Moon and Pollux

When astronomers compare the brightness of different stars, they use a scale known as absolute magnitude. That’s how bright the stars would look if they were lined up at the same distance: 10 parsecs, which is 32.6 light-years.

One star they’d barely have to nudge is Pollux, the brightest star of Gemini. It’s only one light-year farther than that distance. So if it moved to exactly 10 parsecs, you’d have a hard time telling any difference in its appearance.

That includes its color. Its orange glow tells us that its surface is thousands of degrees cooler than the surface of the Sun.

Originally, Pollux would have shined almost pure white – an indication that its surface was much hotter than it is today. But the star used up the hydrogen fuel in its core. That triggered a series of changes that caused its outer layers to puff up to giant proportions. As the gas expanded, it cooled, making the star orange. Today, Pollux is the closest giant star to the Sun – a bit more than 10 parsecs away.

Pollux is close to the Moon as they climb into good view by about 10 o’clock tonight. The moonlight will wash out some of the star’s color. Gemini’s other twin, the star Castor, will stand farther to the upper left of the Moon. They’ll be high in the sky at first light. By then, the Moon will line up half way between Pollux and an even brighter orange light: the planet Mars. We’ll have more about Mars and the Moon tomorrow.

Script by Damond Benningfield