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If you draw Earth’s orbit around the Sun, it looks like a circle, with the Sun right in the middle. But that picture is a bit misleading. Earth and the Sun actually orbit a common point — the center of mass for the system. Since the Sun is about 330,000 times heavier than Earth, that center is deep within the Sun itself. So the conventional picture of Earth’s orbit is just fine for everyday uses.

For objects of similar masses, though, it’s a different story. Consider Capella, the brightest star of Auriga the charioteer. It consists of two pairs of stars, although only one pair is bright enough to see without a good telescope.

The stars in the bright pair are almost equal — about two-and-a-half times the mass of the Sun — so the center of mass is half-way between them. So when you trace out their orbits, it looks like a pair of toy trains going around and around on the same circular track.

Because the stars are the same mass, they also have the same life history. They’re both puffing up to become giants. And they face the same fate — both will blow off their outer layers and end their lives as the faint cinders known as white dwarfs. That won’t happen for tens of millions of years, though, so Capella will continue to shine brightly for a long time to come.

Right now, look for it low in the northeast at nightfall, and high in the northwest at first light. It’s one of the brighter stars in the night sky, so you can’t miss it.

Script by Damond Benningfield

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