$$\require{cancel}$$

# 5.1: Introduction

[ "article:topic", "authorname:tatumj" ]

As radiation struggles to make its way upwards through a stellar atmosphere, it may be weakened by absorption and scattering. The combined effect of absorption and scattering is called extinction. Scattering may simply be by reflection from dust particles. If the radiation interacts with an atom, the atom may be excited to a higher energy level and almost immediately (typically on a time-scale of nanoseconds) the atom drops down to its original level and emits a photon of the same frequency as the one it absorbed. Such a process - temporary absorption followed almost immediately by re-emission without change in wavelength - is probably best described in the present context as scattering. Individual atoms in a stellar atmosphere generally radiate dipole radiation; however, since many randomly oriented atoms take place in the process, the scattering can be regarded as isotropic. If, however, the excited atom collides with another atom before re-emission, the collision may be super-elastic; as the atom falls to a lower state, the energy it gives up, instead of being radiated as a photon, goes to kinetic energy of the colliding atoms. The radiation has been converted to kinetic energy. This process is absorption.