Atoms floating in the interstellar medium, including carbon and silicon, combine into tiny particles about the dimension of a visible light wave, causing scattering of light.
The scattering is more efficient for blue light than red light, so the interstellar clouds such as this one form reflection nebulas of blue color when starlight bounces from the clouds.
Within the Iris, dusty nebular material surrounds a hot, young star.
The dominant colour of the brighter reflection nebula is blue, characteristic of dust grains reflecting starlight.
Central filaments of the dusty clouds glow with a faint reddish photoluminesence as some dust grains effectively convert the star's invisible ultraviolet radiation to visible red light.
Infrared observations indicate that this nebula may contain complex carbon molecules known as Polycyclic Aromatic Hydrocarbons (PAHs). The blue petals of the Iris Nebula span about six light-years.
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