APOD: Hubble 25th Anniversary: Pillars of... (2015 Jan 07)

[geckzilla]

Can anyone explain why the sky in the background is blue instead of the usual black?

Lots of glowing gas. It's also important to know that these colors are representative. The wavelengths coming out of the gas are not pure enough to have such vivid colors.

[Chris Peterson]

Ot's also important to know that these colors are representative. The wavelengths coming out of the gas are not pure enough to have such vivid colors.

The wavelengths coming from the gas are extremely narrow, and if they were bright enough (as from a reference tube) would appear intensely vivid and saturated. The colors seen here are representative in the sense that they have been shifted to different colors. "Pure" doesn't apply here.

[geckzilla]

Sure, but they are mixed with all the pure wavelengths so only with careful filtering and processing does it come out looking so vibrant. Any wideband images of glowing gas tend to be rather drab. Anyway, I don't think there is any way to explain it in a way that is easy to understand.

[Chris Peterson]

Sure, but they are mixed with all the pure wavelengths so only with careful filtering and processing does it come out looking so vibrant. Any wideband images of glowing gas tend to be rather drab. Anyway, I don't think there is any way to explain it in a way that is easy to understand.

Okay. But this has nothing to do with the wavelengths being emitted by the gases, which are, by definition, very pure colors. You're comparing a narrowband image to one where continuum light sources serve to reduce the saturation of the emission lines.

[geckzilla]

Yes, because almost everyone asks these questions thinking of light in terms of a continuum as their own eyes see it so I just go to that point automatically.