A bright star in Cassiopeia is in a short but impressive phase of life: it’s beginning to puff up like a giant balloon. Beta Cassiopeia is in the northwest early this evening, at the bottom of a sideways letter W formed by some of the constellation’s brightest stars. Beta Cass is probably less than half the age of the Sun. But it’s almost twice as massive as the Sun, so it burns through its nuclear fuel much more quickly. In fact, it’s probably already consumed the original hydrogen fuel in its core, converting it to helium. Gravity is causing the core to shrink and get hotter, which eventually will allow it to start burning the helium to make even heavier elements. In reaction to the changes in the core, Beta Cass’s outer layers are starting to puff up. Right now, the star is roughly four times the diameter of the Sun. That’s big, but not nearly as big as the star will be. Over millions of years, it’ll puff up to dozens of times the Sun’s diameter. It’ll then shrink and cool a little
