Final answer:
The colour of a star reflects its temperature. The hottest stars are blue, and cooler stars are d) red. Therefore, if an observed star appears white, a star with a cooler temperature next to it will appear red.
Step-by-step explanation:
The question concerns the observation of stellar colours and how they relate to the temperature of stars. When classifying main sequence stars, the colour of a star is an important indicator of its temperature. Specifically, the hottest stars appear blue or blue-white due to their higher temperatures and shorter wavelengths of light they emit, while the coolest stars appear red, as they emit longer wavelengths of light. The primary factor that distinguishes the colour of stars is their surface temperature. A star that is cooler than a white star, which is often at a mid-range temperature, will look red, as red is indicative of a cooler star temperature compared to blue. Therefore, the answer to the question is: An observed star appears white. When classifying main sequence stars, a star with a cooler temperature next to it will appear: is d. Red.