Longer answer: It’s actually light diffracting around the four arms (called spider legs) holding up the secondary mirror of a telescope! For the stars above it’s four spider legs from the stars because the Hubble Space Telescope took these images and its secondary mirror has four arms holding the secondary – for other telescopes the diffraction could be different depending on how the secondary mirror is held up.
Here’s apicture of the Hubble’s secondary mirror design:
Three of the four “spider arms” are visible and I drew red lines next to them so you could see what they look like.
(Image credit: NASA and ESA
)
While they’re at it, they could point out that almost all the colors in space photos are faked too
“Faked” is a huge simplification of that. Actually most of the space images you see are detections of colors the human eye can’t see… like x-ray, gamma or radio wave light. By converting these colors into pictures visible to the human eye we aren’t lying… we are allowing ourselves to actually see and study the universe – not to mention make a massive fraction of the invisible reality totally visible to the average person. Uh, you’re welcome? 😉 😛
It’s even more complicated than that. Some of the images are not “true color” even though they are actually taken in visible light. Let me explain with an image I made:
This is an image of M101. It *looks* like a true color image, but it’s not. That is because it is a composite of these two images:
The top one is an exposure taken through a B (”blue”) filter while the bottom one is taken through a “V” (”visible”) filter. The B band is centered on 445nm or blue wavelengths whereas the V band is centered on 551 nm or yellow/green wavelengths. These two images when overlaid would not make an easily viewable image, so I made the V image orange so they became complementary colors.
But! It gets more complicated still. You see, when you take an image through a telescope, the CCD (your “camera”) always produces an output in greyscale. The greyscale image’s appearance can be further altered by changing the way the counts on the CCD are displayed as values on your computer screen.
This here is a linear stretch. It would look very different with a histogram equalization stretch, while still actually being the same image.
You can only get color images by combining two or three different greyscale images, and dyeing them different colors. Sometimes, you take an image through a red, green, and a blue filter, and then when you combine them, it roughly approximates a “true color” image. However, as you have control over the intensity and contrast of each component color, the resulting image can be made to look differently.
Two ways of making a composite.
Sometimes you want to combine images taken through filters that focus in on specific spectral lines. Sometimes these spectral lines are in the same region of the color spectrum. For example, the famous Hubble “pillars of creation” image actually contains two red spectral line components, one due to hydrogen, and one due to sulfur. To make the composite image easier to read, one of these red wavelengths was recolored blue.
The image would basically be mostly red if they didn’t do this.
And of course, as @antikythera-astronomy said, there are always false-color images of light wavelengths we cannot see, such as this radio image of mine:
So, what does this all mean for images being faked or not?
The Answer: None of the images of space taken by telescopes are what they would look like to your eyes, but that’s okay, because what is the point of a telescope if it just mimics what your eyes see?
The human eye cannot distinguish colors at all for low surface brightness objects such as galaxies or nebulae. If you ever look through a personal telescope at a galaxy, what you see is a faint smudge. That first galaxy whose image I posted? I’ve seen it with my own eyes, and it looked like a blurry grey blob with faint grey spokes. Even if you were in a spacecraft blazing your way through the Orion Nebula, you wouldn’t be able to see its colors. The surface brightness is just too faint.
Even though none of these images is a “true color” image, none of them are faked either, because each is actually visualization of a scientific measurement. They are merely ways we can use our computers to present to us the results of measurements of photon counts, wavelengths, and position on the sky, it’s like changing the scale on a graph, or a map. It’s not fake, just…different.
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