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ZETA-1 SCO (Zeta-1 Scorpii).Scorpius,the Scorpion, is known for its hot blue-white O and B stars, aswell as for greatAntares. At thesouthwestern bend of the Scorpion's taillies the visually faintest star of the classical figure, one withno proper name, Zeta Scorpii. A closer look reveals Zeta to be aclose naked-eye pair separated by an easy 7 minutes of arc. Thebrighter, fourth magnitude (3.62) Zeta-2 Scorpii, falls just to theeast of fifth magnitude (4.73) Zeta-1, which though lying inseeming obscurity is one of the grandest stars of the whole sky. The pairing is accidental: Zeta-2 is an orange class K giant only150 light years away, while Zeta-1 is nearly 40 times farther! Such a great distance makes distance measurement difficult andproblematic. This hot (21,000 Kelvin) class B (B1) high-endsupergiant is much too far for parallax measurement. Instead, wemust make use of its spectrum, of its membership in the distantScorpius OB-1 association (of hot blue stars), and its possiblemembership in the open star cluster NGC 6231 that lies just to thenorth of it, all leading to a best estimate of an astonishing 5700light years. Buried in the heart of the Milky Way, Zeta-1 Sco isdimmed by over two magnitudes by intervening dust clouds, and alsoreddened to appear a bland yellow-white instead of its truesparkling bluishness. Accounting for distance, dust-absorption,and a great deal of ultraviolet light the eye cannot see, Zeta-1radiates roughly 1.5 million times more energy than does theSun. Even the lowest estimate of luminositycomes in at a million solar, which makes Zeta-1 Sco one of the mostmassive stars in the Galaxy, falling somewhere around 60 solar. Stars like this one burn their interior hydrogen fuel very quicklyand live short lives, Zeta-1 born only a few million years ago, anddestined to explode only a few million years hence. It is sobright as to have gained the appellation "hypergiant." It alsoseems to fall into the category of a potential "luminous bluevariable," the "LBVs" ledP Cygni and bythe southern hemisphere'sEta Carinae, which in 1846 brightened tobecome one of the sky's brightest stars and is now surrounded by avast cloud of its own making. Zeta-1 Sco, now losing mass at arate of about 1/100,000th of a solar mass a year at a wind speed of400 kilometers per second, has a similar reputation of variability. While only varying minimally by a percent or so now, it may haveundergone an eruption and brightened to third magnitude a couplehundred years ago, when it apparently outranked its much closerline-of-sight neighbor Zeta-2. Thanks to Jeff Bryan, who suggestedthis star.
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