Green Star Mystery

Green Star Mystery

The constellation Libra can be seen toward the southern horizon. It sits between the constellation Scorpius and the bright star Spica. Libra is a kite-shaped constellation and one of its dominant features is the Northern Claw and the Southern Claw. They are called claws because once upon a time the two stars were part of Scorpius, but the Romans changed that when they created a new constellation called The Scales, which was meant to represent Justice.

The Northern Claw’s recognized name is Zubeneschamali. Say that five times fast! It is the brightest star in Libra. Long ago the Greek astronomer Erastosthenes, who was the first to measure the diameter of the Earth, recorded the fact that Zubeneschamali was brighter than the star Antares which is in Scorpius. This is surprising because Antares, which sits some 500 light-years away, is a bright red first magnitude star. Even a casual viewer would note that on a summer evening Antares quite outshines the Northern Claw. So the question begs to be asked: has Zubeneschamali dimmed since the time of Erastosthenes? But rather than dimming it is possible that Antares has expanded and brightened over the course of the last 2300 years. The heart of Scorpius is well on its way to becoming a red super-giant.

The Southern Claw is called Zubenelgenubi. It is somewhat dimmer than the Northern Claw. That seems fitting as this difference in luminosity between the two stars would have been suggested an imbalance to the creative Roman who came up with the idea of The Scales. Looking carefully the two stars do appear out of balance.

The image above is taken from the EarthSky website (www.earthsky.org). The article on Libra indicated that Willian Tyler Olcott, a noted astronomer, was quoted in Burnham’s Celestial Handbook as having stated that the Northern Claw is “… the only naked-eye star that is green in color.” I’ve heard this before at a stargazing party in Bristol Springs, NY. Several of us looked through some large Dobsonian telescopes that were present and could not see the green tinge. To me it looked deep blue, so I can see how, under certain conditions, Zubeneschamali might take on a greenish tinge.

Stars populate a zoo of sizes and types, the most common and understood occupying what is called the main sequence. The main sequence can describe a “typical” star’s lifespan. The main sequence shows the phases that a star will go through across the billions of years of its lifespan. For astronomers it is a key to understanding stellar evolution.

It seems impossible that if a star were to move off the main sequence some combination of nuclear burn rate, scale, and pressure could lead, ever so briefly, to a star that looks green. This scenario turns up from time to time in science fiction. I’ll give the authors who create such stars a good deal of credit. They very cleverly explain such a star’s existence as being based on a set of freakish conditions that impact the burn ratio of elements in the core of a star. This leads, ever so briefly, to a green star. Could such a thing exist? Well, across an infinite amount of time and space, perhaps.

Is Zubeneschamali green? Go out on a clear summer night and look for yourself! Maybe you’ll solve the mystery of the green star.

The Scales

The Scales

One summer constellation with an interesting history is the star pattern known as Libra the Scales. On a warm summer evening it can viewed toward the southern horizon near the reddish star Antares. That star is in Scorpius and in ancient times Libra was also a part of the constellation Scorpius. Libra served as the claws of the scorpion and the two brightest stars in Libra are still called the Northern Claw and the Southern Claw.

The Romans were known for many things including their system of laws. It was the Romans who removed the claws from the scorpion and created a new constellation in the night sky. This constellation was meant to represent the scales of justice. In the northern hemisphere Libra is the only figure within the zodiac that represents a man-made object.

Libra was added to the zodiac during the time of Julius Caesar. Caesar established a new calendar called, appropriately enough, the Julian calendar. Caesar’s new calendar remained in use for almost 1600 years. It was replaced by the Gregorian calendar which was promoted by Pope Gregory in the 1580s. This is the calendar we use today. Interestingly Libra is often associated with the nearby constellation of Virgo. The Romans viewed Virgo as a goddess of justice so having the scales of justice nearby is appropriate.

Libra is “home” to the Gliese 581 planetary system. A number of exoplanets have been found within the Gliese 581 system including Gliese 581c which was the first Earth-scale exoplanet found within another star’s habitable zone. Gliese 581c has a number of companion worlds that circle the parent star. Perhaps these could be named after prominent Romans?

libra2
Constellation Libra with diagram and stars labelled.

(Images may be found at Professor James Kaler’s website at the University of Illinois: http://stars.astro.illinois.edu)