Type of Tau Ceti and Toliman

[Jos Bergervoet]

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_Centauri#Alpha_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]]

[Phillip Helbig (undress to reply)]

In article <6030c9f9$0$27912$e4fe...@news.xs4all.nl>, Jos Bergervoet

<jos.ber...@xs4all.nl> 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_Centauri#Alpha_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).