The characteristic absorption spectra of two stars are shown.
Indicated in each is a set of absorption lines due to interstellar
material between us and the stars. Using this information and the
spectral drawings shown below complete parts a - e of this

a. The interstellar absorption lines appear more intense in the
spectrum of star A than star B. What does this mean in terms of the
number of absorbing atoms between the earth observer's line of
sight to star B compared to star A?
b. If we assume that the interstellar material is uniformly
distributed along the direction to each of these stars, which star
is probably farther away? Explain.
c. If these two stars are at the same distance, explain why the
interstellar lines are more intense in spectra A compared to
spectra B.
d. How can it proved that some absorption lines in the spectra
of some stars are due to interstellar material?
e. Since scattering by dust leaves the colors of stars reddened,
we can recognize the existence of interstellar material in the
direction of any particular star by comparing its color to what we
expect for its particular spectral class. Describe how the amount
of "reddening" may depend on the distance to the star and/or the
amount of material between us and the star.
Interstellar Absorption Lines Spectra of star A Spectra of star B
Interstellar Absorption Lines Spectra of star A Spectra of star B

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