Final answer:
The redder light observed by an astronomer from a distant star is likely due to the star moving away from the observer, causing a redshift as a result of the Doppler effect.
Step-by-step explanation:
If an astronomer observes that the light emitted by a distant star appears redder than predicted based on the star's size and temperature, this is most likely caused by the star moving away from the observer. This phenomenon is known as a redshift and is a result of the Doppler effect. As the star moves away, the wavelengths of the light stretch out, making the light appear redder to the observer. When stars or galaxies are receding from us, the light they emit is stretched into longer wavelengths, shifting towards the red end of the spectrum. This redshift can provide valuable information about the speed at which the object is moving away and is an important tool used by astronomers to understand the dynamics of the universe.