neutral hydrogen gas
/ HY-droh-jen /
Hydrogen is the simplest and most common atom in the universe — one proton with one electron orbiting it. Across the vast cool reaches between the stars, hydrogen usually keeps that electron, staying neutral and un-ionized. Astronomers call this neutral atomic hydrogen, and write it as HI (pronounced 'H-one'). It is the most abundant gas in the galaxy, and for decades it was the main way we mapped the unseen structure of the Milky Way.
Neutral hydrogen lives in the cooler, un-ionized phases of the ISM. In the cold neutral medium it sits at around 100 kelvin and a few tens of atoms per cubic centimeter; in the warm neutral medium it is more diffuse, around 8,000 kelvin and well under one atom per cubic centimeter. Crucially, HI emits a faint radio signal at a wavelength of 21 centimeters, produced when the electron flips the direction of its tiny magnetic 'spin.' Because that radio wave sails straight through dust that blocks visible light, HI surveys let us see clear across the galaxy, even through clouds that would otherwise hide everything behind them.
Mapping HI revealed the spiral arms of the Milky Way and the rotation of galaxies, and the deficit of expected rotating mass in that gas was an early hint of dark matter. A useful caution: 'neutral' refers to electric charge, not chemistry — neutral hydrogen is highly reactive and readily forms molecules where it gets cold and dense enough. And HI is only the atomic form; in the densest, coldest clouds hydrogen instead pairs up into molecular H2.
Radio telescopes tuned to the 21-cm line mapped neutral hydrogen across the whole sky and traced the Milky Way's gas spinning around the galactic center — revealing that the outer gas rotates far faster than the visible stars alone can explain.
HI maps trace galactic structure and rotation, hinting at dark matter.
'Neutral' means electrically neutral, not inert. HI is the atomic form of hydrogen; the molecular form (H2) hides in the coldest clouds and is nearly invisible, so astronomers trace it indirectly using carbon-monoxide emission instead.