The Red Giant

By Bob Berman|Friday, December 01, 1995
RELATED TAGS: SOLAR SYSTEM
Despite (or perhaps because of) its peculiar appellation, Betelgeuse is among the best known stars. Its prominent position just on Orion’s shoulder doesn’t hurt, of course. Nor does its fortune in being one of the few bright stars visible in every country in both hemispheres. Hovering above Earth’s equator, its orange light rains down on people everywhere from November through April, when our planet whirls us around to face it.

Betelgeuse (its name is probably derived from an Arabic phrase meaning armpit of the white-belted sheep) is the largest of the night’s major stars, the prototypical supergiant. Its hugeness can be grasped with a model: if this colossus were scaled down so that its diameter equaled the height of a ten-story building, then on that same scale Earth would be the period at the end of this sentence.

It’s hard to imagine something so immense doing anything but lazily holding court like Jabba the Hut, but Betelgeuse is far from lethargic. Expanding and contracting every few years like an anesthesiologist’s oxygen ball, it swells from 400 to 600 times the diameter of our sun. At its most bloated, Betelgeuse, if plopped into the sun’s place, would fill the solar system out to Jupiter.

This kind of yo-yo existence is common to all red supergiants and is a sign that the star is near the end of its life. Indeed, our sun will still be shining long after Betelgeuse has vanished. Such life-styles of the red and swollen occur because of an epic struggle between the outward rush of radiant energy and gravitational collapse, each briefly winning out in a period of about three years. You can watch this drama for yourself: the doomed twinkling jewel betrays its mutating size with distinct changes in brightness over the years.

Although Betelgeuse sits at the daunting distance of 500 light- years from Earth, it’s still the nearest red supergiant and boasts the largest apparent size of any star in the heavens. It’s not surprising that it was the very first to have its diameter measured. The now-common process that achieved this success in 1930 was interferometry, which uses optical interference patterns to calculate a source’s dimensions. Then in 1975 astronomers working with the great 158-inch telescope at Kitt Peak, using computer enhancement techniques, discovered blotches on Betelgeuse that resemble giant sunspots.

If we ever achieve the telescopic thrill of seeing a star as a real disk rather than a point of light, this, too, will most likely first happen with Betelgeuse. The star, which from Earth appears to be some 36,000 times smaller than the full moon, is potentially resolvable with the Hubble Space Telescope. That would be quite a feat for the Hubble; resolving Betelgeuse would be something like discerning a basketball as a disk from about 800 miles away.

In 1978 astronomers found Betelgeuse to be surrounded by a shell of potassium gas. This material originated on Betelgeuse but has now blown into an enormous bubble extending some 300 times farther than Pluto lies from the sun. The outflow demonstrates the way most of the universe’s elements formed--in stellar furnaces rather than in the original Big Bang. Who knows? Perhaps someday fragments of Betelgeuse’s homemade potassium may find their way into the organisms on some distant planet.

In anticipation, we might enjoy snacking on bananas while gazing into the December sky at the largest object most of us will ever see.
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