Interstellar Absorption Lines
The interstellar medium produces absorption lines in the spectra of stars.
These can be distinguished
from stellar absorption
lines through:
a) Absorption from wrong ionization states
Narrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in the background; multiple components
b) Small line width (too low temperature; too low density)
c) Multiple components (several clouds of ISM with different radial velocities)
