September 18, 2007


Recently, Astronomy Picture of the Day featured a stunning image of the constellation Scorpius (with a swath of the Milky Way across the background), and for no, uh, particular reason I can think of (see recent posts...) I thought I would follow up on that with more information about Scorpius which can be found low in the Southern sky this evening. According to Sky and Telescope, the waxing crescent moon can be found in Scorpius tonight near the bright star Antares.

The name Antares literally means Ant-Ares (Like Mars or Rival of Mars) because it resembles Mars in the night sky. The only difference is that it twinkles while Mars usually doesn't. I'll talk about why stars twinkle and planets don't another time if you are nice to me. Antares is a cool, red supergiant. If you put it in the middle of our solar system, it would engulf all four of the inner planets and a decent fraction of the asteroid belt! It is one of the few stars that is large enough and close enough for us to physically estimate its diameter based on its angular size.

Because it is in an obviously late stage in life, it is one of the few candidates (Betelgeuse in Orion is the other that springs to mind) in the sky that could go supernova at any time, perhaps tomorrow, perhaps in a few million years (that's "soon" to Astronomers). Due to its large size, it pulsates a little bit and so the brightness varies on a timescale of a few years by maybe 10-20%. Antares was known to the Romans as Cor Scorpionis ("heart of the scorpion"). Because it reaches its most prominent position in our skies around this time of year, the Persians made it one of their four Royal Stars. Antares is ascendant during the time of the autumnal equinox, so as it is seen in the Southern sky, we know Fall is near.

Antares is a member of the Scorpio-Centaurus Association, established by Kapteyn in 1914 (reference here is the wonderful "Burnham's Celestial Handbook"). This is a sprawling collection of young stars that spans across nearly a quarter of the southern sky. What links them is that they are all moving together toward a common point, meaning this is likely the remnants of some huge, nearby (few hundred light years away) molecular cloud that formed lots of bright stars millions of years ago. The huge angular size is due to the cloud being so close to us.

From Star Tales, we learn about some of the other bright stars in Scorpius: The 2nd brightest (Beta Scorpii) is often called Arcab, which is Arabic for Scorpion. Arcab is not to be confused with the nearby Arkab ("achilles tendon"), a bright star in Sagittarius. The same star (Arcab) is known to many as Graffias (Latin for "claws"). Other bright stars are Dschubba (Arabic for "forehead") and Shaula ("the sting"). Chris Dolan's incredibly useful constellations web page has more information and a handy star chart for this constellation. The Texas Astronomical Society of Dallas has a guide to some of the interesting Deep Sky Objects in Scorpius, of which the reflection nebula Rho Ophiuchi is perhaps the most spectacular.

The mythology of Scorpius is closely linked to Orion. In a couple of variants of the story, Orion is killed for some transgression (against the goddess of hunting Artemis perhaps) by Scorpius, and so the constellations are on roughly opposite sides of the sky. As Orion the Hunter sets, his conqueror Scorpius is rising. Scorpius was once a much larger constellations, but it was divided in the distant past into Libra and Scorpius. The two brightest stars in Libra are Zubenelgenubi and Zubeneschamali, which literally translate to the "northern claw" and the "southern claw".

Being a zodiacal constellation, Scorpius is near the ecliptic plane, which means it is often subject to occultations by the Moon or the planets. When Antares is occulted, observers have reported seeing a faint green companion (the color presumably arising from intervening material as there are no truly green stars, a tale for another time) star nearby when seeing conditions are excellent.

One last thing, if you haven't tried the Wikisky tool, check out the Scorpius entry and have fun.

Posted by Observer at September 18, 2007 10:53 AM

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.

Ok, so it's about 600ly away. If it went supernova 600 years ago, we'd notice it now. Would it have any effect on ISS or our communication sats?

Posted by: Humbaba on September 18, 2007 06:34 PM

Not real sure about communications disruptions. The only real effect would likely be a big spike in our neutrino detectors when the detonation wave gets here. Within a few days of that, it would brighten to around full moon brightness and stay that way for a few months. Could be worse if the pole of the star is pointed at us (unclear if that is so).

The real uncertainty is whether the blast wave, which travels at about 1/3 of the speed of light, would affect us much. Even if it does, that would be 1000 years or so later. The Wikipedia article on Betelgeuse speaks to the immediate effects that travel at the speed of light but not the blast wave.

I don't think it is all that likely that a supernova would have much of an effect on life here. If you run the numbers, I imagine in the 3.8+ Billion year history of life on the planet, we've had supernovae go off within a few hundred light years many times (on the order of 10-100 times at first guess, if supernovae are equally common everywhere in the galaxy). If it were that devastating, it seems like it would be more obvious in the fossil record.

There are mass extinctions in the fossil record, but not THAT many, and there are plenty of other explanations for them. Now if you run the numbers for supernovae occurring within 10 light years, you get on the order of a 1%-10% chance of one.

Posted by: Observer on September 18, 2007 08:40 PM