In article , Jos Bergervoet
writes:

Why is Tau Ceti a G-type star and Toliman (Alpha Cen B) a K-type?

In terms of mass, Tau Ceti is actually the lighter one (78% of the
Sun's mass vs. 90% for Toliman.) So you'd expect the opposite..

https://en.wikipedia.org/wiki/Tau_Ceti
https://en.wikipedia.org/wiki/Alpha_...pha_Centauri_B

Note that Tau Ceti is G8, while the Sun is G2. After G comes K, so the
difference between G8 and K isn't that large. There are different
metallicities, and also Tau Ceti is a single star, so there is a
different history.

In this case, both are main-sequence stars. However, the spectral type
itself says little about the mass; red giants and red dwarfs, for
example, can have the same spectral class (but different luminosity
classes).