The spectrum of light from the galaxy M100 has a prominent dark line at a wavelength of 592.2 nm. The same spectral line observed in a lab on earth has a wavelength of 589.4 nm. How fast is M100 moving relative to earth? (the speed of light = 3.00 x 108ms-1)

Answer 1

(delta L)/L = v/c

L is the wavelength. v is the galaxy radial velocity (in the direction away from earth). Velocity components normal to that direction cannot be measured spectroscopically.

Since the wavelength shift is to the red (longer wavelengths), it is moving away.

Use the equaition provided to calculate V