Tonight look to the south and you’ll see the waxing gibbous moon paired up with two bright “stars”. One’s a real star – Spica in Virgo – and the other is the planet Saturn. If you let your gaze slide further south and east you’ll soon run into a third luminary – the red-orange heart of the Scorpius Antares (an-TAR-eez).
While Antares is one of the sky’s most interesting red supergiant stars, we’re going to turn our attention instead the three stars to its west that forms the head of Scorpius and in particular to the middle star Delta Scorpii. Delta is in the throes of a tantrum that’s lasted more than a dozen years.
Back in June 2000, this unassuming 2nd magnitude star quite suddenly began to brighten until by 2001 and 2002 it had nudged up almost to 1st magnitude, the class which includes includes the brightest. It peaked in 2003-2004 at magnitude 1.6. Delta’s since faded back to about 2.0 magnitude (I checked two nights ago) which is still a third brighter than normal.
What could cause such an outburst? Astronomers think it may have to do with how fast Delta spins. Hot giant stars of its ilk rotate so rapidly – at least 155 miles per second vs. the sun’s 1.2 mile per second – they sometimes fling hot, luminous gaseous masses from their equators like a plump clown tossing candy at a parade.
The hydrogen gas forms a flattened bright disk around the star causing a temporary brightening. Complicating matters, Delta has a very close companion star that circles it every 10.8 years. Searching back through earlier data, astronomers found that a similar though weaker outburst occurred in 1990-91 a short time after the smaller star passed closest to Delta as it did again in 2000.
Every clear night I look to the southern sky to see what Delta’s up to. It’s so easy to do. If you’d like to try it yourself, compare Delta to nearby Beta Scorpii (magnitude 2.6) and Spica (mag. 1.0). If it’s exactly between the two, its magnitude is 1.8. For the moment, our featured star has yet to return to its original brightness, so you never know what’s next. That’s the fun of it of course.