I was checking out this picture of the Andromeda Galaxy and in the caption you can find “This image was taken with an hydrogen-alpha filter”: what is it? Thanks!
In: Physics
When you excite(give energy to) the electrons around an atom and then they drop back down into their base state they give off a photon with a certain energy. This causes gasses to glow certain colors when you excite them, we use this feature for those colorful neon lamps. If you were to run this light through a prism you would see a mostly dark sheet with a few bright lines on it.
[Hydrogen has 4 lines in the visible spectrum](https://upload.wikimedia.org/wikipedia/commons/thumb/6/60/Emission_spectrum-H.svg/2560px-Emission_spectrum-H.svg.png), the bright Red line on the right is called Hydrogen-Alpha (H-alpha) and is the brightest line in the visible spectrum
H-alpha filters will block out all colors except this one so that you can get good contrast just based around the emissions of the hydrogen which is helpful when taking pictures of the [sun where the general glow would otherwise wash out all the details](https://upload.wikimedia.org/wikipedia/commons/d/d7/HI6563_fulldisk.jpg)
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