Spectrum of Stars

Introduction

The only “visible” part of the stars are their surfaces, or which we call atmosphere of stars. The light/ radiation that we observe from stars is primarily emitted from their atmospheres.

Physical Processes

Radiation and Absorption

An atom is a system consisting of a nucleus and surrounding electrons. The radiations from an atom are produced by transitions between its energy states. A bounded energy state has a discrete, quantized energy level, while a free energy state has a continuous energy level(ionized state). Atomic radiations thus can be classified into three types:

1. Bound-bound (line spectrum)

A transition between two energy states emits a photon of energy , or frequency :

where is Planck’s constant.

2. Bound-free (continuum spectrum)

A transition from a bound state to a free state results in emissions of a continuous spectrum:

or inversely a transition from a free state to a bound state results in absorption of light:

3. Free-free (bremsstrahlung)

This is a phenomenon classically explained as bremsstrahlung, where the acceleration of charged particles cause radiations. A classical derivation can be found here.