Hydrogen Alpha

This rather beautiful image is a picture of the Milky Way taken in Hydrogen alpha (H-α) emission. In other words, this is what the Milky Way would look like to you, if the only things you could see were hot (ionised) hydrogen atoms. At least one of the most famous objects in the sky is really quite obvious. A large blob on the right of the image (the lower of the two) is our local star forming cloud the Orion Nebula, illuminated by the young stars forming inside it ionising the hydrogen gas therein.

The smaller spots and smudges in the image are mostly from other nebulae or supernova remnants. I haven’t really looked closely enough to say, but I’m sure you can see most of Messier’s collection in there somewhere.


(click to aggrandise)

Conveniently for astronomers, atomic hydrogen emits light at several characteristic frequencies. H-α, one of the brightest, lies at precisely 656.281 nm. That makes it a rather nice scarlet red which, coincidentally, also happens to be my favourite colour.

Astronomers like to talk about H-α quite a lot. It’s an extremely useful little tool which can be used to calculate a lot of things, regardless which flavour of astronomy you’re working in. For instance, if you observe stars, the H-α line will be slightly shifted, depending on whether the star is travelling towards, or away from us. Anything interesting about the star itself will be shifted by the same amount. If you find something recognisable that’s shifted differently, you’ve discovered something interstellar! H-α peaks can also be used to spot close binary pairs and, if a star is old or massive enough to be puffing off gas, you can sometimes use it to estimate stellar wind speed.

The only trouble with H-α is that hydrogen is one of the most abundant things in the entire Universe. As a result it can be overpoweringly bright and so isn’t always the most accurate thing to use. If you’re studying interstellar clouds, H-α can show you the size and the shape, but emissions from molecules like carbon monoxide are much better if you’re trying to be accurate!

The image is courtesy of the rather marvellous wikisky.org project, which will easily cause you to lose at least one hour of your life to sheer fascination if you go and look at it!

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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