Why do astronomers observe a red shift in the spectra of stars?

A. According to the Doppler Effect, the red shift is caused by stars moving toward us in the contracting universe.
B. According to the Doppler Effect, the red shift is caused by the atmospheric changes on Earth.
C. According to the Doppler Effect, the red shift is caused by stars moving away from us in the expanding universe.
D. According to the Doppler Effect, the red shift is caused by variances in the gravitational pull on Earth’

Question

Why do astronomers observe a red shift in the spectra of stars? 



 A. According to the Doppler Effect, the red shift is caused by stars moving toward us in the contracting universe. 
 B. According to the Doppler Effect, the red shift is caused by the atmospheric changes on Earth. 
 C. According to the Doppler Effect, the red shift is caused by stars moving away from us in the expanding universe. 
 D. According to the Doppler Effect, the red shift is caused by variances in the gravitational pull on Earth’

anonymous · Answer

Light waves travel at different speeds, when an object is receding, people observe a red shift; when an object is approaching, people observe a blue shift.