DSCHUBBA (Delta Scorpii). The middle star of the three-star line
that makes the head of Scorpius, the
Scorpion, Dschubba appears just south of (usually) somewhat fainter
Graffias. The name derives from an
Arabic phrase meaning "the forehead" (jabhat) of the scorpion,
which was originally applied to the whole line, Graffias, Dschubba,
and somewhat fainter Pi Scorpii (which in spite of its brightness
has no proper name, the downside of being in a constellation with
so many bright stars). Dschubba, usually a bright second magnitude
star (previously measured at 2.32) that is 400 light years away, is
undergoing a remarkable change. In July 2000, the star began to
brighten, and during 2001 and 2002 began to close in on first
magnitude as it turned itself into a "B-emission" star rather like
Gamma Cassiopeiae, one with a
surrounding disk produced in part by
rapid rotation (which is at least 181 kilometers per second at the
equator, 90 times that of the Sun). Dschubba received Bayer's
Delta designation, and originally in fifth place in brightness it
is close to the mark, but beat out not by Beta (Graffias) and
Gamma, but by Lambda (Shaula), Theta,
Epsilon, and of course Antares. Currently,
the star is number two right after Antares. Dschubba is also
remarkable for its complexity, and is most likely quadruple. The
main component, the one that is brightening, is a hot class B (B0)
star (almost but not quite class O) at least 14,000 times brighter
(including the invisible ultraviolet radiation) than the Sun and 5 times as large. It is accompanied by
a cooler class B companion over ten times fainter, the two
separated by roughly Mercury's distance from the Sun and taking 20
days to orbit. A third companion two-thirds as bright as the
dominant star was originally lies much farther away from it, at
least at Saturn's distance, and takes at least a decade to make the
trip. At a minimum double that distance lies yet another fainter
star. With masses that range from 12 down to 6 times that of the
Sun, all are probably still fusing hydrogen into helium in their
cores (though the main component, a "subgiant," may be close or
even beyond the end of that stage.) The main component will die
first and may someday look something like Antares does today. It
will produce either a very heavy oxygen-neon white dwarf or will
explode sometime within the next 10 to 15 million years. The
others will follow in their evolution shortly thereafter, the two
fainter becoming heavy, but more ordinary carbon white dwarfs
(rather like Sirius-B is today). Dschubba is part of an
association of O and B stars that includes both Graffias and
Antares, all of which were born within the same complex of
interstellar gas and dust. Dschubba is hot enough to ionize the
surrounding interstellar gas out to a distance of almost 10 light
years. Although fairly close to us, there is so much dust in the
line of sight that the stars of Dschubba are dimmed by about fifty
percent.