Deep-Sky Objects: The Great Hercules Cluster (M13)

Messier 13
Image Credit: Roth Ritter

Also known as the Great Globular Cluster in Hercules, M13 is without doubt the finest globular cluster visible from the northern hemisphere. All told, It contains around 300,000 stars, and spans a distance of 20 arc minutes, which at its distance from Earth, translates into a diameter of 145 light years. This cluster was discovered by the British astronomer Edmond Halley in 1714, and listed in the Messier catalogue in June of the year 1764.

Quick Facts

• Constellation: Hercules
• Coordinates: RA 16h 41m 41.24s |Dec. +36° 27′ 35.5″
• Distance: 22,200 light years
• Object Type: Type-V globular star cluster
• Mass: 600,000 sol
• Radius: 84 light years
• Apparent Magnitude: +5.8
• Age: 11.65 billion years
• Other Designations: NGC 6205

 

Physical Properties

Core Density

The most striking aspect of M13 is the density of its central region, which is fully 500 times more compact than on the fringes of the cluster. In fact, the stars in the core region are so densely packed that about 100 stars will fit into a cube only 3 light years wide on each side, and to put this into perspective, the closest star system to the Sun, Alpha Centauri, is more than 4 light years away, with no other stars found between it and the trinary star system.

Star Types

The brightest star in M13 is a red giant called V11, a known variable with an apparent magnitude of +11.95. However, it also contains the star Barnard 29, a B2-type star that did not originate within M13, but is thought by investigators to have been “sucked up” by the globular cluster during one of its orbits around the Milky Way.

Also of interest are the 15 blue stragglers, and a further 10 or so possible blue stragglers, which are stars that appear to be younger and hotter than their surrounding stars. In the case of M13, the stragglers in the blue horizontal branch of the cluster appear to be “centrally depleted” compared to other types of stars, while as a group, the blue stragglers appear to be more concentrated towards the core of the cluster in comparison to other types of stars in the core, and especially relative to old red giant stars.

While this may seem strange, it appears to be a normal function of the evolutionary process at work in globular clusters. Investigator Frank Grundah et al. have shown that M13 seems to consist of two distinctly different stellar populations on the horizontal branch, and that the level of deep mixing that occurs between the precursor red giant stars in a cluster may very well account for the distribution of the blue-straggler stars in the cluster. This process is perhaps best explained by the astronomers Yoon and Lee, who in their 2002 published paper named “An aligned stream of low-metalicity clusters in the halo of the Milky Way”, describe the process thus:

“One of the long-standing problems in modern astronomy is the curious division of [stars in] Galactic globular clusters, the “Oosterhoff dichotomy,” according to the properties of their RR Lyrae stars. Here, we find that most of the lowest metallicity clusters, which are essential to an understanding of this phenomenon, display a planar alignment in the outer halo. This alignment, combined with evidence from kinematics and stellar population, indicates a captured origin from a satellite galaxy. We show that, together with the horizontal-branch evolutionary effect, the factor producing the dichotomy could be a small time gap between the cluster-formation epochs in the Milky Way and the satellite. The results oppose the traditional view that the metal-poorest clusters represent the indigenous and oldest population of the Galaxy.”

Age

Even though M13 is one of the most intensely studied globular clusters in astronomy, there is some debate about the age of the stars that comprise the cluster. Estimates vary greatly, but the main problem centers on the issue of establishing the parameters of the conditions that obtained when the first stars in the cluster formed. Investigator R. Glebocki (et al) describes the problem in his 2002 paper “Catalog of Projected Rotational Velocities” thus:

“Much theoretical and observational work about the role that rotation plays in stellar evolution has been done. Angular momentum is one of the fundamental parameters in the process of star formation as well as in early life of a star. A considerable amount of research has been done on the stellar axial rotational velocities. Clusters present unique possibility of determination of age of stars.”

In short, Glebocki says that until the role of the rotation of forming stars is more precisely constrained, the exact age of the stars in M13 cannot be stated with any degree of certainty, which means that at best, the stated age of M13 of 11.65 billion   only an estimate.

Visibility

M13 in Hercules
Image Credit: IAU and Sky & Telescope magazine

The constellation of Hercules is visible from latitudes of +90° to -50°, where it can be seen in the night sky between the bright stars Vega in Lyra to the east and Arcturus in Boötes to the west. From mid northern latitudes in the northern hemisphere, M13 can be seen with binoculars and small telescopes for part of the night all year round. During May, June, and July, however, the constellation is visible all night long, while in August and September, Hercules sets only after midnight. Amongst the many colorful descriptions of this beautiful globular cluster over the years is one by Admiral Smyth (1788-1865), who wrote:

“A large cluster, or rather ball of stars, on the left buttock of Hercules, between Zeta and Eta; the place of which is differentiated from Eta Herculis, from which it lies south, a little westly, and 3 deg 1/2 distant. This superb object blazes up in the centre, and has numerous outliers around its attenuated disc. It was accidentally hit upon by Halley, who says, “This is but a little patch, but it shows itself to the naked eye, when the sky is serene, and the moon absent.”

History

Having been discovered only fairly recently, M13 does not have any real cultural significance. However, in 1974 it became the target of a message sent from a radio telescope at the Arecibo Observatory in Puerto Rico that described human DNA, atomic numbers, Earth’s position, and other miscellaneous information about our civilization and planet. This message was beamed toward the core of M13 on the assumption that the high stellar density of the core might correspond to a high density of planets that might harbour intelligent life. While it has been stated that M13 will have moved out of the beam’s path by the time the message arrives, the fact is that the cluster will have shifted by only 24 light years, which means that the message will still arrive at a point close to the centre of the cluster.

M13 has also been mentioned fairly often in science-fiction stories, most notably by the late Isaac Asimov (1920-92), who used it as the setting for his SF novella “Sucker Bait”. In the Dan Simmons novel “Hyperion Cantos”, M13 is the location where a copy of Earth was recreated after the original Earth was destroyed, while in the 1968 Dr. Who story, “The Wheel in Space”, the Cybermen concoct a plot to engineer a star in M13 to self-destruct in a supernova explosion. Author Kurt Vonnegut also mentions M13 in his novel, “The Sirens of Titan”, thus:

“Every passing hour brings the Solar System forty-three thousand miles closer to Globular Cluster M13 in Hercules – and still there are some misfits who insist that there is no such thing as progress.”

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