The Zuben Stars
Published on Sep 21, 2006 at 5:32 pm.
8 Comments.
Filed under constellations, skywatching, stars.
In the western sky, for a few hours after sunset, you can see a very bright object. That is the planet Jupiter. However, Jupiter isn’t really what I want to talk about. Rather I want to talk about two stars in the same area of the sky. One is just to the right of Jupiter, named Zubenelgenubi, and the other is above it a little ways named Zubeneschamali. Actually, there is no universal agreement on the spelling of these names, but these seem to be among the most common. These two stars are part of the constellation Libra. For some reason, I have always had rather a fascination with these stars. Perhaps it is the fact that they have two of the longest star names that there are.
These two stars are the two brightest stars in Libra. Libra has always looked to me sort of like a box, and I never see scales. But, that’s OK. I seldom actually see the patterns that are supposed to be there. Modern astronomers don’t really think of constellations as patterns, anyway, but as regions of the sky.
Libra is one of the constellations that lie along the ecliptic. The ecliptic is the path that the Sun appears to take through the sky. The Moon and planets also have orbits that make them always appear to be close to the ecliptic. That is one reason that Juipter is in Libra this year. Actually, Jupiter takes almost 12 years to orbit the Sun, to it appears to move over close to one constellation per year. Next year, Jupiter will be in Scorpius, and last year it was in Virgo. In particular,the ecliptic passes very close to the star Zubenelgenubi. In fact, it is so close that the Moon or planets often pass directly in front of the star — an event that we call an occultation. This time by, Jupiter passed just about 1/3 of a degree from Zubenelgenubi back on September 10, 2006. Back in May of 2004, the Moon occulted Zubenelgenubi during a total lunar eclipse. The next time a planet will occult the star will be in November 2052, when Mercury will pass in front of it.
Interestingly, Libra is the only zodiacal constellation (constellations along the ecliptic) that is supposed to represent anything other than a living being. This wasn’t always so. Long ago, Scorpius was once a much larger constellation, with the claws of the scorpion extending into the area now recognized as Libra. In fact the names of these stars tell that story: Zubenelgenubi means “Southern Claw” and Zubeneschamali means “Northern Claw.” These aren’t the only stars in Libra whose names harken back to when this was part of Scorpius. Gamma Librae is Zubeneladrab (”Scorpion Claw”). According to my research, the name Zuben Hakrabi has been applied to several stars in Libra, including Eta Librae, Gamma Librae, and Nu Librae.
It is not really clear why the stars of Libra were separated from Scorpius into their own constellation. Perhaps it was because Scorpius was too big, occupying a disproportionate amount of the ecliptic. At any rate, it is now its own constellation. As for why it is a pair of scales, this may be because of its proximity to the Autumnal Equinox. The equinoxes precess along the ecliptic, and about 3000 years ago, when many of these things were getting the names that we use today, the Autumnal Equinox fell in Libra rather that in Virgo as it does today. Several writes of ancient Greece and Rome made reference to the scales of Libra being used to ballance the days and nights at the Autumnal Equinox.Â
So, just to round out my discussion of these stars, I’ll give some physical data on them. Zubenelgenubi is Alpha Librae. It is a magnitude 2.75 type A3 giant or subgiant star about 77 lightyears away. Through a small telescope, you can see that it is actually a binary star. Zubenelgenubi is also known as Kiffa Australis (the “Southern Tray” of the scales).
Zubeneschamali is a spectral type B8 star of magnitude 2.61. Though a tiny bit brighter than Zubenelgenubi, Zubeneschamali bears the Bayer designation of Beta Librae. I have two references to this star one of which says that it is a giant star at a distance of 277 lightyears and the other says that it is a main sequence star only 160 lightyears distant.  I regard the latter as more likely correct, as it is based on more recent data. But, this isn’t the only confusion as to this star. I have also seen its name written many different ways. It is usually Zubeneschamali, as I have used it here, but I have also seen Zubeneschemali, Zuben Elschemali, and Zuben el Chamali. It is also known as Kiffa Borealis (the “Northern Tray” of the scales).
So, there you have some skylore. Go out and enjoy Libra while you can. In another couple months, the Sun will be sitting in the middle of it, and you won’t be able to see it at all!
-Astroprof
(Image generated by Starry Night Pro software)
jake on December 7, 2006 at 6:13 pm: 1
unbelievable
Nick Stafford on June 19, 2007 at 4:29 am: 2
Is it true that zuben elschemali is also green? If so do you know any references to this as this is an unusual anomoly.
Astroprof on June 19, 2007 at 7:19 am: 3
No, it isn\’t green. There are no truly green stars. However, sometimes a star that has little obvious color of its own will appear green when seen next to a definitely red star (like the companion to Antares). That is an optical illusion, though. I posted something about that here.
Steve on September 8, 2007 at 8:40 pm: 4
I’ve been looking at the “box” that is supposed to be “scales” (I don’t see the scales either). I’ve been trying to determine which is which.
When I look up and see the box, the top left is brighter than the rest, and the bottom left seems to flicker a red or orange colour. Which two stars are the very bright and the oddly red ones? Could you help me, please? My curiousity is getting to me.
Astroprof on September 9, 2007 at 2:17 pm: 5
I’d have to be standing next to you to see exactly which stars you are pointing to. Imagine Libra as a box. The upper right star would be Zubeneschamal and the lower right one would be Zubenelgenubi. Those are the two brightest stars. The other two stars in the “box” that makes up Libra are Gamma Librae in the upper left and Sigma Librae in the lower left.
Steve on September 18, 2007 at 8:35 pm: 6
I’m looking at the sky from southern Ontario this evening. The almost-half moon is relatively low, perhaps 30 degrees above the horizon at 830pm. To the right are the two stars I mentioned. Almost directly right (just slightly lower) is the reddish twinkling star I spoke of previously. Up and right is a very bright, whitish star, one of the brighter stars out this evening. About as bright as Arcturus in Bootes.
Hopefully this helps identify the two stars of which I write, and if indeed they are Zuben stars.
Thanks again! =)
Astroprof on September 18, 2007 at 10:30 pm: 7
Ah!!! I think that I know the two that you mean, now.
The red one is actually Antares, in Scorpius. The really bright one is the planet Jupiter. They are just a bit to the east of Libra.
Steve on September 19, 2007 at 12:55 pm: 8
Thank you. =)