The line width dependence of zero-phonon lines and phonon sidebands oil temperature, bath dissipation, and electron-phonon coupling is studied for an underdamped Brownian oscillator model with an Ohmic dissipative bath. Factors determining the line widths vary from the zero-phonon lines to the phonon sidebands. The control-parameter space of line broadening has been mapped out, revealing that the line widths of the zero-phonon lines and phonon sidebands are linearly dependent on both the temperature and the Huang-Rhys factor. It is also found that the dependence of the line widths oil the bath damping factor is linear for the zero-phonon lines and quadratic for the phonon sidebands.