July Feature – The Hercules Cluster (Messier 13)

“My god, it’s full of stars.”

– 2001:  A Space Odyssey (novel), by Arthur C. Clarke

In the beginning, some 13.87 billion years ago, there were no stars in our universe. Even atoms did not exist in the beginning. The infant universe started out as a hot, dense point of matter, energy, and fundamental cosmic forces, all compressed into a single point known as a “singularity”.

Then, for reasons we do not yet understand, this singularity suddenly began to inflate, initiating what we now refer to as “the Big Bang”.

Within a fraction of a second of this sudden expansion, the four fundamental forces that shape and govern our universe (gravity, electromagnetism, and the strong and weak nuclear forces) became their own separate entities.   Within an hour of this expansion the infant universe had cooled to a point where subatomic particles could form.

After 380,000 years of more expansion and cooling, the universe’s temperature dipped below 3,000 degrees Kelvin (4,490 degrees Fahrenheit) and the first, simplest atoms of hydrogen and helium formed. By around 300 to 500 million years after the Big Bang, conditions were right for the formation of the very first stars and galaxies out of vast, cold clouds of molecular hydrogen.


At around 13.21 billion years ago there was a certain giant cloud of hydrogen gas collapsing under the force of gravity to form multiple clusters of stars. As this material collapsed, the vast clouds of cold gas began to spin and flatten into a disc, a disc that would, over time, become our Milky Way Galaxy.

But as the disc began to take shape, there were about 150 or so giant, spherical masses of stars that got left behind in the process. These dense, globular concentrations of stars, all held together by their mutual gravity, contained thousands to millions of individual stars and they took up a spherical distribution around the galaxy’s bright, central hub.

Today, after billions of years of both cosmic and biological evolution, there exists within a quiet corner of the galaxy, carbon based, bipedal lifeforms known as “amateur astronomers” who love to observe these ancient star cities in their telescopes and one of the finest examples is dubbed by them as “Messier 13” or, “The Hercules Cluster”.


Hercules constellation

To find the Hercules Cluster we must first find the constellation of Hercules.

During July, look for the pattern of faint stars representing the famous strongman of mythology almost directly overhead by around 10 or 11 P.M. Use a star map or app to help you locate him.

Another way to find Hercules is to use the handle of the Big Dipper and then “arc to Arcturus”.

After locating the bright star Arcturus in Bootes, the Herdsman, look for the slightly less bright star Vega within the constellation of Lyra high in the eastern sky. Hercules lies in between these two bright stars (hint: he’s closer to Vega than he is to Arcturus).


Hercules and M13The most prominent stars in Hercules form a lopsided square known as “the keystone”, after the stone architectural feature located at the top of an arch which is narrow at one end and broad at the other.

If you draw an imaginary line from Vega back to Arcturus then you will find the keystone about a third of the way along its length. Now, look on the Arcturus side of the keystone. Two stars denote the corners of the keystone here, Eta Herculis and Zeta Herculis (again, a star map or app will be your best friend).

The Hercules Cluster is located about one third of the way in between Eta and Zeta.

M13 is one of three globular clusters that can be seen by the naked eye but it requires very dark skies and good eyesight as the cluster has an apparent magnitude of 5.8, putting it right at the threshold of naked eye visibility.

A pair of 10x or 15x binoculars will show the Hercules Cluster as a fuzzy cottonball of light under dark sky conditions. Small telescopes with an aperture of 4 inches or more and good optics will begin to show individual stars at the outer edges of the cluster. Mid to large aperture telescopes (8 inches or more) will begin to resolve individual stars closer towards the center and a complex spider-like structure within the cluster. The core is so densely packed with stars that you will never be able to resolve them all the way to the center.


During the 18th century many famous comet hunters of the day took note of this “nebulous patch” upon the sky, including Sir Edmond Halley in 1714 and Charles Messier in 1764. Messier made it number 13 in his famous catalog of “not a comet” objects in the night sky.

But the telescope technology of their day only permitted them to see a fuzzy “nebula” without any associated stars.   It wasn’t until 1789 that British astronomer and composer, Sir William Herschel had a telescope capable of showing that M13 was a spherical mass of stars. In fact, it was Herschel who coined the term “globular cluster” in his “Catalogue of a Second Thousand New Nebulae and Clusters of Stars”.


In 1920, American astronomer Harlow Shapley, discovered that globular clusters were distributed in a vast, spherical halo around the center of our galaxy and that the halo extends both above and below the galactic nucleus. By mapping out the distribution of globular clusters such as M13, Shapley calculated the dimensions of our galaxy (although he was off by a factor of 2) and realized that our Sun was not placed at the center of the then known universe.

Located 25,890 light years away, the Hercules Cluster spans some 160 light years across and is jam packed with an estimated 300,000 individual stars. I say, “jam packed” but keep in mind that most stars in clusters like these are at least a tenth of a light year apart. While that is a sizeable distance to specks of animate stardust like you and me, it is still quite close in astronomical terms.



This high stellar density might prompt one to ask, “what would the sky look like from a planet located within a globular cluster?” No doubt, the sky from such a vantage point would be filled with hundreds of thousands of bright stars. Here on Earth, you would be lucky to see just two or three thousand individual stars on a dark, moonless night. So, a sky filled with hundreds of thousands of bright stars means that you would probably never get to experience a truly dark sky. The prolific science fiction author (and all around polymath) Isaac Asimov once played around with the idea of such a situation and the result was his classic short story (later expanded into a novel) “Nightfall”.

But are there planets inside these spherical masses of stars? To the best of my knowledge there has only been one exoplanet detected so far inside a globular cluster (Messier 4, in the constellation of Scorpius) and it orbits around a pulsar (a fast spinning, very dense, very compact core of a dead giant star).   That planet is not likely to support life.

One of the common features of globular clusters is that they are very low in metals. Now, keep in mind that “metals” to an astronomer is any element heavier than hydrogen and helium. This low metallicity might be favorable for the formation of rocky planets but probably reduces the chance of life evolving on a planet inside a globular cluster.

Arecibo_Observatory_Aerial_ViewStill, this did not deter astronomers Carl Sagan and Frank Drake from beaming a message to the Hercules Cluster using the Arecibo Radio Telescope in Puerto Rico back in 1974.

Given that it will take over 25,000 years for the signal to get there and another 25,000 years for a return signal (if any E.T.’s are there to receive and send it), the conversation is going to be awkward to say the least.

The low metallicity of globular clusters also tells us that these stars formed out of nebulae that were never seeded with heavier elements formed within the cores of older stars that would have spewed their chemically rich innards back out into space upon their deaths in supernova explosions. A vital clue that tells us that globular clusters like M13 are very, very old.


So, how do we know the ages of stars? Well, astronomers cannot tell the age of any individual star, but we can get a good estimate on the age of star clusters. It’s safe to assume that most of the stars within a cluster were all born at roughly the same time.

The basic physics of how stars convert the hydrogen in their cores into helium, how they generate their energy, and how long it takes them to do all of this is straightforward and well understood. Low mass stars go through their fuel supply slowly and live longer lives, high mass stars go through their fuel supply quickly and live comparatively shorter lives.

In a star cluster, there are going to be stars with different masses and different stages of their evolution, consequently we can construct a plot of the stars making up the population of a cluster and we can see how many of the high mass stars are coming to the ends of their lives as main sequence stars (stars that are fusing hydrogen within their cores) and are now entering the red giant phase of their lives (stars that have expended the supply of hydrogen in their cores).

There is about a 10% to 20% amount of uncertainty with this method so we can’t get absolute ages with any certainty but our best guestimates puts us within a ballpark range of 12 billion to 15 billion years of age for globular clusters such as M13. That’s about as old as the universe itself. So, when you observe M13, you are looking at the oldest thing you are likely to ever see.

Sky Inside A Cluster

Oddly, astronomers have found blue, youthful stars near the centers of many globular clusters. What we think is happening here is that some stars are so close together in a cluster that they have either merged with one another or that one star is leaching material off its neighbor (kind of like a stellar vampire). Either way, this stolen material makes the star appear more youthful when compared to all those red, geriatric stars we see making up most of the cluster.

Should you find yourself bitten by the globular bug and want to see more then summer is a great time to observe them as our view of the Milky Way now is towards the inner regions of our galaxy. Just to the northwest of M13 is M92, another fine globular in Hercules while southerly views will provide you with over a dozen more globs in and around Sagittarius. Get outside this summer in gaze in awe and wonder at these spherical masses of stars from the dawn of the universe itself.

Posted in: News, Night Sky

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