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)
