Saturn-moon Engagement Plus A Peculiar Star Worth Watching

Look south this evening and you’ll see Saturn and Spica on either side of the moon. Farther down in the southeast, Antares sparkles with a reddish hue. Our featured star Delta is just in front. Created with Stellarium

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.

The hot subgiant star Achernar, very similar to Delta Scopii, is flattened into an oval shape by its extreme rotation. Credit: Wikipedia

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.

3 Responses

  1. Edward M. Boll

    Speaking of Saturn, I understand that on Sunday morning, Nov. 24 Saturn and Mercury will appear close together. Make that a triple conjunction. ISON will appear near the pair, hopefully as bright. Hey, make that a quadruple! Encke will appear 1 degree from ISON. One ephemeris puts Encke around magnitude 5. The 4 should rise over an hour before sunrise.

  2. Phil Moran

    I live in Springfield, MO. Tonight (8/15/2014) at 11:30 I noticed a peculiar star in the west maybe 10 degrees or so above the horizon. In my binoculars it appeared to be changing color rapidly – red & blue. I saw no movement so ruled out airplane. Any ideas???

    1. astrobob

      Hi Phil,
      That was the bright star Arcturus. It flashes and flickers from atmospheric turbulence which is stronger closer to the horizon compared to when a star is higher in the sky.

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