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.