Doplar effect calculation of date of Super Nova

[DLH112]

In my astronomy lab we (part A) calculated how many arc seconds a super nova remnant had expanded from 1942 to 1976, and in further calculations (Part B) used the doplar effect to calculate a velocity, which was then used with the results of part A to calculate the distance from Earth to the supernova remnant.
My problem is with how part B was done. I don't know if I'm misunderstanding something or what. there was a spectrum with the red shifted and blue shifted lines of oxygen, we found how many angstrums were between them. Then it said to calculate the relative speed of the red and blue shifted lines. (delta lambda C)/lamda0. My professor insisted that delta lamda was 1/2 the range of angstrums between the red and blue shifted lines(e.g. if blue shifted was 50A and redshifted was 100, use 75), and that lamda0 is what it should be when unshifted.
My problem with this method is that, aren't you calculating the speed of the either shifted end relative to the unshifted value, instead of the speed of the shifted lines relative to each other(like the experiment said)? I'm only really asking this here because i ended up with pretty darn close to 1/2 the known value of the distance.

 

[lippyka]

Is this just a coincidence or did the professor misguide us here?It is possible that your professor has misguiding you here. The method you described assumes that the relative speed of the red and blue shifted lines is equal to the velocity of the source of the radiation (e.g. the supernova remnant). However, it is more likely that the lines are shifting due to the Doppler effect from the expansion of space. Therefore, the relative speed of the two lines should be calculated as the difference between their velocities, not the difference between their wavelengths. For example, if the red line has a velocity of 100 m/s and the blue line has a velocity of 50 m/s, then the relative speed of the two lines would be 50 m/s, not 25 m/s (which would be the difference between their wavelengths).It is also possible that your professor was trying to simplify the calculation by assuming that the two lines have equal velocities, in which case your calculation would be correct. However, this is unlikely to give an accurate result since the velocities of the two lines would depend on their distance from the source.In any case, it is worth double-checking the calculation with your professor to make sure that you are using the correct method.