THE USM 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:  2458659.5
                  "The pessimist sees difficulty in every opportunity. The
optimist sees opportunity in every difficulty."
                             -Winston Churchill

THE DAILY ASTRONOMER
Tuesday, June 25, 2019
 The Keystone

There is never an inconvenient time to explain the difference between
"constellation" and "asterism," because the latter is a part of the former
even though the terms are often used synonymously.     A constellation is
one of the 88 star patterns desginated by the International Astronomical
Union (IAU).   These constellations include Sagittarius the Archer; Ursa
Major the Great Bear and Hercules, also called the "kneeling one."

Within each of the three aforementioned constellations one would find
"asterisms," well known patterns within these constellations.
Sagittarius' asterism is the "teapot," Ursa Major's is "the Big Dipper,"
and Hercules' is the "Keystone."

[image: herculescentered-5b2b08c304d1cf00362a834b.jpg]
*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
constellations's other features.    Often, observers are only able to find
the asterism, as the legs and arms are lost amongst the surrounding stars.

[image: 34672316993_727379e78f_b.jpg]
*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, a
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.


The Hercules Keystone is a splendid sight to begin our annual summer star
tour now that we're about to start astronomical summer.




*In some versions, Hercules merely eluded the dragon instead of killing
it.  In another version, Hercules didn't know the Dragon's location and in
desperation consulted Atlas, the Titan who had been condemned to hold up
the world following the Titan's defeat in the Titanomachy (war against the
gods.)  Atlas offered to fetch the apples if Hercules agreed to take his
place under the world.  Hercules readily consented.     Soon thereafter,
Atlas returned with the apples, as promised.  However, having been relived
of the burden, Atlas told Hercules that he would take the apples, himself,
and leave Hercules with the world on his shoulder.   Apart from being
angry, the shrewd Hercules expressed relief.    "I am almost at the end of
my labors," Hercules explained to the bemused Atlas. "Each labor has proven
more difficult that the one before it and I knew that the final labor would
be the hardest of them all and would likely have killed me.  If I am
condemned to hold the world now during my penultimate labor, I will have to
forego the last one and can thereby live forever without ever having to
face whatever peril awaits me.    My friend, you may have the apples and
with it you have my eternal gratitude.     I was promised immortality if I
completed the labors, but now I have it without having to finish them.
One thing, though....my shoulder is a bit sore due to this burden.  May I
ask you to take the world for a moment so I can fasten a cushion to my
shoulder?"    Atlas promptly took the world back and watched helplessly as
Hercules picked up the apples and fled.

**The M indicates "Messier Catalog."  Charles Messier (1730-1817) compiled
a catalog of celestial objects that resembled comets.  His aim was to offer
a list of nebulae and other structures that other astronomers might
mistakenly classify as comets.   Comet hunting was a real king's sport in
the 18th and 19th century.     Each object Messier included in his
compilation is assigned an "M" designation.