Main-sequence rule: Going down the main sequence spectral types from O stars to M stars and going down each spectral subtype 0,1,2,3,...,9, stars get:

1. Less luminous.
2. Lower in photospheric temperature: the conventionally the effective temperature which is a sort of average temperature for the photosphere layer.
3. Redder in the sense that the peak of the approximate blackbody spectrum of main-sequence stars shifts redward. Also the color B-V increases which is referred to as "getting redder".
4. Smaller in mass.
5. Smaller in radius.
6. Longer lived on the main sequence and as nuclear-burning stars in general.
7. More numerous.

Note for the OBAFGKM spectral classification, early type stars are those stars that are A stars or earlier in the sequence and late type stars those that are K stars or later in the sequence. The terms early and late are also used in a relative sense for the OBAFGKM spectral classification.

Note, early type stars are rare, but are bright, and so we notice them readily. On the other hand, late type stars are numerous, but faint, and so we do NOT notice them readily. In fact, NO main-sequence M star can be seen by the naked eye (see Wikipedia: Class M), NOT even Proxima Centauri (distance 1.30197 pc = 4.2465(3) ly) the closest neighbor star to the Sun.

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