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According to Wikipedia:

Alpha Centauri A: $M=1.100 M_\odot$ and $L = 1.519 L_\odot$

Alpha Centauri B: $M=0.907M_\odot$ and $L=0.5002L_\odot$

They are 5-6 billion years old (you can assume they are 5.5 billion years old). How long will they live?

It would be good if you could also give some dates about the red giant phase, thanks.

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You can estimate the main sequence life of a star by an approximate formula

$$ \frac{\text{Mass}}{\text{Luminosity}}×10^{10}\text{years}$$

Where the mass and luminosity are both in solar units.

so for Alp-Cen A you get $7.2×10^9$ or about 1.7 billion years left. For Alp-Cen-B you get $1.8×10^{10}$ or about 12 billion years left.

The Red Giant phase will then last about a billion years. It will first build up an inert Helium core, then experience a "Helium flash" followed by an "asymptotic-giant" stage, with an inert carbon-oxygen core, and shells of helium burning, and hydrogen burning.

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    $\begingroup$ α Cen B currently has a mass of $0.907M_\odot$, and is expected to lose $\sim 0.3 M_\odot$ before a possible helium flash, putting it above the $0.5 M_\odot$ threshold for helium fusion. $\endgroup$
    – fasterthanlight
    Aug 16, 2021 at 13:54
  • $\begingroup$ Right, I was getting luminosity and mass confused in my brain $\endgroup$
    – James K
    Aug 16, 2021 at 14:00
  • $\begingroup$ So Alpha Centauri A will become a red giant a few billion years before the Sun does. Of course, the Earth will be uninhabitable well before 1.7 billion years. $\endgroup$
    – PM 2Ring
    Sep 15, 2021 at 20:53
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    $\begingroup$ Note that the Sun will not be a red giant for another 7 and a bit billion years. $\endgroup$
    – ProfRob
    Sep 21, 2021 at 7:33

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