Stellar surface convection introduces an overall blueshift in the light emitted from the photosphere.
a) estimate the doppler shift to the hα line as a result of granular upflows with speeds of 500 m/s.
b) estimate the doppler shift to the hα line as a result of granular downflows with speeds of 1000 m/s.
c) provide an explanation for why the overall effect is a convective blueshift even though the granular downflow speeds are of greater magnitude.

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Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.