Using the Doppler formula to find radial speed of a star

1. Oct 3, 2012

owengmcdonnel

1. The problem statement, all variables and given/known data

High resolution spectroscopy reveals that the H line is located at a wavelength
of 656.250 nm for this star. The wavelength of H measured in the lab is 656.280 nm.
Calculate the line-of-sight velocity of the star.

2. Relevant equations

Here's the Doppler formula:

Vr = (Δλ/λ0)c

where Vr is the radial speed of the star, Δλ is the Doppler shift, λ0 is the rest wavelength or the wavelength measured in the lab, and c is the speed of light.

3. The attempt at a solution

I'm just having trouble with units. I need the answer (the radial speed of the star) in km/s, but i don't know what units to put the other variables (Δλ, λ0, c) in.

2. Mar 31, 2014

BlackStar

λ= 656.28 nm
Δλ=656.28-656.25=.03 nm
c=3*108 m/s=3*105 km/s

so finally ,
v=13.7 km/s

The units of Δλ and λ cancel, so v has basically same units as c