How can binary stars orbit each other so fast?

In a News Note (S&T: November 2004, page 16) you described WR 20a, a binary star in Carina with components of 83 and 82 solar masses and an orbital period of 3.686 days. How can two huge balls of gas move so fast?

Grab any introductory astronomy textbook and turn to the chapter on binary stars. There you’ll find some simple formulas relating the two stars’ masses to their separation and orbital period. These equations come from Isaac Newton’s theory of universal gravitation and Johannes Kepler’s laws of orbital motion, which have served astronomers well for more than three centuries.

When I plug in the masses and period given in our News Note, I find that the stars are about two-thirds as far apart as Mercury and the Sun, and that they whirl around each other with a relative speed of more than 700 kilometers per second (11/2 million miles per hour).

That is very fast, and it may strike you as outlandish, but it’s completely consistent with orbital mechanics. It’s also within the realm of our own experience. We’re actually zipping along at about 240 km per second right now as our solar system circles the galactic center!

The components of WR 20a are Wolf-Rayet stars, extremely hot, luminous bodies about 20 times larger than the Sun and only a hundredth as dense. Even whirling around as they do, they have no trouble holding themselves together. Such is the awesome power of 80-plus Suns’ worth of gravitating mass.

— Richard Tresch Fienberg

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