Have A Cosmic Year With Hercules’ Great Cluster

Hercules the Hero is a “fist” to the right of Vega and near the top of the sky around 10-10:30 p.m. in mid-July. M13 is a spectacular star cluster in the figure’s torso also known as the Keystone. Maps created with Stellarium. At right is the mythological figure of Hercules. Credit: Urania’s Mirror

When you look up near the top of the sky around 10:30 p.m. in mid-July you can’t miss the brilliant white star Vega. It sparkles just to the left of due south. Shoot a line from Vega a little more than one outstretched fist to the right and you’ll arrive at … nothing!

OK, I’m exaggerating, but at first glance, it looks pretty empty up there. A closer look will show a pattern of faint stars that make up the large, sprawling constellation Hercules the Hero. Known in ancient Greek myth as Heracles, this heroic figure is perched upside-down near the zenith at nightfall in July.

At the center of Hercules is something you can focus on – a group of four stars in the shape of a trapezoid called the Keystone. Along its right side is one of the finest star clusters in the summer sky.

Some 300,000 stars reside in the starry metropolis M13. They’re gravitationally bound to each other as the cluster orbits the core of the Milky Way galaxy. Credit: N.A. Sharp, REU program, NOAO, AURA, NSF

The cluster’s name, the Great Globular Cluster in Hercules, sounds practically generic, but it’s snappier than M13, the number 18th century astronomer Charles Messier gave to it in his catalog of fuzzy objects that resembled comets.

From rural skies, M13 is a little fuzzy patch visible to the naked eye if you know just where to look.

Binoculars bring out its two-part structure – a brighter core surrounded by a fainter halo. In telescopes as small as 4 inches you begin to see what all the excitement’s about as the outer haze begins to resolve into a starry sprinkles. Bring an 8-inch or larger scope to the task and M13 scintillates with hundreds (thousands?) of stars. Let’s just say too many to count.

A close up view showing just the central part of Hercules called the Keystone. M13 is about 1/3 the way between the naked eye stars Eta and Zeta.

Hercules’ Great Globular was discovered by Edmund Halley of Halley’s comet fame in 1714 and belongs to a class of dense, spherical-shaped assemblies of stars called globular (GLOB-you-lur) star clusters.

I always tell people to buy a telescope – or at least find one to look through – if only to see the planet Saturn, the single most amazing sight in the universe. Globulars come in a close second. The throng of stars that meets the eye through 8-inch and larger instruments is nothing short of stunning.

The inner core of the Great Hercules Globular resolved into colorful stars by the Hubble Space Telescope. The clusters is 145 light years across and 25,000 light years from Earth. Click for a high-res version. Credit: NASA/ESA

Hercules is also home to a very special spot called the solar apex, shown in the first map above. This is the direction in which the sun is traveling as it orbits the nucleus of the Milky Way galaxy. As seen from the galaxy’s north pole, the sun, with planetary bling in tow, circles around the galaxy in a clockwise direction at 137 miles per second. Sounds fast, doesn’t it? Not fast enough. Our galaxy’s so big it still takes the sun 225 million years to go around once. Astronomers call one full cycle a cosmic year. Life on Earth began only 15 years ago – cosmic years that is.

A model of the Milky Way galaxy showing its bar-like center and spiral arms. The sun orbits the galactic center in 225 million years or one cosmic year.  We currently reside in the Orion-Cygnus arm. Credit: NASA/JPL-Caltech/R. Hurt. // Wikipedia

Other stars and gas clouds in the sun’s vicinity travel at nearly the same speed, moving very little with respect to one another. It’s not unlike a multi-lane road where everyone is traveling at 35 miles an hour in the same direction. Very frustrating if you’re trying to pass.

Aside from this general streaming motion around the galaxy, the sun has a slight additional motion of 10 miles per second in the direction of the constellation Hercules southwest of Vega. When you go out the next clear night and gaze almost straight up at nightfall, picture yourself on a very long journey with our hero Hercules.

8 Responses

  1. Mike


    Congrats on DNT front page, NBC Nightly News and Newseum.

    All that aside, it is an awesome picture, well done!


  2. the younger Mrs. America

    You are amazing, such great info, sorry I missed the most likely only aurora show this summer the last few days.
    I see you were even mentioned in the Pioneer Press!
    from the 45th parallel…………..

    1. astrobob

      Thanks very much! A sweet poem, too. Seven sisters is a great many, indeed enough to form your own star cluster.

  3. the younger Mrs. America

    I entered a poetry contest, here is my inspiration: I do have seven sisters!
    Pleides and me

    The seven sisters are near each other
    Year after year in any weather
    That group of stars in the late night sky
    Give me hope as time goes by

    As they are above, they are as below
    Seven sisters of mine are on the go
    Moving across this world we’re in
    Different paths, through thick and thin

    We stick together like the Pleides
    My seven sisters and also me.

  4. Jed

    The Hercules cluster is a great sight indeed! I viewed it for the first time just a month or so ago under dark skies in the desert. It took over an hour to locate (still not good at spotting the constellations in a starry sky) but that made finding it even more exciting. I’m curious to see if I can find it/see it at home near the city.

    1. astrobob

      Hi Jed,
      One of best sights in the sky and worth the time to find for sure. I visit it a couple dozen times each spring/summer. Thanks for sharing your observing experience.

  5. brandon anthony

    hi it been a long time im sorry about my crazy meltdown before im so sorry i appreicate what your doing keep up what your doing

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