Calculating relative velocity of Crab Nebula

[LavaLynne]

I'm using the the Doppler formula to calculate the relative velocity between the approaching and receding filaments of the crab nebula: Δλ/λnaught = v/c Change in wavelength/ wavelength = velocity/ speed of light

I have reworked the formula as v= c(Δλ/λnaught)

When I plug in the values I get: 300,000 km/s (38.336 angstrom/ 3727 angstrom) = 3085.81

As this is nowhere near the velocity of the Crab's expansion I asked a cosmologist friend for help. He said that I should do the formula as: v= c(Δλ/λnaught) / 2

This gives me: 300,000 km/s (38.336 angstrom/ 3727 angstrom) /2 = 1542.90 km/s

Now this result is very close to the actual velocity of the expansion. What I'm wondering is...why am I dividing by two?

Keep in mind that I'm a mature student and this my first year of school in a very long time! Please go easy on me! :)

 

[Bystander]

You're looking at the difference between approaching and receding filaments. Half that difference is assigned to the shift resulting from approach, and half to the shift due to recession.

 

[LavaLynne]

Thank you very much for that! That make quite a lot of sense! :)