ZETA-1 SCO (Zeta-1 Scorpii). Scorpius, the Scorpion, is known for its hot blue-white O and B stars, as well as for great Antares. At the southwestern bend of the Scorpion's tail lies the visually faintest star of the classical figure, one with no proper name, Zeta Scorpii. A closer look reveals Zeta to be a close naked-eye pair separated by an easy 7 minutes of arc. The brighter, fourth magnitude (3.62) Zeta-2 Scorpii, falls just to the east of fifth magnitude (4.73) Zeta-1, which though lying in seeming obscurity is one of the grandest stars of the whole sky. The pairing is accidental: Zeta-2 is an orange class K giant only 150 light years away, while Zeta-1 is nearly 40 times farther! Such a great distance makes distance measurement difficult and problematic. This hot (21,000 Kelvin) class B (B1) high-end supergiant is much too far for parallax measurement. Instead, we must make use of its spectrum, of its membership in the distant Scorpius OB-1 association (of hot blue stars), and its possible membership in the open star cluster NGC 6231 that lies just to the north of it, all leading to a best estimate of an astonishing 5700 light years. Buried in the heart of the Milky Way, Zeta-1 Sco is dimmed by over two magnitudes by intervening dust clouds, and also reddened to appear a bland yellow-white instead of its true sparkling bluishness. Accounting for distance, dust-absorption, and a great deal of ultraviolet light the eye cannot see, Zeta-1 radiates roughly 1.5 million times more energy than does the Sun. Even the lowest estimate of luminosity comes in at a million solar, which makes Zeta-1 Sco one of the most massive stars in the Galaxy, falling somewhere around 60 solar. Stars like this one burn their interior hydrogen fuel very quickly and live short lives, Zeta-1 born only a few million years ago, and destined to explode only a few million years hence. It is so bright as to have gained the appellation "hypergiant." It also seems to fall into the category of a potential "luminous blue variable," the "LBVs" led P Cygni and by the southern hemisphere's Eta Carinae, which in 1846 brightened to become one of the sky's brightest stars and is now surrounded by a vast cloud of its own making. Zeta-1 Sco, now losing mass at a rate of about 1/100,000th of a solar mass a year at a wind speed of 400 kilometers per second, has a similar reputation of variability. While only varying minimally by a percent or so now, it may have undergone an eruption and brightened to third magnitude a couple hundred years ago, when it apparently outranked its much closer line-of-sight neighbor Zeta-2. Thanks to Jeff Bryan, who suggested this star.
Written by Jim Kaler. Return to STARS.