In the lexicon of astronomy, Pollux is known a class K-zero-3 star. That tells us that the star’s surface is a little cooler and redder than the Sun’s. It tells us that the star has puffed up to many times its original size. And it tells us that the star is nearing its end.

Pollux is the brightest star of Gemini. It’s quite close to the Moon tonight. Its “twin,” the star Castor, and the brilliant planet Jupiter are a little farther from the Moon.

The system that astronomers use to classify stars was developed more than a century ago. It groups the stars into classes O, B, A, F, G, K, and M. That system is based on a star’s surface temperature or color – hotter stars are bluer, while cooler stars are redder. O stars are blue-white, while M stars are red or orange.

Each class is subdivided using the numbers zero through nine. At K-0, Pollux is just across the line from class G – the class that includes the Sun.

The classification ends with the Roman numerals one through five. A “five” means the star is in the main phase of life. A “three” means it’s moved on to the giant phase. It’s converted the hydrogen in its core to helium. Pollux is now fusing the helium to make carbon and oxygen. That change has caused it to puff up; it’s nine times the diameter of the Sun.

Over time, Pollux will get even bigger, cooler, and redder. It may evolve into class M – a brilliant star at the end of its life.

Script by Damond Benningfield