Doppler effect and hydrogen alpha distributions

[aspodkfpo]

Homework Statement

https://www.asi.edu.au/wp-content/uploads/2015/03/PhysicsASOE2014solutions.pdf
q14 b) i)

Relevant Equations

n/a

https://www.asi.edu.au/wp-content/uploads/2015/03/PhysicsASOE2014solutions.pdf

q 14b) i)

Assuming that the planet is rotating at a constant rate, shouldn't the distribution be even across all wavelengths, or do I have something very wrong with my model.
I take the graph as the summation of rays from points on a planet which rotates. Each point similarly rotates and causes an equal amount of each wavelength.

 

[haruspex]

Assuming that the planet is rotating at a constant rate, shouldn't the distribution be even across all wavelengths, or do I have something very wrong with my model.
I take the graph as the summation of rays from points on a planet which rotates. Each point similarly rotates and causes an equal amount of each wavelength.

Planet? Am I looking at the right question?
The question is poorly explained. I see no mention of gas until we come to the solution, so the student will naturally think these emissions are from the stars. Neither is it clear whether we are concerned with the spectrum from some specific part of the galaxy or from the galaxy as a whole. The mention of "fast rotating" region makes it sound like the former, but I think they want the expected spectrum for the galaxy as a whole when taking into account the contributions from the fast regions.
This is important because if it is for the whole galaxy then we have both fast towards us and fast away from us, leading to the symmetric widening in the answer graph.

Does that answer your question?

 

[aspodkfpo]

Planet? Am I looking at the right question?
The question is poorly explained. I see no mention of gas until we come to the solution, so the student will naturally think these emissions are from the stars. Neither is it clear whether we are concerned with the spectrum from some specific part of the galaxy or from the galaxy as a whole. The mention of "fast rotating" region makes it sound like the former, but I think they want the expected spectrum for the galaxy as a whole when taking into account the contributions from the fast regions.
This is important because if it is for the whole galaxy then we have both fast towards us and fast away from us, leading to the symmetric widening in the answer graph.

Does that answer your question?

nope, I don't see why it wouldn't be the same % for each wavelength, i.e. straight line distribution.

 

[haruspex]

nope, I don't see why it wouldn't be the same % for each wavelength, i.e. straight line distribution.

On one side of the galaxy you have stars and hot gas clouds rushing towards you (relative to the galaxy as a whole) at great speed, giving a blue shift, while on the other side they are are rushing away. I'm not sure which regions move fastest... if it is like planets around a star, the ones nearest the galactic centre will give the greatest shift.
But there's a limit to how fast they can go, and relatively few can be near the centre, so most of the shifts will be modest.