# Calculating relative velocity of Crab Nebula

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1. Mar 3, 2015

### 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! :)

2. Mar 3, 2015

### 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.

3. Mar 3, 2015

### LavaLynne

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