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Moon and Regulus

June 3, 2014

A star that’s barely holding itself together perches to the upper left of the Moon this evening.

Regulus is the bright heart of Leo, the celestial lion. It’s several times bigger and heavier than the Sun, and hundreds of times brighter. Its most interesting feature, though, is its shape. Instead of a nice, round ball, it’s shaped more like a round jelly doughnut — it’s a third wider through the equator than through the poles.

That odd shape is the result of the star’s rotation. It makes one turn on its axis in less than a day, compared to one turn every 24 days for the smaller Sun. That pushes the gas at its equator outward, making Regulus fat but short. In fact, if the star were spinning just 10 percent faster, it would tear itself apart.

Regulus probably had help to reach that dizzying speed. A dead companion star known as a white dwarf orbits quite close to Regulus. It’s unusually small, suggesting that it somehow lost a good bit of its original mass.

The companion probably was born much heavier than Regulus. As a result, it aged more rapidly, quickly puffing up to giant proportions. As it did so, it lost the grip on its outer layers of gas. Some of the gas funneled onto the surface of Regulus. That not only transferred hot gas from one star to the other, it also transferred momentum. So as material poured on to the surface of Regulus, the star began spinning faster and faster — almost fast enough to rip itself apart.

Script by Damond Benningfield, Copyright 2014

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