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Moon and Regulus
The evolution of a star is dominated by one thing: the star’s mass. Heavy stars live short but brilliant lives. Lightweight stars are less showy, but they live a long time. And when there are two stars in a single system, a difference in mass can change the evolution of both.
A prime example is Regulus, the brightest star of Leo, the lion, which is just a whisker away from the Moon tonight.
The bright star we see as Regulus has a tiny companion — a white dwarf. It’s the dead core of a once-normal star. But when the stars were born, the companion was the more impressive one.
Recent studies, in fact, suggest that the companion originally was about 2.3 times as massive as the Sun. Today’s bright star was only 1.7 times the Sun’s mass. The extra weight squeezed the companion’s core tightly, speeding up its nuclear reactions. So the star evolved more quickly.
As it used up the nuclear fuel in the core, it puffed up to many times its original size. The two stars were so close together that the expanding star began dumping gas on the other. When that process was done, the heavier star had transferred most of its mass to the present-day bright star. So while the star we see is now almost four times the Sun’s mass, its flashier companion is a faint ember — just one-third the mass of the Sun.
The transfer had another important effect. It made bright Regulus spin much faster — almost fast enough to rip itself apart.