THE SOUTHWORTH PLANETARIUM 207-780-4249   www.usm.maine.edu/planet
70 Falmouth Street   Portland, Maine 04103 43.6667° N                   70.2667° W  Altitude:  10 feet below sea level Founded January 1970 Julian Date:  2459362.18 

The Hercules' Keystone:  a four star quadrilateral marking the Hercules' mid-section.   The "Keystone" is an asterism, or a star pattern within a larger constellation.   One can locate the Keystone by drawing an imaginary line connecting Deneb and Vega in the Summer Triangle. Extend the line slightly westward and it will intersect the Keystone.  

Today's focus is the "Keystone," a quadrilateral centered on Hercules.    The image seen above  shows Hercules "upside down," as he is portrayed as kneeling on Draco the Dragon's head.     Draco is often identified mythologically as the dragon who guarded the golden apples within the Garden of the Hesperides.     The eleventh of Hercules' twelve labours required him to steal these apples, a task rendered all the easier once he slew the dragon.


Though it consists of no bright stars, this keystone is somewhat easy to find provided one first locates the Summer Triangle.    Vega, the brightest Summer Triangle star, is just east of the Keystone.    Extend a western line connecting Deneb, the Summer Triangle's eastern star, and Vega.   This line slices neatly through the Hercules asterism.      The Keystone stands out because,though not brilliant, is still brighter than the constellation's other features.    Often, observers are only able to find the asterism, as the legs and arms are lost amongst the surrounding stars.

M13: The Hercules Star Cluster    A globular star cluster "located" in the Hercules Keystone.   Approximately 25,000 light years distant, this cluster contains more than 100,000 stars within a "globe-shaped" arrangement.      Image:  Sara Wager

Apart from being Hercules' sole prominent section, the Keystone also marks the location of M13,** a globular star cluster.   The Hercules Cluster is a gorgeous globular cluster consisting of more than 100,000 stars located in the galactic halo.     So named due to their resemblance to globes,  globular clusters are significantly older and larger than the open (galactic) clusters that swarm about the galaxy's spiral arms.       Of course, we can easily see a couple such open clusters (Praesepe and the Pleiades), while the much more distant Hercules cluster is only visible through a telescope.

TUESDAY, JUNE 8:  CIRCUMPOLAR GALLERY

Some have described the night sky as a comforting constant in an ever changing world.  In some respects the night sky seems immutable.  Throughout the year, the same constellations appear each season.  For instance, Orion the Hunter will be due south in the early evening in February as the Summer Triangle looms high overhead in the early evening now.   We even have a retinue of constellations that one can see throughout the year at any time of night.   We refer to this subset of constellations as "circumpolar."   They appear to describe circles around the North Celestial Pole and, at our latitude, never set.  

We recognize six circumpolar constellations:

Camelopardalis the Giraffe

Cassiopeia the Queen

Cepheus the King

Draco the Dragon

Ursa Major the Great Bear

and

Ursa Minor the Little Bear

Three of these patterns - Ursa Major, Ursa Minor and Cassiopeia- are quite easy to observe provided one's night sky is reasonably dark.      

Two of these constellations - Cepheus the King and Draco the Dragon- are not so easy to locate.   Their stars are generally fainter and Draco meanders through the sky, making it difficult for any star gazer to discern the pattern.

One of the circumpolar constellations (Camelopardalis the Giraffe) is almost impossible to observe.  

We'll quickly examine each circumpolar constellation in turn:

CAMELOPARDALIS:  The Giraffe

The circumpolar constellation that hardly anyone has ever seen.     Invented by Petrus Plancius in the early 17th century, this constellation was a "filler star pattern" meant to occupy a patch of dark sky.   Of all the circumpolars, Camelopardalis is the only "modern constellation," meaning that it doesn't date from antiquity.  The constellation name derives from "Camel Leopard" a term once applied to the animal we now call the giraffe.

CASSIOPEIA: the Queen
A queen of ancient Ethiopia, Cassiopeia resembles an elongated "w." This constellation is identified mythologically with Cassiopeia, the mother of Andromeda.    Cassiopeia's boast that her daughter was more beautiful than Poseidon's Nereids incited the god's ire.  Poseidon summoned Cetus the Seamonster from the depths to ravage Cassiopeia's village.  A Delphic oracle instructed Cassiopeia to sacrifice her daughter to Cetus in order to mollify Poseidon.  The miserable queen followed these orders.  Fortunately, however, Perseus the warrior intervened by slaying Cetus before he could devour Andromeda

CEPHEUS: the King

When you refer to Cepheus was Cassiopeia's husband, you've said just about everything about him. His role in the Cassiopeia-Andromeda debacle was one of a quiet spouse who dutifully helped his wife tie Andromeda to the rocks as an offering to Cetus.    Mythologically, he does little else but look on.

DRACO: the  DRAGON

Dragons do almost everything.   One cannot cite just one adventure or one task.  Dragons have guarded sacred apples, reclined on stolen treasure, eaten hapless maidens, inadvertently sent castles into orbit ("Dragons is so stupid!"), what have you.  Of all chimerical creatures, dragons are so common throughout world mythologies as to be almost ubiquitous.    Draco's curvaceous body weaves through the northern sky.  To find it, either look first to Hercules's keystone asterism as Draco's head is just to the north of it.  Or, look between the dippers to trace the serpentine star line representing the monster's body.  

URSA MAJOR: The Great Bear

The "Big Dipper" is only part of this constellation.    The Big Dipper's bowl represents the bear's body and the handle forms the tail.    The rest of the bear consists of stars surrounding the dipper.     

URSA MINOR: The Little Bear

The "Little Dipper" is the main part of this constellation.  Polaris, the north star, is the end-handle star in the Little Dipper.