Delta Scorpii - Dschubba

Dschubba is the middle star of the three-star line that makes the head of Scorpius, the Scorpion. It appears just south of (usually) somewhat fainter Graffias. The name derives from an Arabic phrase meaning "the forehead" (jabhat) of the scorpion, and was originally applied to the whole group of β, δ, and π Sco. δ Sco also has the traditional names Iclarcrau or Iclarkrav.

Because Delta Scorpii is near the ecliptic it is occasionally occulted by the Moon, or (extremely rarely) by planets.

Outburst

Usually measured at magnitude 2.32, δ Sco was originally fifth brightest in the constellation. Recently it underwent a remarkable change. In July 2000, the star began to brighten, and during 2001 and 2002 it began to close in on first magnitude as it turned itself into a "B-emission" star, like Gamma Cassiopeiae, with a surrounding disk produced in part by its rapid rotation. (δ Sco's rotation rate is at least 181 km/sec at its equator, 90 times faster than the Sun's.) At its peak in 2003-2004, the star was second brightest in the constellation right after Antares itself. As of 2008,it remains in an erratic state near magnitude 2.1, about 0.2 magnitudes brighter than normal.

Components

Dschubba is remarkably complex, and is most likely a quadruple system. The main component, the one that is brightening, is a hot class B0 IV subgiant at least 14,000 times more luminous than the Sun (including the invisible ultraviolet radiation), and five times as large. It is accompanied by a class B star that orbits the primary every 20 days, at a distance comparable to the distance from the Sun to Mercury. There is a third component that takes about 10 years to circuit Dschubba, in a highly eccentric orbit that takes it close in to the primary once a decade. The last close encounter of these two stars happened in mid-2000, and may have triggered the outburst of the primary star. A possible fourth companion star lies at about twice the distance again from the main star.

Evolution

With masses that range from 12 down to 6 times the Sun's, all of δ Sco's components are probably still fusing hydrogen into helium in their cores (though the main component, a subgiant, may be close to the end of that stage.) The main component will evolve to a red giant first and may someday look something like Antares does today. Sometime within the next 10 to 15 million years, it will produce either a very heavy oxygen-neon white dwarf, or will explode as a supernova. The others will follow in their evolution shortly thereafter, the two fainter ones becoming heavy carbon white dwarfs.

Dschubba is part of an association of O and B stars that includes both Graffias and Antares. All 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, at 400 light years away, there is so much dust in the line of sight that the components of Dschubba are dimmed by about 50%.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]