Which type(s) of main-sequence stars constitute(s) about 90% of the total?
A) O-type
B) O-type and B-type combined
C) A-type, F-type, and G-type combined
D) K-type and M-type combined
E) M-type
To determine which type(s) of main-sequence stars constitute about 90% of the total, we need to understand the classification system of stars and their distribution in the main sequence.
Main-sequence stars are classified based on their spectral types, which are determined by their surface temperatures. The main spectral types, from hottest to coolest, are O, B, A, F, G, K, and M.
Based on the information provided, we can rule out options A, B, and C since they involve O-type stars, which are the hottest and rarest type of main-sequence stars. Option E also only includes M-type stars, which are the coolest and most common type of main-sequence stars.
Therefore, the correct answer is option D) K-type and M-type combined. K-type stars are cooler than G-type stars but warmer than M-type stars, while M-type stars are the coolest main-sequence stars. Together, K-type and M-type stars make up a substantial portion of the total main-sequence stars, constituting about 90% of them.