Are all nebulae/galaxy photos false colour? Even NASA ones?

Most of the nebulae and galaxy photos are what we’d call false colour, yes - although it’s probably much more fair to the people who make these images to call them “exaggerated colour”, or perhaps “reconstructed colour”. These images do not usually reflect what we would see if we looked at them ourselves.

The human eye has a really bizarre sensitivity pattern to light. We’re pretty good at seeing things in the yellow-green range, orange we can usually do, but once you get into reds and blues, our eye suddenly gets really bad at registering the deep reds and dark purples, and our brain translates those colours into “black”, or more accurately as, “there are no photons here that I can deal with”. To anything outside the range of visible light we are completely blind. This odd sensitivity pattern means that it’s really hard to make a camera with exactly the same sensitivity as our eye. This is the same reason why it’s sometimes hard to get your camera to pick up the colours we can see by eye. Most cameras have settings nowadays to help change the sensitivity towards a specific colour, but they won’t perfectly replicate the eyeball.

Furthermore, from a scientific standpoint, replicating the eyeball isn’t an incredibly useful thing to do. We’re usually more interested in either a specific colour of light (usually one that corresponds to the colour certain atoms release) which helps us tell how much of that atom is present in the nebula or galaxy. Alternately, we can go after broader “colour” - the relative contribution of blue versus red light tells us things about the stars and dust in a galaxy. If we’re interested in these total values, and in trying to compare red and blue, then introducing the handicaps of the human eye into the equation will only serve to complicate our situation more than necessary.

Given that we’re detecting light at much better sensitivities than the human eye, and that we’re usually doing it discrete chunks instead of one (very complex) curve as the eye does, putting these chunks of light back into a single image is tricky business. Even when all the light was taken from the narrow range of light that we could see, it must still be reconstructed and tweaked to reflect the brilliance of the nebula in the colours we’ve observed. Hubble has produced many beautiful images (such as the one above) labeled as ‘visible light images’. What this means is that the narrow ranges of colours that Hubble observed all fall within the the visible range - but they have still been patched together, the colour of each set of data overlaying on top of each other to build an image in full colour. This particular image had 6 colours to work with, and it’s made a lovely and vivid image, but it is still only six colours. The colours here aren’t really “false”, but they have been “reconstructed” from six black and white images.

“Exaggerated colour” images can be used to extend our sight much beyond what we can actually see. Perhaps a galaxy is rather unimpressive in visible light, but has an impressive brilliance in the ultraviolet or X-Ray - to our eyes this is dark; but if the telescope can look at ultraviolet or X-Ray light, we can put it into our image, and reconstruct an image that we will never see with our own eyes.

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With false-color images, how do we know which parts of the image go with which elements?

In false-color images, where elements are different colors, how do astronomers know what parts of the image are those different elements?

Most false color images are made entirely for illustration, and the creation of them is more an art than a science.

If you want to make a false color image, you have to start with a series of black and white images. All the cameras attached to telescopes are effectively just photon counters - if a photon makes it through the telescope and into the camera, it adds 1 to the number of photons that arrive from that patch of the sky. Now, generally astronomers are more interested in specific colors of light than they are in the total amount of light of any kind that arrives at the telescope. In order to limit the kind of light that actually makes it to the camera, then, we put a filter in front of the camera. This works in the same way as red-blue 3D glasses and images; the red lens only lets through red light, and the blue lens only lets through the blue, so each eye gets a different picture, and your brain reconstructs the depth of the image.

Filters for astronomy purposes are usually of a very specific color that corresponds to a physical process that you’re interested in. So, if you’re interested in looking at where the ionized hydrogen is, you create a filter that lets in only the light produced by ionized hydrogen, stick it on your telescope. Then you can produce an image which counts all the photons from the hydrogen and their locations on the sky. This image, which is in black and white, is what an astronomer would use to determine where these elements are in the sky.

To make a false color image, you’d get a bunch of these black and white images that correspond to different elements, and order them according to the wavelength of the light the elements give off. You then give the bluest wavelength the bluest color, and the reddest wavelength the reddest color, and patch them all together. Some images are quite scientifically useful to see where elements appear together relative to each other, but most of the spectacular images we have of the universe come from more artistic manipulations.

The ones that you see on the Hubble Heritage site have been tweaked so that they are more visually appealing. Mostly this involves playing with the colors and saturation levels assigned to the original black and white pictures, but it can result in much more dramatic images than if you just plopped all the images on top of each other and colored them in.

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