The number of particles in a column of unit area along the line of sight, measuring gas abundance.Column density is a measure of how many particles are present in a column of unit cross-sectional area along a given line of sight. It is typically expressed in units of particles per square centimeter (cm⁻²) and is a fundamental quantity in astrophysics, where it quantifies the amount of gas or dust between an observer and a distant source. For example, the column density of hydrogen in the interstellar medium tells astronomers how much material lies along the path to a star, affecting the star's observed brightness and spectrum. Column density is distinct from volume density (particles per cubic centimeter) because it integrates along the line of sight, making it directly relevant to observations of absorption and emission.
The concept of column density has been used in astronomy since the early 20th century, particularly in studies of stellar atmospheres and interstellar absorption lines. It became a standard tool with the development of radio astronomy and ultraviolet spectroscopy in the mid-1900s.
To obtain column density, one integrates the volume density n(r) along the line of sight from the observer to the source: N = ∫ n(r) dr. In practice, astronomers infer column density from spectral line measurements—the strength of absorption or emission lines depends on the number of absorbing or emitting particles along the path. For instance, the hydrogen column density is often derived from the Lyman-alpha absorption line, and dust column density from infrared extinction maps.
Column density is used to map the distribution of gas in galaxies, to correct for extinction in astronomical observations, and to assess the shielding of molecular clouds from ultraviolet radiation, which affects star formation.
The column density of the Earth's atmosphere at sea level is about 10²⁵ molecules per square centimeter, which is why the sky is blue—Rayleigh scattering is efficient because the column is so thick.
| N | variable | column density — column density per square metre |
| S_ν | variable | line flux density — spectral flux density joule per square metre per second per he |
| Δν | variable | line width — frequency interval hertz |
| Q | variable | rotational partition function — partition function |
| T_rot | variable | rotational temperature — temperature kelvin |
| A_ul | variable | Einstein A coefficient — einstein a coefficient per second |
| Ω | variable | solid angle of source — solid angle steradian |
| h | variable | Planck constant — planck constant joule second |
| c | variable | speed of light — speed of light metre per second |
| g_u | variable | degeneracy of upper level — degeneracy |
| E_u | variable | energy of upper level — energy joule |
| k | variable | Boltzmann constant — boltzmann constant joule per kelvin |
| N | variable | column density — column density per square metre |
| EW | variable | equivalent width — equivalent width metre |
| f | variable | oscillator strength — oscillator strength |
| λ | variable | wavelength — wavelength metre |
