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    The starting point I would like to begin at are the empirical observations of Kirchoff in the mid 19th century. These observations are summarized in Kirchoff's Laws as illustrated below.


A bright hot source emits a continuum of radiation as observed when the light from that source is broken down into a spectrum.
When a light source passes through a tenuous gas consisting of elements, ions (plasma) or molecules, certain discrete wavelengths are removed from the continuum giving rise to dark absorption lines which fingerprint an element (and/or its ionic state) or molecule. The light from a star passing through the outer atmosphere layers of the star give rise to this effect.

When a  tenuous gas consisting of elements, ions and/or molecules is excited in some manner (collisional, electrically, heat or by light itself), the gas emits certain discrete wavelengths. Certain gaseous nebula or planetary nebula are good examples of this.

These laws formed the basis for the beginning of Astrophysics where celestial objects could succumb to analysis of their makeup. However, an understanding of how the the light interacted with matter was lacking. Not until the early 20th century did we begin to grasp what was going on. The first model, the Bohr model, helped to explain the simplest atom, hydrogen. As a model to help visualize what is going on, it remains useful, but the actual explanation for both hydrogen and more complex atoms, ions and molecules had to wait for quantum mechanics.

In the Bohr model, atoms can be thought of as small solar systems where electrons orbit the nucleus in discrete orbits-not all orbits or levels are allowed or even exist. Electrons can jump from one level to another by either absorbing or emitting discrete wavelength

of radiation. Thus, absorption of radiation gives rise to dark lines in the continuum spectra whereas emission of radiation gives rise to bright lines. Another way to look at this is shown below. There are many possible absorption-emission transitions which can occur with hydrogen. The Balmer series of transition occur in the visible part of the spectrum and correspond to transitions from higher orbits to the 2nd orbit (emission) or the 2nd orbit to higher orbits (absorption). Also shown below is the spectrum of an A type star indicating

absorption lines for the Balmer series in hydrogen. Thus hydrogen is present in stars!

Absorption and emission features, at least for hydrogen and hydrogen like ions(He+, O+7 for example), can be understood with to a large measure with classical physics.


Continued construction. Please stay tuned especially if your mathematically inclined.