June - The Hercules Globular Cluster

The Hercules Globular Cluster in the Sky

Imagine you are outdoors in the northern hemisphere on a clear June night. If you look almost directly overhead around 10 PM, you can pick out the constellations of Cygnus and Lyra. Not far from them is a large trapezoid pattern of not-very-bright stars called the "Keystone" - part of the constellation Hercules, named after the powerful, belabored hero in Greek mythology.

If, as Edmund Halley put it, "the Sky is serene and the Moon absent," you may spot a barely-visible blob along the western side of the Keystone. This is the Hercules Globular Cluster - one of the brightest globular star clusters in our own Galaxy that can be observed from north of the equator. With a 4- to 6-inch telescope, this dim, hazy object can be resolved into hundreds of distinct starry dots. Follow these easy observing tips from the June IYA Discovery Guide to find the Hercules Globular Cluster.


Globular Clusters: Ancient, Starry Globes

A globular cluster looks like a celestial "snow globe" where hundreds of thousands of shimmering stars are jammed into a ball-like region about 100 light years across and held together for billions of years by gravity. Our Milky Way galaxy plays host to about 200 globular clusters. Imagine that you could somehow transport yourself well out of the Galaxy. You would then notice the globular clusters forming a spherical halo around the center of the Galaxy rather than congregating along the disk. Imagine further that you could somehow watch this panorama for several hundred million years. You could track the globular clusters whirling around the center of our Galaxy in non-circular, randomly oriented paths, every once in a while plunging with abandon through the Galaxy’s disk. In fact, the origin and lives of the globular clusters surrounding us are still a topic of great interest to astronomers. Were some clusters originally part of one of the Milky Way’s satellite galaxies? Are others being torn apart by the tidal forces of the galaxy? As astronomers gather more observations, both our knowledge of our galaxy as well as its mysteries grow deeper.

A Stellar Metropolis

Discovered by Halley in 1714, the Hercules Globular Cluster (also known as Messier 13 or M13) is at a distance of about 25,000 light years from us. Being so bright and relatively close, M13 was one of the first globular clusters to be discovered. It remains a pivotal target of observation for astronomers seeking to understand the intricacies of the lives of stars, and indeed, the origin of globular clusters themselves.

Home to several hundred thousand stars, M13 spans about 150 light years and tips the scales at a trillion trillion trillion kilograms or about 600,000 times the mass of our Sun. M13, like other globular clusters, contains little gas and dust. At the center of this colossal globular cluster, the stars are packed more than a hundred times as tightly as in the neighborhood of our Solar System. Even so, M13 is not so dense as some other globular clusters such as Messier 15 and 47 Tucanae.

Imagine for a moment that you lived on a planet around one of the cluster’s central stars. If you stretched out your arm with your palm against the sky, you would see a pretty bright star at the tip of every finger. Basking in an everlasting twilight, you would never experience the serenity of truly dark skies and the joy of sighting “deep sky” objects, such as the fuzzy globular cluster siblings of M13, with the naked eye.

However, the stars of M13 are unlikely to host planets and life. Like in other globular clusters of the Milky Way Galaxy, the stars in M13 are among the oldest in the universe, clocking in at twelve billion years old, give or take a billion years. These ancient stars formed from pristine gas that was “uncontaminated” by heavy elements, such as carbon, oxygen or silicon, into it. Due to this deficiency of heavy elements in the stars, planets like the Earth could not form around them.

What’s more, just like in a crowded metropolis where you time and again bump into other people, globular cluster stars interact sporadically with their close-in stellar neighbors. The gravitational fracas in such a rowdy neighborhood may not be suitable for the formation and evolution of stable planetary systems. Scientists are still grappling with the question of what kind of planets may be found around stars crowded together in space.

Such gravitational tug-of-war has other interesting outcomes. Within the ancient stellar population in M13 are many red giant stars whose cool atmospheres balloon out tens to hundreds of times the size of our Sun. On rare occasions, when two such stars merge with each other, they can create a strange new star called a “blue straggler.” Scientists are still figuring out why the number of blue straggler stars in M13 is low compared to within its sibling globular clusters.

M13 Globular Cluster

Image using a 20-inch telescope and a CCD camera, as part of the Advanced Observer Program at the Kitt Peak National Observatory’s Visitor Center. Credit: Tom Bash and John Fox/Adam Block/NOAO/AURA/NSF

M13 Globular Cluster

The view from space - Hubble Space Telescope multicolor image of the inner 36 light years of M13. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Artist’s conception

Artist’s conception of the sky from a hypothetical planet near a globular cluster. Credit: David A. Aguilar (Harvard-Smithsonian Center for Astrophysics)

Venus transit

Hercules Constellation

Right Ascension: 17h
Declination: +30ş