Hertzsprung-Russell diagram of stars within 100 parsecs of the Sun, with distances measured by the Hipparcos satellite. The right scale on the vertical axis shows the luminosity in units of the Sun's luminosity. Spectral types of the stars are along the top.
Doppler shift formula from Box 10.1:
Wavelength shift/original wavelength = &Delta &lambda/&lambda
= V/c = source speed/light (or sound) speed.
The speed V is the motion directly towards the observer (&lambda gets shorter: blueshift) or away (&lambda gets longer: redshift).