A star appears to be red in the night sky. Is the star traveling toward the earth or away from the earth? How do we know this? (Hint: Think about the Doppler effect)

The star is travelling away from the earth.

Explanation:

A star's temperature largely determines its color. Stars with higher temperature have a bluish color and stars with low temperature appear red in color. But the color of star that we observe from the earth is different from its actual color.

This variation in color is due to Doppler effect. When the movement of a star is towards the earth the frequency shifts to the higher frequency region and when the movement is away from the earth the frequency of the light emitted will shift to the lower frequency region. Color of light depends on the frequency.

The color corresponding to the lower frequency region is red and color corresponding to high frequency region is blue. So when the star moves away from the earth it appears red in color.