Libra – the scales

April 2, 2017

The constellation of Libra represents a set of scales and it is by far the newest of the zodiacal membership. It may seem odd that there is a machine for weighing stuff in the weird line-up of monsters and zoo critters. On the other hand its two brightest stars have my most favorite star names; Zubenelgenubi, and Zubeneschamali, aka alpha and beta Librae.

You should definitely learn these two star names because you can impress your friends and your family. Take them outside at night, and look up, and just point to any two stars, they won’t know the difference, and say “Look, there’s Zubenelgenubi and Zubeneschamali.” From then on they’ll assume you know everything about the stars.

The backstory on Libra starts at a time when the Romans were mucking about with the calendar. Those guys were always stealing days from February and other shenanigans. Then they started trying to divide the sky up into perfect 30 degree sections. Well that didn’t work. The star groups are all different sizes and shapes. The Sun is in Virgo for 45 days. It was in the scorpion for a neat and tidy 30 days. But the Romans didn’t like how Scorpius had such huge gangly claws because you see at that time, there was no Libra. The stars of Libra were the scorpion’s original claws. So the Romans cut off his claws. Then they gave him some little tiny claws, and used the left over space to add the scales. The Romans actually stole the idea of the scales from some old Babylonian drawings.

Chopping off the claws is why the Sun is now in Scorpius for only 7 days and why we have 12 birthday signs instead of 11, which is way more cool anyway because 12 can be divided easily. If you think long division sucks, just try doing it with Roman numerals. But here’s the thing, Zubenelgenubi is an ancient Arabic phrase that means “the southern claw,” and Zubeneschamali means “the northern claw.” And now you know the rest of the story.

Zubenelgenubi (south claw) is a member of the Castor Moving Group (Castor, Fomalhaut, Vega, α Cephei, and α Librae). These are similar stars that are all headed in a similar direction at a similar speed so for a while astronomers figured they were born together, that they are all sisters. Finer evaluation has revealed the group’s association is recent and temporary so they are probably not sisters. Plus they are all getting along and never argue or steal each other’s sweaters.

Zubeneschamali (north claw) is sometimes known as the only green star. Occasionally, throughout the ages, people have reported a bit of a greenish tinge. A star’s peak output of color can be pretty much anything in the rainbow but there is a good reason they never look green. The peak output of a star is rather “flat” which is geekspeak obviously and what it means is that if a star peaks in the green, it also puts out almost as much blue and red. If it peaks in the blue, it looks blue. If it peaks in the red, it looks red. But if it peaks in the green, it looks RGB, which is white. So it takes several measurements through special filters to sort it all out. Anyway, Zubenschamali doesn’t look the least bit green at all to me. I don’t know what all those crazy star hoppers back then were smoking, something green probably.

For all you historians concerned with such things as what the sun was up to prior to 730 AD, it could be found in Libra on the autumnal equinox back then. Scales… Equinox. It sort of makes sense. But by 730 the earth had wobbled Virgo over to that position. Young Maiden… Equinox…? You’re going to have to figure out how that one balances on your own.

Another interesting star in Libra is HD 140283, also known as Methuselah. It’s the oldest known star in the whole damn universe, 14.46 billion years old. That’s weird because the Universe is only 13.77 billion years old. So the star is .69 billion years older than the universe. What this discrepancy is actually revealing is that we astrophysicists are not as good at math as we pretend to be.