Which is the correct order of spectral types from hottest to coolest?

Which is the correct order of spectral types from hottest to coolest?

• A. O, B, A, F, G, K, M
• B. M, K, G, F, A, B, O
• C. A, F, G, K, M, B, O
• D. O, A, B, G, F, K, M

Answer: (A)

Stellar spectral types reflect surface temperature, so the sequence from hottest to coolest follows the order O, B, A, F, G, K, M. O-type stars are the hottest and appear blue, with temperatures around 30,000 K or higher; B-type stars are blue-white; A-type stars are white; F-type are yellow-white; G-type are yellow (our Sun is a G-type); K-type are orange; and M-type are the coolest, appearing red. Each step down in this sequence corresponds to a cooler surface temperature and a change in the observed spectral features, such as the strength of hydrogen lines and the appearance of metal lines or molecular bands. That’s why the correct answer places O first and M last: it matches the natural ordering by temperature. Other orders mix hotter and cooler types, which would misrepresent the temperature progression and the associated spectral characteristics.

Stellar spectral types reflect surface temperature, so the sequence from hottest to coolest follows the order O, B, A, F, G, K, M. O-type stars are the hottest and appear blue, with temperatures around 30,000 K or higher; B-type stars are blue-white; A-type stars are white; F-type are yellow-white; G-type are yellow (our Sun is a G-type); K-type are orange; and M-type are the coolest, appearing red. Each step down in this sequence corresponds to a cooler surface temperature and a change in the observed spectral features, such as the strength of hydrogen lines and the appearance of metal lines or molecular bands.

That’s why the correct answer places O first and M last: it matches the natural ordering by temperature. Other orders mix hotter and cooler types, which would misrepresent the temperature progression and the associated spectral characteristics.