Fred the Cat wrote:
I don't suppose Hubble's camera has a red-eye mode.
The Rosette Nebula in Hubble palette.
Photo: Filipe Alvez.
Apparently it does!
The picture on the left is admittedly not a Hubble image
, but it is a Hubble palette
image of a nebula that is completely dominated by red Hα emission. It must be said that not a single red Hα nebula actually looks red to the human eye through the telescope. Nevertheless, I find it noteworthy that many red Hα nebulas are made to look predominantly bluish in Hubble palette images (and in some bona fide Hubble images
, too), because there is some
OIII emission in the inner parts of them.
Planetary nebulas are different from "ordinary" emission nebulas in that there is extremely little hydrogen in the inner parts of very many of them (depending on their evolutionary state, admittedly, but still), but there is often quite bright
OIII emission there. That is why some planetary nebulas really look colored (green) to the visual observer, although basically all other deep-sky objects look colorless to the human eye.
As for the Cat's Eye nebula, it is dominated
by OIII emission:
However, when Huggins looked at the Cat's Eye Nebula, he found a very different spectrum. Rather than a strong continuum with absorption lines superimposed, the Cat's Eye Nebula and other similar objects showed a number of emission lines. The brightest of these was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known element.
nanometres is of course the green color of OIII.
Besides, planetary nebulas are called planetaries because the term was coined by William Herschel, shortly after he had discovered Uranus!
The term "planetary nebula" is a misnomer. It was coined by William Herschel, who also compiled an astronomical catalog. Herschel had recently discovered the planet Uranus, which has a blue-green tint, and he thought that the new objects resembled the gas giant.
Planet Uranus. NSA/JPL-Caltech.
Does Uranus often look red through the telescope? I don't think so!
My software, Guide, sometimes quotes an observer, Steve Coe, when describing deep-sky objects. This is what Steve Coe said about the Cat'e Eye Nebula, when he observed it one night back in 1995:
(...) great night near the Grand Canyon, light green, elongated 2X1 in PA 45, central star obvious, small dark donut around the central star. This donut of material is neon green, it really does seem to glow.
So any predominantly red portrait of the Cat's Eye Nebula by Hubble (or by anyone else) is not
And as for Hubble, it will sometimes show red objects as bluish and green objects as red!