what is an hydrogen-alpha filter in photography?

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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

The lyman-alpha emission line occurs when atomic hydrogen relaxes from the first excited state to the ground state, emitting a photon [n(2) -> n(1)], emitting a photon with an energy equal to the gap between the two energy states. This energy gap is equivalent to a photon with a wavelength of 121.6 nm. There is so much atomic hydrogen in the universe that it’s very bright when viewing 121.6 nm, so it is often appropriate to filter 121.6 nm out of the picture to provide contrast and resolve structures.

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)

Thanks u/Snakeinbeaver, I’ll just go and to try to ELI5.

You know how when metal goes really hot it glows red? That’s because everything that has heat gives off light, all the time. But the ‘color’ of the light is related to the temperature of the object, and human eyes can only see light produced by things that are thousands of degrees in temperature because our human eyes can see only certain colors.

I said the ‘color’ was based on the temperature *but* it’s also based on the elements that are in the hot material. So all together, this is why stars that contain a lot of hydrogen, and are really, really hot, are all that kind of whitish/orangish/yellow. That color comes from a mix of the hot-star-temperature AND the hydrogen the star is made from.

Now, that whitish/orangish/yellow color is actually what our eyes see from a bunch of different different individuals all on top of each other, like using half a box of crayons to draw the same Sun on top of itself in a picture. It’s chaotic and confusing. So you can use a special filter, called a “hydrogen-alpha” filter that removes 99% of the different colors and leaves you with a single reddish-orange color, a sun drawn from a single burnt-sienna crayon. It’s much easier to see and understand just this one color vs. all those crayons all stacked up on top of each other.