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# astrophysics question?

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1. https://ec36d9d695804abc0313cb02cd29.../Cosmology.pdf

https://ec36d9d695804abc0313cb02cd29...ology%20MS.pdf

q1bii - can someone explain to me why the wavelength is only 0.0131 an not double that? also why is the period 240 days?
2. (Original post by liverpool2044)
https://ec36d9d695804abc0313cb02cd29.../Cosmology.pdf

https://ec36d9d695804abc0313cb02cd29...ology%20MS.pdf

q1bii - can someone explain to me why the wavelength is only 0.0131 an not double that? also why is the period 240 days?
You're after the speed of the star in it's orbit... minimum wavelength occurs when it's on the RHS of the diagram (it's velocity vector is pointing at the observer) and maximum wavelength occurs when it's on the LHS of the diagram (velocity vector is pointing away from the observer)... orbital speed is half the difference.

compare to observing a car going round and round a roundabout at constant 30 mph speed... at some point in time it'll be coming towards you at 30mph and at another time it'll be going away from you at 30mph.... it's speed is still only 30mph (not 60!)

---
period is the time for a full cycle - the time between maximum and minimum is the time for half a cycle
3. thank you!

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