A Capella

By Bob Berman|Friday, March 01, 1996
RELATED TAGS: NEW PLANETS
Last October a star named 51 Pegasi, previously of academic interest only, suddenly became a news item when astronomers detected a wobble in its movement through space. That wobble, they determined, was being caused by a large planet racing around the star in a tight, four-day orbit. Not only was this evidence for a planet outside our solar system, but 51 Pegasi was intriguingly similar to our own sun. No wonder backyard observers across the country began struggling to find the dim celebrity.

Those observers would do well to take a look this month at the far more easily seen Capella, which dominates the constellation Auriga. Capella is now high overhead and at its best, the fourth-brightest star above North American skies. Its creamy white dazzle is bested these nights only by blue Sirius much lower in the south.

These two stars--Capella and 51 Peg--offer a fascinating comparison. They boast the same temperature and color, and both, like our sun, are G-class stars.

Then why does Capella so outshine barely visible 51 Peg? Because G-class stars come in two varieties. Our sun and 51 Peg are the more common midget brand (astronomers call them main sequence stars), while Capella is the nearest example of the giant variety. It’s a larger billboard, catching our notice by emitting 100 times the light of its tinier compatriots.

This most northerly of the night’s bright stars lies close enough to the celestial pole that it never sets for people in the northern United States, all of Canada, and most of Europe. No night is ever completely empty of Capella.

Or, rather, both Capellas. There are two of them, merged into a single brilliant star point. The pair of giant stars tumble around their common center of gravity every three and a half months. (This unseen choreography, first discovered in 1921 at Mount Wilson Observatory in California, surrendered its final secrets just two years ago at that same facility--as if to prove that the world’s onetime largest telescope can still yield amazing discoveries.)

Using optical interferometry, astronomers split the two. This was not easy: from Earth their separation is the width of a penny as viewed from 30 miles away. The Capella twins look like one star even through the Hubble Space Telescope.

Yet the investigators were able to determine not just their separation (less than Venus’s distance from the sun) but their individual diameters. Capella A (which, unlike earthly a cappella endeavors, tells us nothing without accompanying instruments) is 7.5 million miles across-- nearly nine times the sun’s width. At 6 million miles across, Capella B is almost as big.

If Earth orbited the pair at the same distance it now orbits the sun, they’d appear separated by 45 degrees in our sky, or by about the radius of a rainbow. Each would be the size of a small plum held at arm’s length. But this is impossible. Capella offers a friendly sunlike environment on paper only: such large and massive stars swinging so closely and rapidly about each other make stable planetary orbits nearly impossible.

Things are no better over at 51 Peg: its newfound companion orbits just 5 million miles from the star--nearly eight times closer in than Mercury is to our sun--and is scorched into an 1800-degree lifeless cinder. It may be a planet, and its star may be like our sun, but neither is anything quite like home.
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