If a star's spectrum is blue-shifted, what does this indicate about its motion relative to Earth?

If a star's spectrum is blue-shifted, what does this indicate about its motion relative to Earth?

• A. Moving toward Earth
• B. Moving away from Earth
• C. Stationary
• D. Rotating

Answer: (A)

Blue shift means the light from the star is compressed to shorter wavelengths because the star is moving toward us. This Doppler effect shows there’s a line-of-sight component of the star’s velocity toward Earth. If the star were moving away, we’d see redshift (longer wavelengths). If it were stationary, there’d be no shift. Rotation can broaden lines, but it doesn’t by itself cause the whole spectrum to shift toward blue. Moving toward Earth is the best interpretation.

Blue shift means the light from the star is compressed to shorter wavelengths because the star is moving toward us. This Doppler effect shows there’s a line-of-sight component of the star’s velocity toward Earth. If the star were moving away, we’d see redshift (longer wavelengths). If it were stationary, there’d be no shift. Rotation can broaden lines, but it doesn’t by itself cause the whole spectrum to shift toward blue. Moving toward Earth is the best interpretation.