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it takes many years to see the change of arms of a galaxy. so how was the rotation of arms measured?
but it is said that stars in the arms are moving at fast rate.
the arms are nothing but collection of stars. so, how can arms have a different speed and if so, how can it be measured?
They are indeed going faster than general relativity predicts, hence the assumption that there is hidden (dark) matter.but it is said that stars in the arms are moving at fast rate.