Example (II):
Take l0 of the Ha (Balmer alpha) line:
l0 = 656 nm
Assume, we observe a star’s spectrum with the Ha line at l
= 658 nm. Then,
Dl
= 2 nm.
We find Dl/l0 = 0.003
= 3*10-3
Thus,
vr/c = 0.003,
or
vr = 0.003*300,000 km/s = 900 km/s.
The
line is red shifted, so the star is receding from us with a radial velocity of 900 km/s.
