Which spectral class corresponds to the coolest stars on the Main Sequence, typically displaying an orange to deep red color?
Answer
K and M stars
The stellar spectral classification system, ordered by decreasing surface temperature (O being hottest, M being coolest), directly correlates with observed color. O and B stars are blue/blue-white, A and F are white, and G stars (like the Sun) are yellow-white. The coolest stars on the Main Sequence fall into the K and M spectral types, which radiate most strongly at longer wavelengths, resulting in observable orange or deep red hues.
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