Neutral Hydrogen

Electrically neutral hydrogen atom with one proton and one electron. It is commonly referred to as HI (pronounced H-one), and is located throughout galaxies as HI clouds or external to galaxies as part of the intercloud gas. Neutral H is detected via the spin-flip transition at 21 cm, and HI clouds were used to determine the structure of our Galaxy from our location within it.

Some or all content above used with permission from J. H. Wittke.