Electrically neutral Lightest and most common element in the universe (~92% by atoms; ~75% by mass). Hydrogen's isotopes are: • 1H (99.9885 %)
• 2H (0.0115 %), also called deuterium.
• 3H, also called Tritium, is a radioactive (t½ = 12.32 y) by-product of atmospheric thermonuclear tests in Earth's hydrosphere and atmosphere.
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 Origin of the 21-cm emission line that originates with a neutral 1H atom. The proton and the electron each have a quantum “spin,” which points either “up” or “down.” Spins can be parallel (both of them “up” or “down”), or antiparallel (opposite states). The antiparallel state has slightly less energy at 21 cm, and HI clouds were used to determine the structure of our Concentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are four main types of galaxies: • Elliptical
from our location within it.
Some or all content above used with permission from J. H. Wittke.