Lars Lading has written:

'Remote Laser Measurement of Temperature in the Atmosphere on the Basis of the Doppler Broadening of the Molecular-Spectrum'

Perhaps this broadening of the spectral line might be due to different propogation speeds under Doppler conditions when the emitting object is moving towards us? Therefore increasing the frequency shift and spreading the spectrum over a wider bandwidth.

Or perhaps

Take an amplitude modulated carrier wave with a fixed modulating frequency. Un-shifted the sidebands would remain constant in width. If the signal source was moving towards us the modulating frequency would also increase. The increased modulating frequency would move the sidebands further away from the carrier, increasing bandwidth or broadening the spectrum.

There are two primary mechanisms that broaden spectral emission (or absorption) lines: Doppler broadening and collision-induced broadening.

Doppler broadening occurs because of the relative thermal motions of the molecules in a gas. Simply put, the molecules are all bouncing off each other, so some are moving towards you and some away, some fast and some slow. Each molecule's spectrum is Doppler shifted by it's current velocity. The composite spectrum from all the individual molecules has its lines smeared out or broadened as a result. As you can guess, the amount of broadening depends on the temperature of the gas.

Collision-induced broadining, sometimes called pressure broadening, is is a result of the deformation of the molecules when they bounce off each other. For example, they may not be as symmetrical after a collision as they were before. These deformations perturb the quantum mechanical energy levels of the molecule, slightly shifting the frequencies of the emission or absorption lines. Just like Doppler broadening, the composite spectrum's lines are therefore broadened. This effect depends on both the pressure and temperature of the gas.

See spectralcalc for complete details and online simulations.

Starlight can be reddened by the Doppler Effect and by the gravity well (that light from a star finds itself in).

Doppler echocardiography

Doppler echocardiography

Doppler echocardiography

Doppler echocardiography

The change in frequency is caused by the Doppler Effect