APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

[Chris Peterson]

Yes, but now look at the two spectra you posted in a previous post.

Note, however, that this spectrum is completely useless for understanding the color of the Owl Nebula. M57 has less reflected light compared with emitted light (although you can still see some continuum light). A spectrum of the Owl Nebula would not look like a flat zero line with spikes at the emission sources, but like a low hot blackbody curve with spikes sitting on top of it. You're going to have a very difficult time assessing the color that someone would perceive when you look at a spectrum like that.

That said, consider the M57 spectrum. The 501 nm line will not appear as green or blue at all, because that line is not in isolation. We can simplify the spectral output to a 501 nm line and a 656 nm line. Visually, individually, these would appear as cyan and red. Present these two lines to the human eye, however, and we'll perceive yellow. In reality, we see something pushed down towards green because our eyes are more sensitive to the shorter wavelength line. The important point, however, is that we'll see a color substantially different than what we'd see with just the 501 nm line in isolation.

[Ann]

Yes, but now look at the two spectra you posted in a previous post.

Note, however, that this spectrum is completely useless for understanding the color of the Owl Nebula. M57 has less reflected light compared with emitted light (although you can still see some continuum light). A spectrum of the Owl Nebula would not look like a flat zero line with spikes at the emission sources, but like a low hot blackbody curve with spikes sitting on top of it. You're going to have a very difficult time assessing the color that someone would perceive when you look at a spectrum like that.

Good point. It is obvious from the appearances of these nebulas that there is more reflective dust behind the hot blue central star of the Owl Nebula than behind the central star of the Ring Nebula, so there must be more continuum light in the Owl than in the Ring.

That said, consider the M57 spectrum. The 501 nm line will not appear as green or blue at all, because that line is not in isolation. We can simplify the spectral output to a 501 nm line and a 656 nm line. Visually, individually, these would appear as cyan and red. Present these two lines to the human eye, however, and we'll perceive yellow. In reality, we see something pushed down towards green because our eyes are more sensitive to the shorter wavelength line.

I have a quibble with that. Visually, hardly anyone appears to be able to see 656 nm line when it is emitted by objects in space. We are much more sensitive to green than red light. If we were to see an object in space that emitted only two wavelengths, at 501 and 656 nm, in more or less equal amounts, it seems certain that this object wouldn't appear yellow at us. If we were to see color at all in it, it would be green to our eyes.

Ann

[Chris Peterson]

That said, consider the M57 spectrum. The 501 nm line will not appear as green or blue at all, because that line is not in isolation. We can simplify the spectral output to a 501 nm line and a 656 nm line. Visually, individually, these would appear as cyan and red. Present these two lines to the human eye, however, and we'll perceive yellow. In reality, we see something pushed down towards green because our eyes are more sensitive to the shorter wavelength line.

I have a quibble with that. Visually, hardly anyone appears to be able to see 656 nm line when it is emitted by objects in space. We are much more sensitive to green than red light. If we were to see an object in space that emitted only two wavelengths, at 501 and 656 nm, in more or less equal amounts, it seems certain that this object wouldn't appear yellow at us. If we were to see color at all in it, it would be green to our eyes.

Sorry if that was confusing. When I said "visual" I wasn't implying this is how we'd see the object directly. I just meant that if we mixed those two wavelengths of pure light, bright enough for us to see the color, we wouldn't see either color individually. And in the case under discussion here, we're talking about the photographic appearance given RGB images, which approximate what our eyes would see if they had more sensitivity.

Through the telescope, the only emission line that typically stimulates our color vision is oxygen, triggering all three of our color sensors. Ha only triggers red and green, but is usually not bright enough to do so significantly. So to the extent we see color through the eyepiece, it's nominally cyan, but pushed visually towards green by "white" light, which includes Ha and some continuum sources, or towards blue by scattered blue starlight. That's why when you read about the visual appearance of planetaries, they vary from blue to green.