Pollux, the brighter of the “twin” stars of Gemini, is a giant. It’s expanded to about nine times the diameter of the Sun — a beach ball to the Sun’s golf ball. That’s made Pollux much cooler than the Sun, so it looks orange. And it’s more than 30 times brighter than the Sun.
Careful measurements of its diameter, combined with measurements of subtle “ripples” across its surface, have yielded the mass of Pollux as well: 1.9 times the Sun’s mass. Astronomers have combined that with measurements of its weak magnetic field to learn about the history of Pollux.
In its prime, the star was class “A” — two classes brighter and heavier than the Sun. Its surface was much hotter then, so Pollux glowed pure white. And it rotated much faster than it does now, so it generated a stronger magnetic field. That means Pollux would have been much more “active” than it is today — producing more magnetic storms on its surface, and more explosions of particles and energy.
Because Pollux was so massive, though, it burned through the hydrogen fuel in its core in a big hurry. That triggered the changes that make the star what it is today. So even though Pollux is less than 20 percent the age of the Sun, it’s moved beyond the prime of life into “gianthood” — a stage the Sun won’t reach for billions of years.
Look for Pollux quite near the Moon this evening. Its “twin,” the star Castor, is a good bit farther from the Moon.