Castor

The star Castor represents one of the twins of Gemini. But it takes the business of “twinhood” to extremes. Although we see it as a single point of light, Castor actually consists of three sets of stellar twins — a total of six stars in all.

All six stars in the system really are related, because they were born from a single giant cloud of gas and dust, probably around 200 million years ago.

Two pairs of Castor’s twins are more fraternal than identical. One star in each pair is a good bit bigger, brighter, and heavier than the Sun, while the other star in each pair is smaller, fainter, and less massive than the Sun.

The stars in each pair are so close together that they orbit each other in just a few days. At such close range, there’s no way for telescopes to see them as individual stars. Instead, special instruments separate the “fingerprints” of the stars as they go around each other, providing details on each star.

The third set of Castor twins is identical. Each star is smaller