Final answer:
Spectral type O corresponds to the hottest stars with temperatures over 28,000 K. Spectral type M includes the coolest stars with temperatures below 3,500 K. The sequence of spectral classes from hottest to coolest is O, B, A, F, G, K, M, with L, T, and Y for cooler star-like objects. Therefore the correct answer is A) O.
Step-by-step explanation:
The hottest stars correspond to spectral type O, which denotes stars with temperatures over 28,000 K. On the other end of the spectrum, the coolest stars are of spectral type M, with temperatures below 3,500 K.
The Sun, classified as a type G star, has a surface temperature of approximately 5,800 K, placing it cooler than type O stars and hotter than type M stars.
The spectral types of stars, arranged from hottest to coolest, are O, B, A, F, G, K, M, with the classes L, T, and Y being added to describe cooler objects such as brown dwarfs.
Spectral types do not only indicate temperature but also give insight into a star's luminosity, with the most massive and luminous being of spectral type O, whereas the lowest-mass and least luminous stars fall under spectral type M.