The absorption of starlight by dust and gas in the space between the stars; also known as interstellar extinction. The dust and gas are strongly concentrated towards the plane of our Galaxy. Interstellar absorption increases towards the shorter-wavelength (blue) end of the spectrum, and hence makes stars appear redder, so that the terms interstellar absorption and interstellar reddening are used almost interchangeably. As well as continuous absorption there are diffuse interstellar bands, such as that at 443 nm, due mostly to polycyclic aromatic hydrocarbons and other large carbon-bearing molecules. There are also atomic absorption lines such as the calcium H and K lines at 393 and 397 nm, the sodium D lines at 589.0 and 589.6 nm, and other weaker lines.