Delta Scorpii Still Showing Off

For the fourth summer in a row, the head of the bright constellation Scorpius carries a note of excitement. Delta Scorpii, normally magnitude 2.3, became a variable star in July 2000, flaring in a matter of weeks to magnitude 1.9. In the next few months it dipped almost back to normal, then rebrightened. It has fluctuated between about magnitude 2.0 and 1.6 ever since. As of a few days ago it was hovering at 1.6 or 1.7, barely under the official definition of a 1st-magnitude star (1.5). "The outburst keeps gaining strength," writes its discoverer, Sebastián Otero of Buenos Aires, Argentina. The naked-eye look of Scorpius is visibly changed from its age-old state, and Delta stands out as the brightest thing in the big swath of sky between Antares and Spica.

Delta Sco is a large, hot, rapidly rotating star of spectral type B0 about 400 light-years away. Spectra show that it began throwing off luminous gas from its equator when the outburst began. A fainter companion star orbits it in a very eccentric, looping orbit; the two stars swing close by each other (separated by about 20 times the primary star’s diameter) every 10.6 years. The last time this happened was in the summer of 2000, suggesting that this particular pass somehow triggered the flare-up. No one knows what Delta Sco will do next.