We show how the recent discovery of a likely close white dwarf companion to the well known star Regulus , one of the brightest stars in the sky , leads to considerable insight into the prior evolutionary history of this star , including the cause of its current rapid rotation .
We infer a relatively narrow range for the initial masses of the progenitor system : M _ { 10 } = 2.3 \pm 0.2 ~ { } M _ { \odot } and M _ { 20 } = 1.7 \pm 0.2 ~ { } M _ { \odot } , where M _ { 10 } and M _ { 20 } are the initial masses of the progenitors of the white dwarf and Regulus , respectively .
In this scenario , the age of the Regulus system would exceed 1 Gyr .
We also show that Regulus , with a current orbital period of 40 days , has an interesting future ahead of it .
This includes ( i ) a common envelope phase , and , quite possibly , ( ii ) an sdB phase , followed by ( iii ) an AM CVn phase with orbital periods \lesssim 1 hr .
Binary evolution calculations are presented in support of this scenario .
We also discuss alternative possibilities , emphasizing the present uncertainties in binary evolution theory .
Thus , this one particular star system illustrates many different aspects of binary stellar evolution .