Final answer:
Color is an essential characteristic when studying stars as it indicates temperature, with blue signifying hotter stars and red cooler ones. This information helps astronomers deduce the types of stars within a galaxy, using the color index to quantify temperature differences and the galaxy's color to infer its stellar makeup.
Step-by-step explanation:
Color is a useful characteristic to study when viewing stars because it provides insight into the temperature of the stars. According to Wien's law, the color of a star is correlated to its temperature; hot stars typically emit blue light and cooler stars emit red light. This relationship does not depend on the distance of the star, making it a reliable measure. When we look at the color of a galaxy, we can infer information about its stellar population because the collective light is the sum of all the stars' light within the galaxy.
Astronomers use a color index as a precise way to measure and express a star's temperature. This index is calculated by the difference in magnitudes measured at two different wavelengths. For galaxies, observing their overall color can help deduce the proportion of hot blue stars to cooler red stars, which may indicate the age and evolutionary state of the galaxy, since younger galaxies tend to have more blue stars.
Therefore, color serves as a sort of "thermometer" for gauging stellar temperatures. By extension, it aids in assessing the composition and lifecycle of stars within galaxies. As stars have different colors based on their temperatures, with blue indicating hotter stars and red cooler ones, the overall color of a galaxy gives astronomers clues about the types of stars it contains.