At rest, hydrogen has a spectral line at 126 nm. If this line is observed at 141 nm for the star Sirius, how fast is Sirius moving in km/s?
z = (obs - emit )/emit
= (141-126)/126 = .119 (positive = red shift)
if v is much less than c then z~~ v/c
.119 = v/3x10^8
v = 3.57 x 107 m/s = 3.57 x104 km/s (ignoring the Lorentz factor)
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