H is for Hydrogen and Helium

H is for Hydrogen and Helium, pretty much most of the matter of the universe.

For an astronomer, the Periodic Table of the elements goes like this: hydrogen,  helium, everything else.  (Okay, it really goes, hydrogen, helium, 'metal', starkiller (iron), supernova poop.)

Hydrogen is the most abundant element in the Universe at a whopping 74% of all baryonic matter. Helium takes up about 24%, which leaves about 2% for everything else. Most of that hydrogen and helium float about as Interstellar Medium (ISM), gas clouds like nebulae and gravitationally collapsed into stars.

Why's this? Because in the early seconds of the Big Bang, hydrogen was the easiest atom to synthesis through Big Bang nucleosynthesis. Helium wasn't too hard to synthesise either. Heavier elements tend to be nucleosynthesised in stars through fusion or by supernovae. (This is why they tend to be called "metals" by astronomers, regardless of what they are. If you're not hydrogen or helium, you're a metal.)

A hydrogen atom: one proton, one electron.
Doesn't get any more basic than that. 

How do we know there's so much hydrogen out there? Because hydrogen has its own little radio broadcast channel along the 21cm wavelength line or 1,420 MHz. (Okay, it's more in the range of microwaves, than actual radio waves, but still cool to a radio astronomer.)

In fact, once we were able to identify hydrogen, we were able to see so much more of the Universe, as microwaves at that 21cm frequency can easily penetrate the clouds of dust that tend to block visible light. It was by mapping the hydrogen of the Milky Way that led us to discover it was a spiral galaxy.

Hydrogen hardcore: observing the universe through 21cm.

Do you think we'll run out of hydrogen eventually? How?

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Her Grace once used hydrogen to fill up party balloons because she'd run out of helium.