ASTRO:
     type=globular star cluster
     mag=5.8
     const=Hercules
     dist=22.2 k ly; size=168 ly
     other=estimated to contain >300 k stars
IMAGE:
     location=EB Driveway
     BrtlCls=5
     exposure=DSLR OSC; 20x180s (1.0h), ISO1600
EQUIPMENT:
​     optics=ES102CF APO / Stellarvue 1.0x fld-fltnr, FL=714mm, f/7.0
     camera=Nikon D90 (mod)
     filter=Optolog L-Pro LPS
     mount=Celestron AVX
     guiding=Orion 60x240mm, ZWO ASi224MC
SOFTWARE:
     acquisition=APT, PHD2, Stellarium
     processing=DSS, Photoshop, LrC

Messier 13 or M13, also designated NGC 6205, and sometimes called the Great Globular Cluster of Hercules, is a globular cluster with apparent location in the constellation of Hercules.  It was discovered by Edmond Halley in 1714, and cataloged by Charles Messier in 1764.  He included it into his list of objects NOT to be mistaken for comets.  Messier 13 is often described by astronomers as the most magnificent globular cluster visible to observers in the northern hemisphere.

A globular cluster is typically a large symmetrical and spheroidal formation of stars that are bound together by gravity, with a higher concentration of stars toward its center.  Globulars, as they are often called, can contain anywhere from tens of thousands to as many as millions of member stars, all orbiting their central region in a stable, compact formation.  Their name is derived from the Latin word, "globulus" meaning small sphere.  This is as opposed to the other type of star cluster, an Open Cluster which tends to have lower concentrations of stars, are NOT gravitationally bound nor spaced apart as symmetrically, and with their grouping commonly associated with a nebula cloud where active star formation is taking place.  The stars in open clusters are typically much younger, and generally not nearly as 'organized' as their globular cousins. 

No known globular clusters display active star formation.  They are thought to be some of the oldest objects in the Milky Way galaxy, and among the first collections of stars to have formed in the universe.  Globulars have also been found inside the neighboring galaxies of Andromeda and Triangulum.  Consistent with this theory, and typical of most CG's in the MW,  M13 is extremely old with an estimated age of 11.7 B years, making it almost as old (at 14.5 B yrs) as the universe itself.  

Many of the Milky Way's globular clusters have retrograde orbits (meaning that they revolve around the galaxy center in the reverse direction from that of the rest of the galaxy), including the most massive GC currently known, Caldwell 80, or C80, and commonly called Omega Centauri; whose retrograde orbit suggests it may be a remnant of a dwarf galaxy captured by the Milky Way.  Omega Centauri is only visible from latitudes in the southern hemisphere.

About one third of the way from Vega to Arcturus, four bright stars in the constellation Hercules form a keystone-shaped asterism, making-up the broad torso of the hero.  M13 can be seen in this asterism appx 2⁄3 of the way north from stars Zeta to Eta Herculis.  Although only telescopes with great light-gathering capability can fully resolve the stars in the cluster, M13 may be visible to the naked eye as a "fuzzy patch" depending on sky clarity and light pollution levels.  With a low-power telescope, M13 looks like a fuzzy snowball. The cluster is circumpolar, and therefore visible throughout the year from latitudes greater than 36 degrees north, with the 'best' visibility being during northern hemisphere spring and summer.

The Hercules Globular Cluster contains the estimated mass of about 1/2 million solar masses.  It is located appx 22,200 light-years away from Earth, and is one of 147 known GC's that orbit the center of the Milky Way.  (Sometimes referred to as, "Sentinels of the Galaxy" by the scientific community, there may be more that have not yet been discovered.) 

Messier 13 is a class 5 globular cluster, which means its central concentration of stars is higher than some other GC's, but not as high as others.  ...It has an "intermediate" concentration.   But still, its central region is densely packed, with up to a hundred stars populating a cube only 3 light years on a side.  To illustrate, our closest neighbor, Alpha Centauri, the nearest star system to Earth, is just over 4 light years away.  So stars in M13’s core region are about 500 times more concentrated than those in our immediate interstellar neighborhood.  They are so close together that they sometimes collide and break apart to form new stars.  The resulting newly formed, young stars, so-called "blue stragglers", are particularly interesting to astro-scientists.

The Hercules Globular Cluster was the target of a 1974 effort to send an extra-terrestrial radio message, beamed from the Arecibo Observatory in Puerto Rico, which contained information about the human race, Earth’s location and other data. The message was sent from the Arecibo extra large radio telescope array in the direction of M13 as a way of potentially contacting extraterrestrial civilizations.  However, due to astrophysical characteristics of M13 that are better understood today, the message will never reach its intended target.  It will arrive at the past position of M13 in about 25,000 years, but the cluster will no longer be there at that point in time; it will have already "moved-on" in its orbital path around the center of the Milky Way.

210318 - M13 "Great" globular cluster of Hercules