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Basic Principles of Stellar Spectra
Electromagnetic spectra are produced by means of a spectroscope. Light enters a spectroscope through a narrow slit. Beyond the slit is a collimator lens to form a parallel beam of light sent to a triangular prism (or grating) that separates the light into its component colors.
If white light, produced by the filament (a hot solid material) of a light bulb, passes through the spectroscope, it is dispersed into a continuous spectrum where the colors blend into each other from red to blue without interruption, just like the colors in a rainbow. Isaac Newton showed that the prism doesn't add color to light but rather that color is already contained in white light and that the prism merely separates (or disperses) the light into its colors. A glowing liquid or gas under high pressure also produces a continuous spectrum.
The situation changes by replacing the light bulb with a glowing gas under low pressure. When viewed through a spectroscope, the light from the gas forms instead a series of bright lines of various colors rather than a continuous spectrum. Each line is an image of the slit in the spectroscope; the colors represent discrete wavelengths of light that are emitted by the gas. The hot, low pressure gas is not capable of emitting all of the colors of visible electromagnetic spectrum, and therefore it produces a bright-line or emission spectrum. The bright lines are part of the continuous spectrum, just like a few pieces of a jigsaw puzzle, placed in position, are part of the whole puzzle.
In 1814 Joseph von Fraunhofer, a German optician, used a spectroscope to produce a spectrum of the Sun. He noticed that the spectrum contained a number of dark lines on a continuous spectrum. Fraunhofer had no explanation for these dark lines, but it was later discovered that they had resulted from sunlight passing through cooler gases in the Sun's atmosphere. A dark-line spectrum, called an absorption spectrum, is like a jigsaw puzzle with a few missing pieces. All stars show this type of spectrum.
In the 1860's Gustav Kirchhoff formulated a set of rules, now called Kirchhoff's laws, which summarize how the three types of spectra are produced:
The dark lines in an absorption spectrum are images of the slit of the spectroscope; the missing colors represent discrete wavelengths of light that are absorbed by the intervening cooler gas. Thus, the gas is opaque to the affected wavelengths and transparent to all other wavelengths in the continuous spectrum. The dark lines have the same wavelength as the bright lines that are emitted if the same gas is heated.
Refer to your reading assignment for more on the three types of spectra.
The visible surface (the photosphere) of the Sun emits a continuous spectrum because it is a hot, dense gas. Before the light from the Sun gets to us on Earth, it must pass through the relatively cool atmosphere of the Sun. As the light passes through these gases, atoms in the gases absorb some of it.
Electrons in atoms move about a nucleus of protons and neutrons in levels, similar to people moving around on floors in a building. An electron can move from one level to another by absorbing or emitting discrete amounts of energy, defined by the difference in energy between the levels. Upper levels have more energy than lower levels. If an electron makes a transition (change) in energy to an upper floor, it must be given energy from the outside. One way to do this is to absorb light energy. A downward transition on the other hand requires that the electron gives up energy. In this situation, an electron can emit a flash of light (called a photon) at a specific wavelength. The energy carried away by the photon is the energy that the electron gives up by moving to a lower level. The process is not unlike people moving between floors in a building. People must gain gravitational potential energy if they climb a set of stairs to an upper floor. The potential energy is derived from metabolic energy (stored food energy) in each person. People give up the gravitational potential energy if they use stairs to go to lower floors. As a person goes down a set of steps, the potential energy is removed by friction as heat is generated in the joints of the skeletal structure. The heat is then emitted by the body in the form of infrared light.
The absorption of light energy in the Sun's outer atmosphere moves electrons to upper atomic levels and therefore to higher levels of energy. Since each atomic level has a certain energy requirement, only certain amounts of energy can be absorbed by the electrons. Nature does not allow an electron to absorb more than is necessary to raise its position in the atom.
If white light is passed through hydrogen gas that is too cool to emit light, the gas will absorb some wavelengths of this light. Almost immediately, light is reemitted at the same wavelengths as electrons begin to cascade towards lower levels, but the reemitted light is sent out in all directions. So certain wavelengths of the light that were originally coming towards us are scattered by the gas; the intensity of the bright line spectrum due to remission is therefore dim and cannot replace all the light that is removed by absorption in a specific direction. The result is an absorption line spectrum.
During a total eclipse of the Sun, light from the photosphere is blocked from view, and astronomers can see the emission spectrum of the reemitted light. As expected, the bright line in the emission spectrum corresponds precisely to wavelengths of the dark lines in the absorption spectrum of the Sun. It was by examining the emission spectrum from the upper atmosphere of the Sun that astronomers discovered helium before it was found on Earth.
The Sun, and other stars as well, has a number of chemical elements in its atmosphere (that is, different kinds of atoms, each with its own distinct set of energy levels). As white light passes through stellar atmospheres, many wavelengths of light are absorbed that correspond to the various chemical elements of the atmospheres. By examining the complicated absorption spectrum that results, astronomers are able to deduce which elements are in the star's atmosphere. Thus we answer a question, that just a century ago, was thought to be unanswerable. We now know what stars are made of!
Refer to your reading assignment for more on atomic structure and stellar spectra.
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1. Dark lines in an absorption spectrum are not totally black. How is this
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