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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Are the pictures with millions of stars & nebulae in them real?

The photos in which millions of stars can be seen alongside several nebulae- are these real or edited? If the former, are they taken from space? Can you see them from the naked eye?

While there are certainly a number of edited space pictures floating around the internet, there are also a lot of real pictures like what you describe. The most famous of these tend to be of the Orion Nebula - it’s a very beautiful nebula (along with being scientifically fascinating), so lots of people - both professional astronomers and amateur astronomers - have taken pictures of it. (The Trifid Nebula is another common target.)

Why are there so many stars in these pictures? Almost all of the gas (and most of the stars) in our galaxy is contained up in a very thin disk - proportionally, the Milky Way is thinner than a razor blade. This means that all these nebulae - which are clouds of glowing gas - are also found within this very thin disk. If we look towards them, we are looking through the disk towards the greatest concentration of stars in the sky. The thousands and thousands of stars captured in these images are just the handful of the stars in our galaxy that happen to be in the same direction as the nebula. The fainter the nebula you’re trying to observe, the more stars you’ll capture.

These nebulae can be imaged either from the ground or from space - it depends on what we need the pictures for. The pretty Hubble images you see are usually not the main goal of pointing Hubble at that patch of sky - scientists are usually concerned with how much light there is of a certain colour in a certain region, and whether or not there is more of a certain colour than a second color - these differences in colour can help us understand the structure of the nebula, and how warm or cool it is, along with a number of other properties. This information can be used to make a colour image of the nebula, but it often isn’t done right away, or at all.

If, however, you’re simply wanting to make a really good-looking picture of a photogenic patch of the sky, you would do that from the ground. There are so many things that only space telescopes can do, that we can’t spend much (or any) time just to get good pictures. But on the ground, your time is less constrained, and a lot of amateur astronomers have their own telescopes and cameras attached to them, so they can just go out and take pictures whenever the sky is dark and clear. In this case, these detailed pictures are taken over a long period of time, to make sure that there’s enough time for the light from all those stars to reach the camera.

Most nebulae are too faint to be seen with the naked eye - but the Orion nebula is a notable exception. If the constellation Orion is up, look for the middle “star” in Orion’s sword - it’s actually the Orion nebula. In a very dark sky, it might even look a bit pink.

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