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Most stars are born in bunches — great swarms that form from the collapse of a single cloud of gas and dust. The stars tend to stay together for a while, forming clusters. Over time, though, the gravity of the galaxy’s other stars and gas clouds tends to pull a cluster apart, so its stars go their own ways.
Even then, many of the stars can share a common motion through the galaxy — they move at the same speed, and in the same direction. Such associations are known as “moving groups.” And the title star of one of these groups rises into view in late evening at this time of year.
Zeta Herculis is at the top right corner of the Keystone, a pattern of four stars at the center of Hercules.
Zeta Herculis actually consists of two stars, which are about 35 light-years from Earth. One of the stars is bigger, brighter, and heavier than the Sun, while the other is less impressive than the Sun.
Decades ago, it was discovered that the system shares its motion through space with several other stars, spread across several constellations. That suggested that the stars were born together, so they were called the Zeta Herculis Moving Group.
Over the years, though, follow-up observations have been unable to confirm whether the stars are all the same age. So the stars of the moving group could be related — or they could just happen to share the same motion through the galaxy.
We’ll talk about a group of stars in Hercules that has stuck together tomorrow.
Script by Damond Benningfield