The twins of Gemini climb high across the evening sky at this time of year. Pollux and Castor are well up in the east at nightfall, with Castor standing a little above its brighter “twin.”

Pollux is an orange giant — an old, bloated star that’s about 35 light-years from Earth. It has at least one planet — a world at least three times as massive as Jupiter, the giant of our own solar system.

Castor is about half again as far as Pollux. And it consists of six known stars. All of them are related — they were born from a single giant cloud of gas and dust, probably about 200 million years ago.

The sextet is split into three pairs of twins. Two sets are more fraternal than identical. One star in each pair is a good bit bigger, brighter, and heavier than the Sun. Both of their companions are smaller, fainter, and less massive than the Sun.

The stars in these pairs are so close together that they orbit each other in just a few days. With so little space between them, there’s no way for telescopes to see them as individual stars. Instead, special instruments separate the “barcodes” of the stars as they orbit each other, providing details on each star.

The third set of twins is identical. Each star is smaller and less massive than the Sun, and much fainter. And while the other two sets of twins are close to each other, these twins are so far away that it takes them thousands of years to orbit the others.

Script by Damond Benningfield