Why does the Orions nebula look green to the eye, but turns out purple in pictures?

Answer 1

There's a prominent red emission line of hydrogen that dominates the color profile of emission nebulae for CCDs and most films. The eye, however, is not very sensitive at that wavelength, so it's the blue-green hydrogen emission that you see. Astrophotographs are generally color-balanced to show the contributions of significant spectral lines, rather than to reproduce any sort of "realistic" color.

Answer 2

The human eye is more sensitive to some colors and less sensitive to others, whereas fill can be engineered to capture forms of light that the eye has trouble locking onto. The human eye is less sensitive to the color red. That is why red lights are often used by people working at night or in the dark. The eye does not contract its pupils in red light like it does for other colors.

Answer 3

I've never noticed the Orion nebula looking green myself, but those beautiful, multi-colored pictures you see of it and other celestial bodies are taken with special filters that highlight the colors from all the different elements present.

Answer 4

Because the eye integrates quickly and film can integrate slowly