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Type of Tau Ceti and Toliman
Why is Tau Ceti a Gtype star and Toliman (Alpha Cen B) a Ktype?
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  Jos [[Mod. note  One plausible reason is that stars change in spectral type as they age. Wikipedia gives Tau Ceti's age as 5.8 Gyr, Alpha Cen B as 5.3 Gyr. The two stars also have somewhat different metallicities (which affect spectral types).  jt]] 
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#2




Type of Tau Ceti and Toliman
In article , Jos Bergervoet
writes: Why is Tau Ceti a Gtype star and Toliman (Alpha Cen B) a Ktype? 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 mainsequence 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). 
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