Editing Area density (section)

===Astronomy===
{{expand section|date=August 2015}}

In astronomy, the column density is generally used to indicate the number of atoms or molecules per square cm (cm<sup>2</sup>) along the line of sight in a particular direction, as derived from observations of e.g. the 21-cm [[hydrogen line]] or from observations of a certain molecular species. Also the [[Extinction (astronomy)|interstellar extinction]] can be related to the column density of H or H<sub>2</sub>.<ref>{{Cite web|url=http://astronomy.swin.edu.au/cosmos/C/Column+Density|title = Column Density &#124; COSMOS}}</ref>

The concept of area density can be useful when analysing [[accretion disk]]s. In the case of a disk seen face-on, area density for a given area of the disk is defined as column density: that is, either as the [[mass]] of [[chemical substance|substance]] per unit [[area]] [[integral|integrated]] along the vertical path that goes through the disk [[Sightline|(line-of-sight)]], from the bottom to the top of the medium:

<math display=block>\sigma = \int \rho \, \mathrm{d}z,</math>

where <math>z</math> denotes the vertical coordinate (e.g., height or depth), or as the number or count of a substance—rather than the mass—per unit area integrated along a path (column number density):

<math display=block>N = \int n \, \mathrm{d}z.</math>