What is a Globular Cluster?

To achieve the spatial distribution of stars in a Globular Cluster, as has been observed, a black hole fraction of 22% is necessary.

Globular clusters are fascinating astronomical objects composed of densely packed groups of stars. They are distinct from open clusters, which typically contain a few hundred stars, as globular clusters can host hundreds of thousands to millions of stars.

Globular clusters are roughly spherical in shape and can range in diameter from around 10 to 200 light-years. They have extremely high stellar densities, with stars typically separated by only a fraction of a light-year at their cores. Globular clusters consist mainly of older stars, usually more than 10 billion years old. They are composed mostly of low-mass stars, although they also contain higher-mass stars, variable stars, and even exotic objects such as pulsars and black holes.

The oldest globular cluster has been around for about 103 billion years, and scientists believe studying them may reveal secrets about the universe’s creation.

The Andromeda Galaxy (M31) is known to host a rich population of globular clusters. With its proximity to the Milky Way, it has been extensively studied, revealing the presence of over 400 globular clusters within its halo.

Globular clusters have low metallicities, meaning they contain smaller amounts of elements heavier than hydrogen and helium. This suggests that they formed early in the history of the universe.

Globular clusters contain 100,000 to 10 million stars and stretch over 300 light-years across. The stars are most densely collected in the center, tightly bound by strong gravitational forces. Within a cluster, the stars move randomly to avoid colliding with one another. The cluster moves as a whole, orbiting its galaxy.

Stars within globular clusters exhibit coherent motion, with individual stars orbiting the cluster’s center of mass due to gravitational interactions. The precise formation mechanism of globular clusters is still an area of active research, but the leading theory suggests that they formed from the collapse of giant molecular clouds, which are regions of high-density gas and dust. The collapse of these clouds leads to the formation of a dense stellar cluster.

Through a telescope, a globular cluster appears as a round ball of light. Its center is tightly packed and made of an indistinguishable myriad of stars. Open clusters are much more irregular in shape, less densely packed, and unsymmetrical. Open clusters are considerably smaller, containing only hundreds of stars rather than billions. A younger type of cluster, their life span is only a few hundred million years. Their stars form inside a giant molecular cloud and are all roughly the same age. Spiral and irregular galaxies where star formation is actively occurring contain open clusters. Due to the age of globular clusters, star formation ceased in them long ago.

The details of this process are not yet fully understood, but it likely involves various physical processes such as turbulence, gravitational interactions, and stellar feedback. Globular clusters are found in the halos of galaxies, surrounding their galactic disks. They are more commonly observed in elliptical and lenticular galaxies, although they can also be present in spiral galaxies. The Milky Way, our home galaxy, contains about 150 known globular clusters, with the largest and most well-known being Omega Centauri.

Some Popular Globular Clusters
  • Messier 3 (M3) – The globular cluster M3 was the first object in the Messier catalog to be discovered by Charles Messier[1] himself. Messier spotted the cluster in 1764, mistaking it for a nebula without any stars. This misunderstanding of M3’s nature was corrected in 1784 when William Herschel was able to resolve the cluster’s individual stars. Today it is known to contain over 500,000 stars. M3 is notable for containing more variable stars than any other known cluster. The brightness of a variable star fluctuates with time. For some variable stars, their period relates to their intrinsic luminosity, so astronomers can use those stars’ brightness fluctuations to estimate their distances. This makes them extremely useful for measuring distances to deep-sky objects. M3 contains at least 274 variable stars.
  • Messier 5 (M5) – With your eye alone, binoculars, and a telescope, you can stairstep your way to the beauty of globular star cluster M5, or Messier 5. With the unaided eye, M5 is barely detectable and appears to be a faint star. Binoculars show it a bit more clearly. But turn a small telescope on it, and you’ll see one of the finest globular clusters north of the celestial equator. Unlike many of the “faint fuzzies” in the night sky – nebulas, galaxies, and clusters that still appear as dim, blurry smudges even through telescopes – M5 is one of the best globular clusters to observe.
  • Messier 10 & 12 – M10 and M12 are a pair of globular clusters about 3 degrees apart and similar in size and brightness. They are both about magnitude 6.6, making them good targets for telescopes of all sizes. Medium to large scopes will show hundreds of individual stars in both clusters. The stars of M12 are less densely packed, making them easier to resolve all the way to the center of the cluster.
  • Messier 13 (M13) – Also known as the Great Globular Cluster in Hercules, M13 is located in the constellation Hercules and is one of the best-known globular clusters. It contains around 300,000 stars and is easily visible through small telescopes.
  • Messier 22 (M22) – Messier 22 (M22) is a globular cluster located near the Milky Way bulge, the tightly packed group of stars near the galactic center. The cluster lies in the constellation Sagittarius. It is one of the brightest globular clusters in the sky and was one of the first objects of this kind to be discovered and studied. M22 is also one of the nearest globulars to the solar system. The only globular cluster closer to us is Messier 4, located in the neighboring constellation Scorpius. M22 is elliptical in shape and lies at a distance of 10,600 light-years from Earth. Its designation in the New General Catalogue[2] is NGC 6656. With a visual magnitude of 5.5, M22 is the brightest globular cluster visible from the mid-northern latitudes. However, as it lies in the southern constellation Sagittarius, M22 never rises very high in the sky and can’t really be observed in all its glory from the northern hemisphere. It doesn’t offer a view as impressive as those of Messier 13 in Hercules and Messier 5 in Serpens.
  • Messier 92 (M92) – Located 27,000 light-years from Earth in the constellation Hercules, this globular cluster — a ball of stars that orbits our galaxy’s core like a satellite — was first discovered by the German astronomer Johann Elert Bode[3] in 1777. With an apparent magnitude of 6.3, M92 is one of the brightest globular clusters in the Milky Way and is visible to the naked eye under good observing conditions. It can be most easily spotted during the month of July. The cluster is very tightly packed with stars, containing roughly 330,000 stars in total. As is characteristic of ancient globular clusters — of which M92 is one of the oldest — the predominant elements within M92 are hydrogen and helium, with only traces of others, so it belongs to a group of metal-poor clusters. To astronomers, metals are all elements heavier than hydrogen and helium.
  • Omega Centauri (NGC 5139) – Located in the constellation Centaurus, Omega Centauri is the largest and brightest globular cluster in the Milky Way. It is visible to the naked eye and contains an estimated 10 million stars.
  • 47 Tucanae (NGC 104) – Situated in the constellation Tucana, 47 Tucanae is the second-largest globular cluster in the Milky Way. It has a dense core and is easily visible from the southern hemisphere.

The History

The first known globular cluster, now called M 22, was discovered in 1665 by Abraham Ihle[4], a German amateur astronomer. The cluster Omega Centauri, easily visible in the southern sky with the naked eye, was known to ancient astronomers like Ptolemy[5] as a star, but was reclassified as a nebula by Edmond Halley[6] in 1677, then finally as a globular cluster in the early 19th century by John Herschel[7].

The French astronomer Abbé Lacaille[8] listed NGC 104, NGC 4833, M 55, M 69, and NGC 6397 in his 1751–1752 catalog. The low resolution of early telescopes prevented individual stars in a cluster from being visually separated until Charles Messier observed M 4 in 1764. Sir William Herschel[9], an eminent astronomer of the 18th century, made significant contributions to the study of globular clusters.

Herschel was a pioneer in using telescopes to explore the heavens and discovered numerous celestial objects, including many globular clusters. His meticulous observations and cataloging efforts led to the discovery and classification of various globular clusters, adding to our understanding of their distribution and characteristics. Herschel’s observations of globular clusters, such as his catalog of 34 clusters published in 1789, laid the foundation for further investigations into their properties, formation, and dynamics.

His work marked a crucial milestone in the study of globular clusters, contributing to our knowledge of these intriguing stellar systems. Globular clusters are generally composed of hundreds of thousands of low-metal, old stars. The stars found in a globular cluster are similar to those in the bulge of a spiral galaxy but confined to a spheroid in which half the light is emitted within a radius of only a few to a few tens of parsecs[10].

They are free of gas and dust and it is presumed that all the gas and dust was long ago either turned into stars or blown out of the cluster by the massive first-generation stars.

Tidal Interactions

Tidal interactions play a crucial role in shaping the dynamics and evolution of globular clusters. These interactions occur when a globular cluster passes close to a massive object, such as a galaxy or a galactic tidal field. Tidal forces exerted by the massive object can cause significant distortions within the cluster, leading to various effects. One important consequence of tidal interactions is the stripping of stars from the outer regions of the globular cluster.

This process, known as tidal stripping, occurs due to the differential gravitational forces acting on individual stars. As a result, the cluster’s outer stars can be pulled away, forming extended tidal tails or streams. Tidal stripping can significantly alter the mass distribution and density profile of the cluster over time. Another effect of tidal interactions is the tidal heating of the globular cluster.

As the cluster moves through the tidal field, stars experience time-varying tidal forces, which can lead to an increase in their kinetic energy. This process can cause heating within the cluster, disrupting the stellar orbits and increasing the velocity dispersion of stars.

Furthermore, tidal interactions can induce dynamic friction on the globular cluster. As the cluster moves through the tidal field, it experiences gravitational drag due to the surrounding matter, causing a gradual loss of energy and angular momentum. This results in the cluster sinking towards the center of the galaxy over time. The tidal interactions of globular clusters have been studied through both observational and theoretical approaches. Observationally, the detection of tidal tails and streams around globular clusters provides evidence of their tidal interactions with the host galaxy.

Theoretical simulations and models are employed to understand the dynamics and long-term evolution of globular clusters under tidal influences.

Black Holes

Black holes in globular clusters (GCs) are intriguing astronomical objects that result from the evolution of massive stars within these dense stellar systems. It is estimated that a significant fraction of GCs may host stellar-mass black holes, with some clusters potentially containing tens or even hundreds of black holes. These black holes form through stellar evolution processes, as massive stars undergo supernova explosions, leaving behind remnants with masses exceeding the Chandrasekhar limit[11], resulting in black hole formation.

Black holes in GCs quickly sink to the center due to something called dynamical friction[12]. Due to dynamical friction, the black holes sit at the center while the stars “float” in the outskirts. Models have been run without black holes and found that those models are less likely. To achieve a stellar stream density[13] as has been observed, a black hole fraction of 22% is necessary.

The dynamics within GCs, including interactions between black holes and other stars, play a vital role in the retention, ejection, and segregation of black holes within these systems. The study of black holes in GCs provides valuable information about stellar evolution, dynamical processes, and the formation of compact objects in dense stellar environments.

Shirtloads of Science Podcast
Globular Mysteries with Prof Geraint Lewis (253)

  1. Charles Messier, a French astronomer of the 18th century, made notable contributions to the study of globular clusters through his comprehensive cataloging efforts. Messier was primarily interested in discovering comets, but he frequently encountered globular clusters during his observations. In an effort to differentiate these objects from comets, he compiled a catalog known as the Messier Catalog, which included 110 celestial objects, of which 29 were globular clusters. Messier’s catalog served as a valuable resource for astronomers, aiding in the identification and study of globular clusters. His work contributed to our understanding of their locations, and appearances, and helped distinguish them from other types of astronomical objects. [Back]
  2. The New General Catalogue (NGC) is a comprehensive astronomical catalog that serves as a compilation of various celestial objects, including galaxies, star clusters, nebulae, and other deep-sky objects. It was compiled by John Louis Emil Dreyer, a Danish-Irish astronomer, and was published in 1888. The NGC contains a total of 7,840 objects, each identified by a unique designation number. Dreyer’s goal in creating the NGC was to rectify and expand upon the earlier works of catalogers such as Charles Messier and William Herschel. The NGC has since become a valuable reference for astronomers, aiding in the identification and study of deep-sky objects, and it continues to be widely used in astronomical research today. [Back]
  3. Johann Elert Bode, a German astronomer of the 18th century, made notable contributions to the study of globular clusters through his observational work and cataloging efforts. Bode is best known for his compilation of celestial objects, including globular clusters, in his “Allgemeine Beschreibung und Nachweisung der Gestirne” (General Description and Presentation of Celestial Bodies). In this work, published in 1774, Bode listed and described various globular clusters known at the time, including some discovered by other astronomers such as Charles Messier and William Herschel. Bode’s catalog served as an important reference for astronomers of his time and contributed to the recognition and understanding of globular clusters as distinct objects in the night sky. [Back]
  4. Abraham Ihle, a German amateur astronomer of the 17th century, made significant contributions to the study of globular clusters through his observational work and descriptions. Ihle’s observations of globular clusters, particularly in the constellation Hercules, were noted for their accuracy and detail. He described the appearance and positions of several globular clusters in his work, “Prodromus Astronomiae” (1627), including what is now known as Messier 13 (M13). Ihle’s meticulous observations and descriptions laid the foundation for future astronomers, such as Charles Messier and William Herschel, to further investigate and understand these fascinating stellar systems. [Back]
  5. Claudius Ptolemy, a Greco-Egyptian astronomer and mathematician who lived in the 2nd century CE, played a pivotal role in the history of astronomy. Ptolemy’s most influential work, “Almagest,” synthesized and expanded upon the astronomical knowledge of his time. In the Almagest, Ptolemy presented a comprehensive geocentric model of the universe, which posited that the Earth was at the center and that celestial bodies, including the Sun, Moon, planets, and stars, orbited around it. This model incorporated intricate mathematical calculations and observations and remained the prevailing cosmological framework for over a thousand years. Ptolemy’s work had a lasting impact on astronomy, serving as a foundation for later developments, as well as inspiring astronomers to challenge and refine his theories. [Back]
  6. Edmond Halley, an English astronomer, and mathematician, made significant contributions to the field of astronomy in the 17th and 18th centuries. Halley is best known for his prediction and observation of the comet now named after him, Halley’s Comet. In 1705, he published the work “Synopsis Astronomia Cometicae” where he analyzed the orbits of comets and used historical observations to deduce the periodicity of certain comets, including the one that would later bear his name. Halley’s prediction of the return of Halley’s Comet in 1758-1759, based on his calculations and understanding of gravitational forces, validated his scientific approach. Halley also made contributions to geophysics and magnetic studies, including his investigations of Earth’s magnetic field. His diligent work and scientific achievements established him as one of the prominent figures in the history of astronomy. [Back]
  7. John Herschel, born John Frederick William Herschel, was a prominent British scientist and polymath of the 19th century. As an accomplished astronomer, mathematician, chemist, and photographer, Herschel made significant contributions to multiple fields of study. He conducted extensive observations and cataloged celestial objects, advancing our understanding of the southern skies. Herschel also played a crucial role in the development of photography, introducing the cyanotype process and experimenting with various photographic techniques. His work extended to mathematics, chemistry, and optics, where he made important contributions to the understanding of light polarization and wave theory. Herschel’s broad range of accomplishments solidified his reputation as one of the leading scientists of his time. [Back]
  8. Abbé Nicolas-Louis de Lacaille was a French astronomer and geodesist who made significant contributions to the field of astronomy in the 18th century. Lacaille is best known for his pioneering work in mapping the southern skies during his expedition to the Cape of Good Hope in South Africa. He meticulously cataloged and observed thousands of stars, documenting their positions and magnitudes with unprecedented accuracy. His catalog, “Coelum Australe Stelliferum,” published in 1763, included over 9,000 southern stars, many of which were previously unknown. Lacaille also named several constellations, such as Norma, Circinus, and Antlia, based on his observations. His systematic approach and precise measurements laid the foundation for future studies of the southern skies and contributed to our understanding of stellar astronomy. [Back]
  9. Sir William Herschel, a German-born British astronomer, is renowned for his groundbreaking contributions to astronomy in the late 18th and early 19th centuries. Herschel is particularly celebrated for his discovery of the planet Uranus in 1781, expanding the known boundaries of the solar system. Moreover, he constructed over 400 telescopes, including the largest of his time, to conduct extensive observations of the heavens. Herschel cataloged thousands of nebulae, double stars, and clusters, and his pioneering work in the field of sidereal astronomy greatly expanded our understanding of the universe. He also proposed a model for the structure of the Milky Way galaxy. Beyond his astronomical accomplishments, Herschel made significant contributions to the study of optics, music, and the scientific community. His lasting impact on astronomy earned him a place among the most influential figures in the history of science. [Back]
  10. Parsecs are a unit of measurement used in astronomy to express vast distances. The term “parsec” is a combination of “parallax” and “arcsecond.” It is defined as the distance at which an object would have a parallax angle of one arcsecond when observed from opposite ends of Earth’s orbit around the Sun. Parallax is the apparent shift in the position of an object caused by the observer’s changing perspective. By measuring the parallax angle of a celestial object and applying trigonometry, astronomers can calculate its distance. One parsec is approximately equal to 3.09 trillion kilometers or 3.26 light-years. Parsecs provides a convenient scale for describing distances between stars, galaxies, and other astronomical objects. [Back]
  11. The Chandrasekhar limit is a fundamental concept in astrophysics that describes the maximum mass that a stable white dwarf star can have before it undergoes gravitational collapse. Named after the Indian astrophysicist Subrahmanyan Chandrasekhar, this limit is derived from the principles of degenerate electron pressure within white dwarfs, which counteracts gravitational forces. According to Chandrasekhar’s calculations, the maximum mass for a white dwarf supported by electron degeneracy pressure is approximately 1.44 times the mass of the Sun, known as the Chandrasekhar mass. If a white dwarf exceeds this mass limit, it will collapse under its own gravity, leading to a supernova explosion or, if the mass is higher, potentially forming a black hole or a neutron star. The Chandrasekhar limit plays a crucial role in understanding the fate of stellar remnants and the formation of compact objects in the universe. [Back]
  12. Dynamical friction is a gravitational process that describes the deceleration of a massive object, such as a star or a galaxy, moving through a medium composed of lighter objects, such as stars or gas particles. As the massive object moves, it perturbs the surrounding medium, causing a wake of density enhancements. These density enhancements exert gravitational forces on the massive object, resulting in a drag-like effect that gradually slows it down. The strength of dynamical friction depends on the mass, velocity, and size of the moving object, as well as the density and distribution of the surrounding medium. Dynamical friction is an essential mechanism in astrophysics, contributing to the orbital decay of satellite galaxies, the merging of galaxy clusters, and the sinking of massive objects toward the center of galaxies. [Back]
  13. Stellar stream density refers to the spatial distribution of stars within a stellar stream—a coherent structure composed of stars that share a common origin, such as a disrupted satellite galaxy or a tidally disrupted star cluster. The density of a stellar stream varies along its length, reflecting the distribution of stars that have been pulled apart and spread out over time due to gravitational interactions. The stream density is highest closer to the progenitor system, where the stars are more tightly packed, and decreases with increasing distance from the progenitor as the stream becomes more diffuse. Stellar stream density is a key observable that provides valuable insights into the formation, evolution, and dynamics of galaxies and their satellite systems, shedding light on the processes of galactic accretion and tidal disruption. [Back]

Further Reading

  • Newspapers
  • “The Formation of Star Clusters” (MAY-JUNE 1998 VOLUME 86, NUMBER 3) https://www.americanscientist.org/article/the-formation-of-star-clusters
  • Djorgovski, S. G. (1993). Globular clusters. In Structure and Dynamics of Globular Clusters (Vol. 50, pp. 373-394). Springer.
  • Harris, W. E. (1996). A catalog of parameters for globular clusters in the Milky Way. The Astronomical Journal, 112(4), 1487-1488.
  • Mackey, A. D., & Broby Nielsen, P. (2007). The bimodal distribution of blue straggler stars in globular clusters. Monthly Notices of the Royal Astronomical Society, 379(1), 151-166.
  • Kruijssen, J. M. D. (2015). The formation of globular clusters in galaxy mergers. Monthly Notices of the Royal
  • “Globular cluster” (Last Edited May 2023) https://en.wikipedia.org/wiki/Globular_cluster’
  • “What is a Globular Cluster?” (2023) https://astrobackyard.com/globular-clusters/
  • Mackey, A. D., et al. (2010). The population of star clusters in the Andromeda galaxy: The influence of mass and environment. The Astrophysical Journal, 717(1), L11-L15.
  • Perretcatalog et al. (2002). The formation and evolution of star clusters in interacting galaxies. Monthly Notices of the Royal Astronomical Society, 337(1), 362-374.
  • Strader, J., et al. (2011). The ancient globular clusters of the Local Group. The Astronomical Journal, 142(1), 8.
  • Woodley, K. A., et al. (2010). Two stellar components in the halo of the Andromeda galaxy. The Astrophysical Journal, 708(2), 1335-1349.
  • Hoskin, M. (2003). William Herschel and the construction of the heavens. In The Cambridge Illustrated History of Astronomy (pp. 196-197). Cambridge University Press.
  • Herschel, W. (1789). A catalogue of 500 new nebulae, nebulous stars, planetary nebulae, and clusters of stars; with remarks on the construction of the heavens. Philosophical Transactions of the Royal Society of London, 79, 212-255.
  • “Dating the Evaporation of Globular Clusters” (August 31, 2018) https://aasnova.org/2018/08/31/dating-the-evaporation-of-globular-clusters/
  • Chatterjee, S., et al. (2017). Stellar-Mass Black Holes in Globular Clusters. Annual Review of Astronomy and Astrophysics, 55, 17-51.
  • Gültekin, K., et al. (2019). The formation and evolution of black holes in globular clusters. Monthly Notices of the Royal Astronomical Society, 482(1), 506-521.
  • Kremer, K., et al. (2021). Population properties of stellar-mass black holes in globular clusters. Monthly Notices of the Royal Astronomical Society, 504(1), 1158-1173.
  • Strader, J., & Chomiuk, L. (2019). Black holes and neutron stars in globular clusters. Annual Review of Astronomy and Astrophysics, 57, 617-664.
  • “Uncovering black holes hiding in a globular cluster” (Jul 10, 2021) https://astrobites.org/2021/07/10/black-holes-in-pal5/

Author: Doyle

I was born in Atlanta, moved to Alpharetta at 4, lived there for 53 years and moved to Decatur in 2016. I've worked at such places as Richway, North Fulton Medical Center, Management Science America (Computer Tech/Project Manager) and Stacy's Compounding Pharmacy (Pharmacy Tech).

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