What are the main general features that make clusters of stars useful to astronomers? a. the stars are at the same distance from earth, were formed at approximately the same time, and were made from the same chemical mix. b. the stars all have the same intrinsic brightness but differ in size and surface temperature. c. the stars all have the same apparent magnitude, the same surface temperatures, and the same sizes. d. the stars are all at the same distance from earth, have the same surface temperature, and joined the cluster at various times.