Class G

The Sun is near the top of its class — class “G.” Such stars all have about the same temperature, so they look yellow. Members of the class that are in the prime of life, as the Sun is, are also close to the Sun’s mass, size, and brightness.

That doesn’t mean the stars are just alike, though. They’re different ages, they spin at different rates, and they have different levels of activity.

A close example is Kappa Ceti, in Cetus, the whale. The star is about 30 light-years away. It’s almost exactly the same size and mass as the Sun. But it’s only 85 percent of the Sun’s brightness, and it spins about three times faster. And those two facts are related.

Kappa Ceti is a few hundred million years old — less than 10 percent the age of the Sun. Younger stars rotate faster than older stars — a result of their formation. They spin fast as the cloud of dust and gas that gave them birth collapses. A rapidly spinning star generates a stronger magnetic field. That creates more and bigger “starspots” — dark storms on the surface. And that reduces the star’s overall brightness. In the case of Kappa Ceti, some of the spots are so big that they cause the star’s brightness to change as the spots rotate into and out of view.

Kappa Ceti is high in the south-southwest at nightfall. Under dark skies, it’s just visible to the unaided eye. It’s well to the left of the brilliant planet Jupiter, and to the lower left of the Moon.

Script by Damond Benningfield

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