When we took those images many years ago, what we really got was a series of pictures through
narrow-band filters corresponding to wavelengths emitted by certain specific ions in the nebular gas.
We then choose three of the images, assign a different color (red, green & blue) to each and combine
them to make a color image. It's quite arbitrary. For example, the wavelength of hydrogen (H-alpha)
is nearly the same as nitrogen (N II) -- they are both red -- but they are distributed quite differently
in the nebula. So if you assign them both to red, you smear out the interesting structures. The one on
my webpage (
http://www.astro.umd.edu/~jph/) uses three filters which apply to oxygen ions. The
most prevalent is the highly ionized O III, which at 5007A does fall in the green part of the spectrum,
and I have assigned that image to green in the color composite. So the colors tend to be pretty arbitrary
-- that's why we call it "false color". The Hubble doesn't have a set of filters that match the response
of the human eye, and even if it did, the results would be rather disappointing, not revealing the
information we see in the false color combinations.
By the way, when we (Harrington & Borkowski) released that original image, we picked a combination
that was put together by Zolt Levay at STScI. The black and white image is one from the nitrogen
filter that I processed to reveal the jets.
When we took those images many years ago, what we really got was a series of pictures through
narrow-band filters corresponding to wavelengths emitted by certain specific ions in the nebular gas.
We then choose three of the images, assign a different color (red, green & blue) to each and combine
them to make a color image. It's quite arbitrary. For example, the wavelength of hydrogen (H-alpha)
is nearly the same as nitrogen (N II) -- they are both red -- but they are distributed quite differently
in the nebula. So if you assign them both to red, you smear out the interesting structures. The one on
my webpage (http://www.astro.umd.edu/~jph/) uses three filters which apply to oxygen ions. The
most prevalent is the highly ionized O III, which at 5007A does fall in the green part of the spectrum,
and I have assigned that image to green in the color composite. So the colors tend to be pretty arbitrary
-- that's why we call it "false color". The Hubble doesn't have a set of filters that match the response
of the human eye, and even if it did, the results would be rather disappointing, not revealing the
information we see in the false color combinations.
By the way, when we (Harrington & Borkowski) released that original image, we picked a combination
that was put together by Zolt Levay at STScI. The black and white image is one from the nitrogen
filter that I processed to reveal the jets.