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Material Type: Notes; Class: Birth/Death of Stars >3; Subject: Astronomy; University: University of Oregon; Term: Unknown 1989;
Typology: Study notes
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Photon Emission from Atoms : We have now completed our preliminary investigation of stellar flux and stellar color and how those are measured. The next available measurement to us is spectroscopy. Spectroscopy is the term given to an instrument that is able to disperse and objects light into its component colors (wavelengths). A prism is a simple example of such a device that splits white light into its component broadband colors. This is done through a process known as diffraction and it’s exactly the same process produced by water droplets in the atmosphere as they interact with sunlight to produce a rainbow. However, we want to disperse the light into much higher resolution wavelength regimes than those that define a single, broad band color. Typically an astronomical spectrum of an object will disperse the light into several thousand discrete wavelength regimes, so as to better define important features that are sensitive to the physical properties of a star (in particular, its chemical composition but also its atmospheric temperature). In this section, we will deal specifically with the formation of spectral lines in the atmosphere of stars. These spectral lines occur as a direct result of the structure of atoms, which we now want to examine in more detail. Here is an alternative overview of this subject that you can refer to in addition to the material below. The entire key to understanding the formation of spectral lines lies in the early 20th^ century discovery that each atom has a unique set of atomic energy levels. The basic ideas of atomic energy levels and transitions will become clearer when you run the simulation. For now, the details are summarized in the diagrams below. We begin with the simple concept of Energy levels in an atom as shown below
The key point is that atomic energy levels are discrete (or quantized). These levels can be occupied by electrons. Therefore, in any given atom, an electron can only be in a certain, discrete energy state (which roughly speaking is like the “orbit” of that electron around the nucleus). As said before – each chemical element has a unique set of energy levels. The concept of discrete is important. A photon is either all or nothing. You can not take a photon, steal part of its energy, and still have that photon left. So, in general, there is a one to one correspondence between photon generation (emission) or absorption and the movement of electrons to different energy levels. Photon Emission: Photons are emitted by atoms whenever an electron movies from an occupied higher energy level to a lower one. For now, we do not care how the electron got to the higher energy state in the first place – its just there. When the electron decays to a lower energy state the following rule applies (always – no exceptions): If the electron in an upper energy level (say E 3 ) falls to a lower energy level (say E 2 ) one photon is emitted whose energy is exactly equal to the energy difference between levels E 3 and E 2.