# Messier 13
**Messier 13** (M13, NGC 6205), also called the Great Globular Cluster in Hercules or the Hercules Cluster, is a gravitationally bound ball of several hundred thousand ancient stars in the constellation Hercules, and one of the brightest and best known globular clusters of the northern celestial hemisphere[1](https://messier.lamost.org/seds/seds.org/messier/en/m/m013.html) • [2](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-13/). It lies about 25,100 light years from Earth[1](https://messier.lamost.org/seds/seds.org/messier/en/m/m013.html), and in 1974 it was chosen as the target of the [Arecibo message](https://www.edgepedia.org/arecibo-message), the first deliberate interstellar radio transmission from Earth[3](https://martzobservatory.org/dso/detail/M13).
| Key fact | Detail |
|---|---|
| Designations | M13, NGC 6205, GCl 45, Hercules Globular Cluster[4](https://simbad.cds.unistra.fr/simbad/sim-basic?Ident=Messier+13) |
| Location | Constellation Hercules; coordinates 16h41m41.4s, +36°27′36″[5](https://britastro.org/section_information_/deep-sky-section-overview/observing-programmes/globular-clusters/the-gc-marathons-part-i-galactic-globular-clusters) |
| Distance and size | About 25,100 light years away; angular diameter 20′, corresponding to about 145 light years[1](https://messier.lamost.org/seds/seds.org/messier/en/m/m013.html) |
| Content | Several hundred thousand stars; core stellar density about 100 times that of the solar neighborhood[2](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-13/) |
| Mass | About \( 6 \times 10^{5} \) solar masses, with a Galactocentric radius of 8.4 kpc[6](https://academic.oup.com/mnras/article/465/3/3515/2417040) |
| Brightness and metallicity | Visual magnitude 5.8[7](https://www.astrovox.gr/applications/core/interface/file/attachment.php?id=42775); metallicity [Fe/H] about −1.53[8](https://www.deepskycorner.ch/catalogs.de.php?cat=MWGC&const=Her) |
| Age | About 12.75 billion years from isochrone fitting (2025 revision)[9](https://arxiv.org/abs/2601.07555) |
## Physical properties
M13 is a classic globular cluster, a spherically symmetric swarm of old stars bound by gravity. At its distance of 25,100 light years, its measured angular diameter of 20 arcminutes corresponds to a linear diameter of about 145 light years, though visually it appears only about 13 arcminutes across[1](https://messier.lamost.org/seds/seds.org/messier/en/m/m013.html). The cluster holds several hundred thousand stars, and near the core the stellar density is about a hundred times greater than in the neighborhood of the Sun[2](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-13/). In such a crowded environment, close stellar encounters occur, and collisions and near-collisions produce [blue stragglers](https://www.edgepedia.org/blue-straggler), stars that appear younger and hotter than the surrounding population[2](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-13/).
The cluster is old and metal-poor. Isochrone fitting of its color-magnitude diagram gives an age of about 12.75 billion years and a distance of 7.39 kpc[9](https://arxiv.org/abs/2601.07555), and its iron abundance measured relative to the Sun, [Fe/H], is about −1.53 in the standard catalog of [Milky Way](https://www.edgepedia.org/milky-way) globular clusters[8](https://www.deepskycorner.ch/catalogs.de.php?cat=MWGC&const=Her). Its total mass is estimated at roughly \( 6 \times 10^{5} \) solar masses[6](https://academic.oup.com/mnras/article/465/3/3515/2417040).
## Multiple stellar populations
Like many globular clusters, M13 is not chemically uniform. Spectroscopy of its red giants shows a population of stars depleted in oxygen, interpreted as material mixed to the surface from CNO-cycled interiors, since the sum of the CNO nuclides is the same in all stars studied[10](https://doi.org/10.1086/115798). The cluster also hosts second-generation asymptotic giant branch stars, the Na-rich, O-poor population characteristic of globular clusters generally[11](https://doi.org/10.1088/2041-8205/815/1/L4).
These subpopulations differ dynamically as well as chemically. Radial-velocity measurements with the WIYN telescope, combined with sodium and oxygen abundances, show that the extremely oxygen-depleted population is more centrally concentrated and rotates faster than the rest of the cluster's stars[6](https://academic.oup.com/mnras/article/465/3/3515/2417040). The populations are not fully mixed, which the study's authors attribute to the cluster's relatively large mass, its Galactocentric radius of 8.4 kpc, and only moderate mass loss from two-body relaxation[6](https://academic.oup.com/mnras/article/465/3/3515/2417040).
## Observation history
M13 has been known as a nebulous object since before systematic cataloging began; a historical account describes it as the sixth and last of the nebulae known in 1716[12](https://www.messier-objects.com/messier-13-hercules-globular-cluster/). [Charles Messier](https://www.edgepedia.org/charles-messier) included it as the 13th entry in his catalog of nebulae and star clusters, compiled in the late 1700s[13](https://science.nasa.gov/wp-content/uploads/2023/07/hubble-litho-globular-m80.pdf), and [William Herschel](https://www.edgepedia.org/william-herschel), who kept a catalog of observations of Messier objects from 1776 to 1813 now held in the Royal Society Archives, re-examined such objects with his own large reflectors[14](https://makingscience.royalsociety.org/items/ms_344_1/catalogue-of-observations-of-messier-objects-by-william-herschel).
Modern imaging has resolved the cluster's heart. A [Hubble Space Telescope](https://www.edgepedia.org/hubble-space-telescope) image taken with the Advanced Camera for Surveys shows the core of M13 and provides an extraordinarily clear view of its hundreds of thousands of stars[2](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-13/). Color-magnitude photometry of the cluster shows its stars falling into distinct groups in the Hertzsprung-Russell diagram, with color index tracing stellar temperature from hot blue stars to cool red ones[15](https://apod.nasa.gov/apod/ap190613.html).
## Observing the cluster
M13 is bright enough to be seen with the naked eye under a very dark sky, appearing as a fuzzy star[16](https://www.space.com/what-to-see-night-sky-june-2026). It is easily visible in binoculars, looks like a fuzzy snowball in a small telescope, and resolves into hundreds of individual stars in an 8-inch or larger instrument; even small telescopes resolve some stars around its perimeter. Observers use the averted-vision technique, looking slightly to the side of the cluster, to resolve more stars, especially near the core[16](https://www.space.com/what-to-see-night-sky-june-2026).
Hercules contains a second bright globular, [Messier 92](https://www.edgepedia.org/messier-92), about 9 degrees from M13, which is often found by observers searching for its brighter neighbor[5](https://britastro.org/section_information_/deep-sky-section-overview/observing-programmes/globular-clusters/the-gc-marathons-part-i-galactic-globular-clusters). The cluster is well placed in northern skies in northern spring and summer evenings[16](https://www.space.com/what-to-see-night-sky-june-2026).
## The Arecibo message
In 1974, scientists at Puerto Rico's Arecibo radio observatory beamed an encoded message at M13, which had been picked as a symbolic destination for whatever intelligent beings might live among its stars[3](https://martzobservatory.org/dso/detail/M13). The choice reflected the cluster's richness of stars, though at 25,100 light years the signal would need that many years to arrive[1](https://messier.lamost.org/seds/seds.org/messier/en/m/m013.html) • [3](https://martzobservatory.org/dso/detail/M13).
## Notable individual stars
Radial-velocity measurements have confirmed the membership of a star whose presence is unusual for such an old cluster; it appears to be a captured field star[1](https://messier.lamost.org/seds/seds.org/messier/en/m/m013.html). Detailed abundance analyses of large samples of M13 giants, covering 21 elements in 25 stars from luminous giants down to the main-sequence turnoff, find good agreement with halo field stars for most elements, suggesting a common chemical history for the cluster and the [Galactic halo](https://www.edgepedia.org/galactic-halo)[17](https://arxiv.org/abs/astro-ph/0409725).
## References
1. [Messier Object 13, SEDS](https://messier.lamost.org/seds/seds.org/messier/en/m/m013.html)
2. [Messier 13 (The Hercules Cluster), NASA Science](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-13/)
3. [Hercules Globular Cluster, DSO Catalog, Martz Observatory](https://martzobservatory.org/dso/detail/M13)
4. [Messier 13, SIMBAD Astronomical Database](https://simbad.cds.unistra.fr/simbad/sim-basic?Ident=Messier+13)
5. [The GC Marathons, Part I: Galactic Globular Clusters, British Astronomical Association](https://britastro.org/section_information_/deep-sky-section-overview/observing-programmes/globular-clusters/the-gc-marathons-part-i-galactic-globular-clusters)
6. [Differences in the rotational properties of multiple stellar populations in M13, Monthly Notices of the Royal Astronomical Society](https://academic.oup.com/mnras/article/465/3/3515/2417040)
7. [Globular Clusters data table, AstroVox](https://www.astrovox.gr/applications/core/interface/file/attachment.php?id=42775)
8. [A Catalog of Parameters for Globular Clusters in the Milky Way (Harris), Deep Sky Corner](https://www.deepskycorner.ch/catalogs.de.php?cat=MWGC&const=Her)
9. [Isochrone Fitting of Galactic Globular Clusters VII: NGC 1904, NGC 4372, and revision of NGC 6205 (M13) and others, arXiv](https://arxiv.org/abs/2601.07555)
10. [High-Resolution Spectra of Stars in Globular Clusters. VI. Oxygen-Deficient Red Giant Stars in M13, Astronomical Journal](https://doi.org/10.1086/115798)
11. [Clear Evidence for the Presence of Second-Generation Asymptotic Giant Branch Stars in Metal-Poor Galactic Globular Clusters, Astrophysical Journal Letters](https://doi.org/10.1088/2041-8205/815/1/L4)
12. [Messier 13: Hercules Globular Cluster, Messier Objects](https://www.messier-objects.com/messier-13-hercules-globular-cluster/)
13. [Globular Cluster M80, NASA Hubble lithograph](https://science.nasa.gov/wp-content/uploads/2023/07/hubble-litho-globular-m80.pdf)
14. [Catalogue of observations of Messier objects by William Herschel, Royal Society Archives](https://makingscience.royalsociety.org/items/ms_344_1/catalogue-of-observations-of-messier-objects-by-william-herschel)
15. [APOD: The Colors and Magnitudes of M13, NASA](https://apod.nasa.gov/apod/ap190613.html)
16. [Night sky June 2026: What you can see tonight, Space.com](https://www.space.com/what-to-see-night-sky-june-2026)
17. [Abundances in a Large Sample of Stars in M3 and M13, arXiv](https://arxiv.org/abs/astro-ph/0409725)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Stars and galaxies › Binary and multiple stars, star clusters › Globular clusters › Massive and individual notable clusters*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license