The red and blue color change of moving stars is named what?

• A. Doppler Shift
• B. Gravitational Redshift
• C. Cosmic Expansion
• D. Refraction

Answer: (A)

The main idea here is the Doppler shift of light: motion toward or away from us changes the color we see. When a star moves toward us, its light’s wavelengths are compressed, shifting toward the blue end of the spectrum. When it moves away, the wavelengths stretch, shifting toward red. This simple, direct link between radial motion and color makes Doppler shift the best answer for why moving stars appear bluer or redder.

Gravitational redshift, by contrast, comes from light losing energy as it climbs out of a strong gravitational field, not from the star’s motion relative to us. Cosmic expansion shifts light from very distant sources overall due to the stretching of space itself, not due to the star’s motion within a galaxy. Refraction changes light when it passes through materials with different densities, not because of motion in space.