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VV Cephei

A star system in Cepheus is big and messy. And every couple of decades, it fades considerably.

VV Cephei is about 5,000 light-years away. It’s a binary — two stars locked in a mutual orbit around each other. Both stars are about 20 times the mass of the Sun. But their sizes are vastly different. One is perhaps 15 or 20 times the diameter of the Sun. But its companion is more than a thousand times the Sun’s size — one of the biggest stars in the galaxy.

The difference is caused by their stage in life. The smaller star is still in the prime of life, “burning” the hydrogen in its core to make helium. But the larger star has moved on to the next stage. That’s caused its outer layers to puff up to gigantic proportions. And a strong “wind” of charged particles blows from the star out into space.

In addition to the wind, a ribbon of gas funnels from the bigger star to its companion, forming a wide disk around the companion.

Every 20 years, the bigger star passes in front of the smaller one. Each eclipse lasts more than 21 months. That makes the system shine a good bit fainter than average. The most recent eclipse ended last year, so VV Cephei won’t fade again until 2039.

When it’s not in eclipse, the system is just visible to the unaided eye. It’s at the center of a four-sided figure that forms the body of Cepheus, the king, which is high in the north-northeast at nightfall. You need dark skies to spot this big, messy star system.

Script by Damond Benningfield

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