Scorpion Season

By Bob Berman|Monday, July 01, 1996
RELATED TAGS: STARS

Not many of the sky’s 88 constellations look like the mythological figures they portray. Of the handful that do, the most compelling formation arguably belongs to Scorpius, now up at nightfall in the southeast.

From the ancient Greeks to the pre-Columbian Maya, civilizations spanning time and geography all saw a scorpion in this grouping. That’s not surprising, given Scorpius’s claws, its fiery red heart, and especially its long, bright curvy tail, which culminates in a blue stinger.

The scorpion’s only serious rival for honors for best constellation is on the opposite side of the sky: the hunter, Orion, which has the unfair advantage of rising higher. But perhaps the two should share the prize, since you can’t compare them directly--one always sets as the other rises. Classical legend says this is because of a feud that began when Orion was killed by the scorpion. It’s a shame they couldn’t get along: each pattern is outlined by hot blue suns, and each boasts one of the sky’s only two bright red supergiant stars--Betelgeuse in Orion, and Antares, the heart of Scorpius.

Antares is slightly smaller than Betelgeuse, but it’s no slouch; its volume is more than a billion times greater than our own sun’s. If Antares were represented by an enormous hot-air balloon, Earth would be the size of the period at the end of this sentence.

Although it may be smaller than Betelgeuse, Antares has something Betelgeuse doesn’t: a strange companion star. The star, Antares B, is 100 times dimmer than Antares and can be made out just to its west with a small telescope in steady air. Strangely, although the pair orbits around a common center of gravity, Antares B never seems to change position. Astronomers think that B’s period of revolution about that center is simply too long (possibly 900 years) for us to have yet detected its snail’s-pace motion.

For observers, though, Antares B’s greatest puzzle--and the subject of controversy for more than a century--is its color: green. The problem? Many astronomers contend that there are no green stars. B’s color, they say, is an optical illusion, a green spot we perceive after our eyes turn away from the rosy brilliance of Antares to B’s white surface.

Yet when the Antares duo are eclipsed by the moon, and the companion reappears first (as it always does, since the moon approaches them from the west), most observers report that Antares B still looks green. Since there is no Supreme Court of color arbitration to rule on the issue, observers are free to continue the debate.

More strangeness: Both Antares stars are embedded in an odd nebula composed of fine metallic dust rather than gas. This cloud blocks out twice as much of our sky as the moon. At Antares’ distance of 500 light-years, that corresponds to some 10,000 times the width of our solar system.

Antares and its companion interact with their surrounding dust to produce intriguing radio signals--the target of continued study. Far beyond the pair, immense dusty hydrogen clouds form an obscuring backdrop, while Scorpius’s leftmost edge marks the direction to the turbulent center of our galaxy.

Binoculars make the Scorpius region a happy hunting ground of smudgy star clusters and mottled dark nebulas, but taking in the entire pattern with the naked eye is no small satisfaction. For observers in the northern United States, where the tail sweeps along the horizon, that’s a bit of a challenge. But it’s worth the effort these warm summer nights, if only to view one of the few celestial patterns in which everyone connects the dots the same way.

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