Exam 7: The Structure and Formation of Stars

Correct Answer:

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Interstellar absorption lines are narrow compared to the width of lines that originate in the photospheres of stars due to several factors related to the conditions of the interstellar medium and the nature of stellar photospheres.

1. **Temperature and Pressure**: The interstellar medium (ISM) is generally much cooler and less dense than the photospheres of stars. In the photosphere, atoms and ions are subject to higher temperatures and pressures, which lead to broadening of the spectral lines through various mechanisms. One such mechanism is thermal broadening, where higher temperatures increase the velocity of atoms and ions, causing a spread in the energies and wavelengths at which they absorb or emit light. Another is pressure broadening, where the presence of other particles affects the energy levels of the absorbing/emitting atoms, again broadening the lines.

2. **Turbulence and Velocity Fields**: In a stellar photosphere, there are often large-scale motions such as convection currents, pulsations, and rotational effects that can cause Doppler broadening of spectral lines. These effects are less pronounced in the interstellar medium, where the gas is more quiescent and the velocity fields are less turbulent on the scales that affect line broadening.

3. **Homogeneity**: The interstellar medium is relatively homogeneous over small scales compared to the complex and dynamic environment of a stellar photosphere. This homogeneity means that there are fewer local variations in temperature, pressure, and velocity that could lead to broadening of the absorption lines.

4. **Natural and Collisional Broadening**: In the dense environment of a star's photosphere, natural broadening (due to the uncertainty principle) and collisional broadening (due to interactions between particles) also contribute to the width of spectral lines. These effects are minimized in the sparse interstellar medium.

5. **Single vs. Multiple Absorption Events**: The absorption lines we see from the interstellar medium are often the result of a single, relatively simple absorption event as light from a star passes through a cloud of interstellar gas. In contrast, the lines from a star's photosphere are the result of many absorption and re-emission events in a complex, stratified layer of gas, leading to a broadening of the lines.

6. **Chemical Composition**: The interstellar medium is primarily composed of hydrogen and helium with trace amounts of heavier elements, whereas stellar photospheres can have a more complex chemical composition. The simplicity of the interstellar medium can contribute to narrower absorption lines, as there are fewer types of atoms and ions to interact with the light.

In summary, the narrowness of interstellar absorption lines compared to those originating in stellar photospheres is due to the cooler, less dense, and more homogeneous conditions of the interstellar medium, as well as the simpler nature of the absorption events that occur there.