hydrogen

Hydrogen (H) is the element with atomic number 1, symbol H. It is the most prominent element in the universe, 75% by mass, more than 90% by number of atoms. Its most common isotope has mass number 1; mass number 2 (called deuterium, ²H or D) is also stable and mass number 3 (tritium, ³H) is unstable and very rare. The symbol X is used to indicate a mass fraction of hydrogen, e.g., X_p for its primordial abundance, its mass fraction of the atoms/nuclei produced by Big Bang nucleosynthesis. (Over time, some hydrogen has fused into more massive elements, e.g., powering main sequence stars, but with only a small effect on its mass abundance over the whole universe.)

Neutral atomic hydrogen is termed HI, and a cloud of it is known as an HI region. A region with excited and ionized hydrogen (HII) is termed an HII region. Hydrogen forms molecules of two hydrogen atoms (HH aka H₂, H2, or molecular hydrogen), which produces very little electromagnetic radiation. These are presumed to form mostly on the surface of dust particles in the interstellar medium. Clouds of it are termed molecular clouds. These do not produce easily-observable molecular hydrogen lines, but can be detected using EMR from their tiny amounts of other compounds, such as carbon monoxide, which is commonly used as a tracer for identifying such clouds, and using the CO to H₂ factor (X_CO), for estimating their mass.

The spectral lines of atomic hydrogen are often used in astronomical observation. They are classified by series (hydrogen line series), each consisting on photons generated by the relaxation of electrons from more excited energy state to a specific lower energy state, each such state associated with an electron shell, numbered as n=1, 2, 3, etc. Among the series:

• Lyman series - transitions down to n=1
• Balmer series - transitions down to n=2
• Paschen series - transitions down to n=3
• Brackett series - transitions down to n=4
• Pfund series - transitions down to n=5
• Humphreys series - transitions down to n=6

Such downward transitions are for emission lines: corresponding absorption lines for each series produce transitions up from these levels. The series' wavelengths adhere to the Rydberg formula:

1/wavelength = RH ( 1/n² - 1/m² )

• n is the lower electron shell number, e.g., 2 for the Balmer series.
• wavelength is the wavelength within the series.
• R_H is the Rydberg constant for hydrogen, 1.0973731568160 × 10⁷m^-1 ≈ 13.6 eV / (hc)
• m is the higher electron shell number.
• h - Planck constant.
• c - speed of light in a vacuum.

Atomic hydrogen also produces spectral lines (and spectral-line variations) from tiny energy-differences within individual shells (fine structure), such as the 21-cm line.