Zero-in on Messier 13, the great Hercules globular – Astronomy Now

Messier 13 is the finest globular cluster in the northern hemisphere. Image: Adam Block.

Messier 13 (NGC 6205) in Hercules is another of those truly celebrated deep sky objects, along with the Orion Nebula, the Pleiades and the Andromeda Galaxy. Its iconic status is reinforced by its alternate and long-standing nameplate, the Great Cluster in Hercules, often changed these days to the Great Globular in Hercules. It is widely regarded as the most beautiful globular cluster in the northern sky and is one of the go-to objects in late spring to early summer skies.

Messier 13 lies to the west of Hercules’ prominent asterism, Keystone. A graphic by Greg Smye-Rumsby.

Ancient Star Balls

Globular clusters are ancient objects, the vast majority of which were born between 10 and 13 billion years ago, when the universe was still in its infancy. They are spectacular spheres of hundreds of thousands of old Population II stars (advanced, low-mass main sequence stars) and red giants crammed into regions typically only 50 to 200 light-years across. Over 150 are known in our galaxy.

Messier 13 is one of the largest known globular clusters, approximately 145 light-years across. Its 500-million-year journey around the galactic center is highly elliptical; it is currently about 23,000 light-years from the Sun, not much further than its closest possible distance to the galactic center of 16,000 light-years, but it may be as much as 80,000 light-years from the Sun. Astronomers believe M13 contains about 300,000 stars, but many more may be gravitationally bound to it. The core of Messier 13 must be a spectacular location for any planet orbiting a star there. The density of the stars is said to be about 100 times greater than in the vicinity of our sun.

A sketch of Messier 13 observed and sketched with a Parks Astrolight EQ6 (150mm f/6 Newton). Sketch: Eric Graff.

Find M13 and watch it

Messier 13 is easy to find and observe. Shining bright at +5.7 magnitude, it is difficult to see with the naked eye under optimal conditions, but is easily located through binoculars as a nebulous speck. Look out for the famous Hercules Keystone asterism, a distinctive quadrilateral of 3rd and 4th magnitude stars, including magnitude +2.8 zeta (ζ), marking the southwest corner. Then moving clockwise, epsilon (ε, +3.5) is in the southeast, pi (π, +3.9) in the northeast, and finally eta (η, +3.5) marks the northwest corner. Messier 13 lies about a third of the way along a line drawn between Eta and Zeta.

Hercules and M13 reached the South Meridian around midnight BST in early June, but both can be seen long before that. A telescope with an aperture of only 80mm (~three inches) and a power of about 20x can represent M13 as a speckled sphere with a condensed core. On a good night, try turning up the power and you might see a hint of dissolved stars at the cluster’s periphery. Messier 13 is a moderately condensed globular (class V in the Shapley-Sawyer classification system of apparent stellar density in globular stars, ranging from I, a very high concentration, to XII, very loose with no central concentration).

You will never forget your first sight of Messier 13 through a medium aperture telescope. A 150-200mm telescope operating at high magnification will resolve the cluster’s individual stars, revealing the splendor of masses of twinkling suns filling the field of view and visible down into M13’s core.

This image, taken by the Hubble Space Telescope’s Advanced Camera for Surveys, shows the core of the large globular cluster Messier 13. Image: ESA/Hubble and NASA. Zero-in on Messier 13, the great Hercules globular – Astronomy Now

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