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Homework Help: Using the Doppler formula to find radial speed of a star

  1. Oct 3, 2012 #1
    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. jcsd
  3. Mar 31, 2014 #2
    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
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