The period of a pulsating star with respect to the star

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To determine the period of a pulsating star as observed from a moving observer, the Doppler effect must be applied. The star has a rest period of 3 seconds, but the observer's speed of 2.4 x 10^8 m/s will alter this measurement. The discussion notes that the direction of the observer's motion is unspecified, but it is suggested that the observer is moving away from the star, which is a common convention in such problems. The change in frequency due to relative motion will affect the observed period. Ultimately, the correct application of the Doppler effect will yield the new period as measured by the observer.
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Homework Statement



A pulsating star has a period of 3 sec as seen from the rest mass frame of the star. What is the period of the star as measured by an observer traveling 2.4 x 108 m/s with respect to the star?


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The Attempt at a Solution

 
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How do you think you should start?
 
I'd think you'd need to use Doppler. The frequency changes due to the motion, although it doesn't say which way, and you have the frequency when there's no relative motion.
 
That sounds good, it should work. And that's a good point, they don't seem to say if they are moving towards or away. I'd guess away, as the standard convention is: negative velocities mean approaching each other.
 
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