Castor and Pollux:  Brotherly Love

Castor and Pollux, or Polyduces, were both twin brothers and half brothers: a most unusual combination.     Making twin half brothers requires two fathers and one rather efficient mother.   (The proper term for this conception is "Heteropaternal superfecundation.")    Their mother was the Ateolian princess Leda who became the Spartan queen when she married Tyndareus, who was Castor's father.     Zeus, however, sired Pollux when she seduced Leda while assuming the form of a swan.     As a result of both couplings, Leda produced two eggs.   One of them contained Castor and his sister Clytmenstra.   (We met her on May 11th.)  Out of the other egg hatched the famous Helen and her brother Pollux.      While Helen and her sister were often estranged, Castor and Pollux were the best of friends almost from birth and remained devoted to one another throughout.  They matured into fine horsemen and warriors.   In fact, the named Polyduces means "Boxer."    These two formidable fighters embarked on many adventures together, notably the Calydonian boar hunt in which Atalanta -we met her on May 7th- figured prominently.     They joined Jason and the other Argonauts in their pursuit of the Golden Fleece.   They also angrily pursued Theseus when he kidnapped the young Helen.   The fame-seeking Theseus had heard the prophecy that Helen's abduction would precipitate the greatest war the world had ever known.     Theseus had hoped to bring that conflict about and then serve heroically in it.   Alas, all he managed to do was incur the wrath of her brothers.      They found Helen at the home of Aethra, Theseus' mother.  Theseus decided to keep her there while he and Pirithous ventured down into the underworld to kidnap Hades' wife Persephone.  We might recall that Hades welcomed them both to his house and had them sit on chairs of forgetfulness.   The god knew full well their intentions and it amused him to keep them in states of utter bewilderment.    This situation proved to be a blessing for Theseus, for he was not present when Castor and Pollux arrived to collect Helen.   He might have been a match for one of the Gemini twins, but not both of them.   It was one of the mythological realm's greatest ironies:  Theseus' life was spared by having been imprisoned in the land of the dead*.    One day, as Castor and Pollux were engaged in a battle within Sparta, Castor was slain by Argive King Lynceus, whom Pollux then promptly killed.     As he embraced his brother's corpse, Pollux, who was immortal by virtue of his parentage, pleaded with Zeus to kill him, instead, and bestow immortality on his brother.    Although Zeus refused to fulfill this request, he did allow Pollux to ascend into Olympus with his brother's shade.      They were both then placed in the sky as the Gemini twins, a constellation prominent in the winter and spring evening sky.   Unlike many of the other disagreeable characters adorning the firmament, the Gemini twins exemplify the finest human traits:  bravery, filial devotion and selflessness.   Resembling conjoined stick figures, the Gemini twins represent the everlasting love of brothers.

*Recall that Hercules rescued Theseus when the former descended into the underworld to abduct Cerberus, his twelfth and last labor.  

 

THE SOUTHWORTH PLANETARIUM

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Founded January 1970

Julian Date: 2459095.16

2020-2021:  III

THE DAILY ASTRONOMER

Wednesday, September 2, 2020

Remote Planetarium 81:   Galactic Survey I

Today and tomorrow we're going to conduct a thorough survey of our Milky Way Galaxy. We can pretend that we've been granted a 100 million year life span and have to figure out ways to spend all that time by composing a galactic travel itinerary.     What to see.  What to avoid.   What to admire from a great distance.  In a sense we will be roaming through the Milky Way Galaxy over the next few courses.       What will we encounter?

The Supermassive Black Hole in the nucleus:

A very good object to avoid!     Tucked away in the galactic nucleus is a supermassive black hole about four million times more massive than our Sun.    While not visible optically owing to the intervening gas and dust clouds within the 26,000 light years of space separating it from us, the central black hole is detectable as a powerful radio source dubbed Sagittarius A*.       Astronomers have determined its diameter to  be about equal to 60 million kilometers.   Scientists were able to estimate its mass by measuring the velocities of stars within its vicinity.  As we learned yesterday in our dark matter discussion, stellar velocities are determined by the mass of the material surrounding the stars.      While life tends to be placid out here on our spur, the region surrounding this black hole is a sight of furious activity.     It is likely every elliptical and spiral galaxy contains such a supermassive black hole at their cores.       We would be well advised to avoid them, as well.

Major Spiral Arms:

The Milky Way Galaxy is a barred spiral, meaning that its center consists of a bar from which protrudes a series of curling spiral arms.    The four major arms are:

• Norma
• Sagittarius
• Scutum-Centaurus
• Perseus

Yes, that is correct.  Each arm is named for a different constellation that defines the region these arms seem to occupy.         Our Sun is actually located on a minor arm, the Orion-Cygnus.  More specifically, the Sun is located within the Orion spur, an offshoot arm of stars approximately 2/3 out from the nucleus.  Spiral arms contain the densest collections of stars, gas and dust.     Here one will encounter most of the star formation as gaseous nebulae condense to create new stars from the galactic material.       Astronomers estimate that  5 - 8 stars are formed each year within the Milky Way.   

Galactic Halo:

We recognize three main parts of the galactic halo, the spherical region surrounding the nucleus:

• Stellar Halo    the halo containing the "metal poor" stars within the globular clusters.   These globe-shaped clusters -hence the name- contain some of the oldest stars that are deficient in heavier elements as they formed from material that wasn't as chemically enriched as the matter from which the Sun was made.    Up to one percent of the galaxy's mass is contained within his halo, which extends up to 100 kiloparsecs (100,000 parsecs) from the nucleus.   According to the most recent census, the halo contains about 150 - 200 globular clusters
  
• Galactic corona    a region of hot ionized gases within the galactic halo.       This rarefied region is  likely replenished by cataclysmic events such as supernovae in and around the galactic nucleus.    These "bubbles" travel up as though a chimney in this corona before being disseminated through the disc region.
• Dark matter halo    a vast region that might encompass the entire galaxy.   This DMH is hypothetical as dark matter has not yet been observed.     However, as we discussed yesterday, the surprisingly rapid stellar motions within the galaxy's outer regions might be explicable by the presence of such a halo.    

We conclude our survey tomorrow...

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