One of the biggest families of stars yet discovered rotates down the northwestern evening sky at this time of year. It’s one of the two most populous star systems yet found.

AR Cassiopeia consists of seven stars. They’re split into three pairs, with a third star tied to one of the duos. But they’re not exactly a close family — the pairs are trillions of miles apart.

The most impressive member of the family is identified as star Aa. It’s about six times the mass of the Sun, and five times the Sun’s diameter. It’s paired with star Ab, which is also more impressive than the Sun. Star B orbits them once every 545 years.

The members of the other two pairs are a little smaller. And they’re way off on their own. It takes each pair several hundred thousand years to complete a mutual orbit with the central trio.

That’s really about the only way for such a busy star system to remain stable. If you pack a bunch of stars close together, gravitational interactions among them kick some of them out. So the systems need to be widely spread. And it’s best if they form smaller groupings, like the stars of AR Cassiopeia — one of only two septuple star systems yet discovered.

The system is in Cassiopeia the queen, which is in the northwest at nightfall. Some of its stars form a big letter W, which is tilted on its side right now. AR Cas is below the bottom point of the W. Under dark skies, it’s barely visible to the unaided eye.

Script by Damond Benningfield

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