1/ This is a wide-field picture of Orion I took last night with a diffraction grating in front of my camera lens. It's a great way to visually compare the spectra of different astronomical sources. Let me explain what we're seeing here, because there's a lot to unpack here... 
A diffraction grating is a piece of glass, plastic or a similar transparent material with lots of lines engraved on it, very close to each other. When light goes through a diffraction grating it gets decomposed into its constituent colors.
Image credit: opentextbc.ca/universityphys…
Image credit: opentextbc.ca/universityphys…
Here I've highlighted three stars: Rigel, Sirius and Betelgeuse. Notice how Rigel and Sirius emit way more blue/violet light than Betelgeuse? That's because they're much hotter: roughly 10,000 degrees vs only 3,500 for Betelgeuse.
Now look at the spectrum of M42, a star-forming region. The emission is brightest at very specific colors, corresponding to different atoms. I've highlighted the hydrogen alpha, beta and gamma lines, as well as one Helium line and two forbidden oxygen lines.
"Forbidden" simply means that these lines can't be easily reproduced on Earth because it's so hard to rarify gases to the extremely low densities found in space.
Finally, we can also see the four lasers we use to correct the atmospheric turbulence. Because these lasers are tuned to a specific frequency, to excite sodium atoms 80-90 km above the ground, their spectrum isn't continuous, it just shows emission at that particular frequency.
Sorry, forgot the picture!
You can buy diffraction gratings in specialized stores or online, for just a few bucks. Or just strip off the cover of an old CD, that works too!
End of thread!
End of thread!
