Alkaid

The Big Dipper hangs by its handle on summer evenings, as though it were about to scoop up a dipper-ful of stars. The handle is anchored by Alkaid, a star that’s far more impressive than our own star, the Sun. It’s bigger, heavier, brighter, and a lot hotter.

The key to all of those traits is the “heavier” business. Alkaid is about six times as massive as the Sun, which puts it in the top one percent of all the stars in the galaxy.

Like the Sun, Alkaid is in the prime of life, steadily “fusing” the hydrogen in its core to make helium, and releasing energy in the process. But heavier stars have stronger gravity, which squeezes them more tightly. That makes their cores much hotter, which in turn revs up the pace of nuclear reactions.

Energy from the core radiates outward through the star, pushing the layers of gas around it. A hotter core generates more energy, hence a stronger “push,” which causes the surrounding layers of hot gas to puff up. So Alkaid is more than three times wider