dependence of the Rayleigh–Jeans law based on classical physical arguments. A more complete derivation, which included the proportionality constant, was presented by Rayleigh and Sir James Jeans in 1905. The Rayleigh–Jeans law revealed an important error in physics theory of the time. The law predicted an energy output that diverges towards infinity as wavelength approaches zero (as frequency tends to infinity) and measurements of energy output at short wavelengths disagreed with this prediction.>>