Close Companions

StarDate: May 9, 2009

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Orion's hunting dogs chase high across the sky on May nights. They're directly overhead late this evening, enwrapped by the handle of the Big Dipper. Their constellation is known as Canes Venatici. Its stars aren't all that bright. Yet one of its stars is the prototype for an entire class of stars.

The star is known as RS Canum Venaticorum. It's actually a system of two stars -- one yellow, the other orange -- that orbit each other once every 4.8 days. When one star passes in front of the other and cuts off its light, the system gets a little fainter.

The short orbital period means that the two stars are quite close together -- much closer than the Sun is to Mercury, the innermost planet in our own solar system.

Because they're so close, each star's gravity pulls strongly on the other. That locks them so that the same hemisphere of each star always faces the other, just as the same side of the Moon always faces Earth. So each star in the system spins once every 4.8 days -- much faster than the Sun, which takes a month for each rotation.

The rapid spinning causes strong magnetic activity that produces large, dark storms that are like sunspots. So as the system turns, astronomers see its light vary even when one star isn't eclipsing the other.

RS Canum is the prototype for an entire class of binary stars. They provide insight into what our own Sun was like billions of years ago, when it spun much faster than it does today.

Script by Ken Croswell, Copyright 2009

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