AstroJargon of the Week: HI

For this week’s (late) AstroJargon, I’d like to point out a bit of jargon I used in my Ada Lovelace post the other day. I talked all about HI (the letter “H” and the Roman numeral one) studies, and before posting, I quickly inserted “neutral hydrogen” as a definition. But why is that important anyway?

We know that the universe is mostly made of hydrogen. But unless it is creating energy by fusion in the core of a star, we can’t really see it in visible light. However, a neutral hydrogen atom floating about on its own can give off some light, though very rarely and with very low energy. The simplest hydrogen atom consists of one proton and one electron, and each particle has a property called “spin.” (They aren’t really spinning like planets, but it can be useful to think of it that way.) The spins of the particles can be aligned or anti-aligned, and each configuration is a slightly different energy. When the electron “flips” from the higher state to the lower state, a bit of energy, in the form of a photon of light, is given off. It’s a teeny, tiny bit of energy, not enough to be visible or even infrared. This light is given off in the radio regime with a wavelength of 21 centimeters, or 1.4 GHz. (The wavelengths of visible light range between 400 and 700 nanometers!)

When this was theorized and then discovered, it became an rich area of study for the newborn field of radio astronomy. After all, hydrogen gas is EVERYWHERE, and now we can see it! With hydrogen, we could finally peer through the dust of our own galaxy and determine its spiral shape.

From here.

We’ve been able to map the gas in spiral galaxies, further out than the stars, and make rotation curves that tell us that there’s some dark matter we don’t see.

We can detect interactions between galaxies that seem fairly normal in visible light.

And by detecting HI from the very early universe, we will be able to observationally probe how the very first stars and galaxies were forming in the first billion years of the universe’s history! That elusive signal is called the “epoch of reionization” which I discuss a bit more in my first 365 Days of Astronomy podcast, and in an earlier post.

So, I hope you see that HI is pretty cool, and everywhere, and it’s one of the cool things that radio astronomy has to offer in a unique way!

2 thoughts on “AstroJargon of the Week: HI

  1. HI is also, indirectly, the reason we look at most pulsars in the 1400 MHz band. Nothing particularly interesting happens to pulsars at 1400 MHz; they’re broadband sources, and if they have a spectral break it’s somewhere below 300 MHz (LOFAR might find it). But so may telescopes are set up to be effective at 1400 MHz that it’s the most convenient band for us to use. Initially we probably used HI receivers, even, though by now we’re looking for 800 MHz bandwidths, which are of no use to HI researchers.

    Redshifted HI – from the epoch of reionization or later – is also motivating the construction of a number of telescopes. In fact, since these generally need to be broadband to handle a range of redshifts, some of them can be made into quite good pulsar telescopes.

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