Originally posted at Forbes!
When we take a picture of an object out in space, exactly what you’re capturing with the camera depends pretty strongly on the type of camera you’re using, and if you’ve selected a specific wavelength of light that you’d like to capture. And while a nebula - which is just a pile of glowing gas - is usually sitting within a galaxy, the pictures we take of a galaxy and the pictures we can take of nebulae will look fundamentally different, if you know what to look for.
The pattern we associate with a galaxy - spiraling tendrils out from a central bright core - is present in both the stars and in the gas. If we take an image of a galaxy with a camera that is sensitive to the same section of light that our eyeballs are sensitive to (like the Hubble Space Telescope), most of what we capture is stars, and dark lanes of dust. Dust in a galaxy behaves like the dust you get on Earth, and blocks any light there might be behind it, so for a galaxy, we can’t see the starlight from behind it. In an image like the one below, you can also see some nebulae - in this case, glowing hydrogen - as a very particular shade of hot pink. But you can also see that the dark brown dust, the hot pink nebulae, and the background brightness of the stars, all follow the same spiraling pattern.
There’s also elliptical galaxies, which look like the very cores of the spirals you might think of first. They tend to be amorphous, roundish fuzzballs of slightly redder stars; where the spiral galaxies are a bluish white, ellipticals tend more towards orange-white. In both cases, the individual stars are totally invisible, blurred together in our images of them. The only stars that you can spot in an image of a galaxy are stars that happen to be close to us, and wind up projected onto the same patch of sky.
If you’re looking at an individual nebula - a small (on astrophysical scales), glowing cloud of gas, the patterns you see are very different. There are many more types of nebulae, but they are tied together as a class because they are all clouds of gas, heated to the point of glowing, and that heat is usually provided in the form of a very nearby star.
Planetary nebulae, which form at the end of a star’s lifetime, are heated by the faded star which remains as a white dwarf in their centers. Other nebulae, like the Orion nebula, are the clouds of gas which enveloped a denser region where new stars could form. Those new stars then heat the surrounding gas, both heating and sculpting it with their radiation and solar wind. Any kind of nebula should, therefore, have visible individual stars within it. Many images of nebulae have had to point the camera at the same patch of sky for a rather long time in order to capture the glowing gas in the image, and the stars have saturated the camera, and wind up with spikes protruding out of them. These spikes are entirely an artifact of how the telescope and detector are constructed, but if you see these spikes, you know you're looking at a star.
Clothing manufacturers are particularly bad about making the distinction between nebulae and galaxies properly - pretty much everything space-like, whether or not it’s even a real image, winds up being labeled as a “galaxy”. But with the above patterns in mind, you should be able to tell if what you’re looking at is more likely to be a nebula or a galaxy. Just look for stars.
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