Star Clusters: Jewels of the Deep Sky
Using data from the VISTA infrared telescope at ESO's Paranal Observatory, an international team of astronomers discovered 96 new open clusters hidden by dust in the Milky Way.
Thirty of these clusters are shown in this mosaic.
Image source: ESO/J. Borissova (new window).
Scientific summary
Astronoo's article presents a classification and detailed description of star clusters in the Milky Way. It distinguishes open clusters, young and sparse, located in the galactic disk (e.g., Pleiades, Double Cluster in Perseus), from globular clusters, old, dense, and orbiting the galactic center (e.g., Omega Centauri, M13). The article explains their formation mechanisms from molecular clouds, their dynamical evolution (dispersion of open clusters, persistence of globulars), and their scientific importance: laboratories for studying stellar evolution (HR diagram), gravitational dynamics (mass segregation), and galactic chemistry (metallicity). It also mentions the contributions of the Gaia and James Webb missions to the study of these objects.
What are star clusters and why are they natural laboratories for astronomers?
Star clusters are groups of stars bound by gravity, born from the same molecular cloud. They are divided into two families. Open clusters, like the Pleiades, are young (a few million to hundreds of millions of years), sparsely populated (a few dozen to thousands of stars), and located in the galactic disk, often in spiral arms where gas is abundant. Their lives are short: massive stars explode as supernovae, and galactic tidal forces gradually disperse them. Globular clusters, like Omega Centauri, are dense spheres of hundreds of thousands of old stars (10 to 13 billion years), orbiting the galactic center. Their low metallicity betrays their primordial origin, contemporary with the formation of the Milky Way. These clusters are time capsules: all their stars share the same age and distance, allowing tests of stellar evolution theories (Hertzsprung-Russell diagram), studies of gravitational dynamics (mass segregation), and tracing the chemical enrichment of the Universe.
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What is a star cluster?
A star cluster is a group of stars bound together by gravity, formed from the same giant molecular cloud. These celestial structures offer astronomers natural laboratories to study the birth, evolution, and death of stars. Depending on their nature, two main categories are distinguished: open clusters and globular clusters, each telling a different story of our galaxy.
The two families of clusters: open and globular
Open clusters are young groups, containing from a few dozen to a few thousand stars, scattered in the galactic disk. They form in the spiral arms of galaxies, where interstellar gas is abundant. Their ages range from a few million to a few hundred million years (e.g., the Pleiades, M45). In contrast, globular clusters are dense spheres of hundreds of thousands of old stars (10 to 13 billion years), orbiting the centers of galaxies like satellites. Their low abundance of heavy elements (low metallicity) betrays their primordial origin, often associated with the formation of the Milky Way itself (e.g., Omega Centauri).
Birth and evolution: the life cycle of clusters
A cluster is born when gravitational forces compress a cloud of gas and dust, triggering the formation of stars within it. In open clusters, massive stars (types O and B) dominate initially, but their short lives (a few million years) doom them to explode as supernovae, gradually dispersing the cluster. Less massive stars, like our Sun, survive longer but eventually escape due to galactic tidal forces. Globular clusters, being more massive, resist these perturbations better and can persist for billions of years. Their study thus reveals clues about the age and primordial chemistry of the Universe.
Why do clusters fascinate scientists?
Clusters are time capsules:
- Uniformity of distance and age: All their stars are at the same distance from Earth and share a common age, allowing tests of stellar evolution theories (Hertzsprung-Russell diagram).
- Stellar dynamics: Their structure reveals the effects of gravitational interactions, such as mass segregation (massive stars migrate to the center).
- Galactic chemistry: Globular clusters, in particular, preserve the chemical signature of the first generations of stars, illuminating the progressive enrichment of the Universe in heavy elements.
Telescopes like Gaia (new window) (ESA) and James Webb (new window) (NASA/ESA/CSA) are revolutionizing their study by precisely measuring their motions, compositions, and even their atmospheres.
Pleiades Star Cluster (M45)
The Pleiades cluster (M45) is an open cluster located in the constellation Taurus, visible to the naked eye. Composed of about 3,000 stars, it is only 100 million years old and is located 444 light-years from Earth. Its blue stars, enveloped in reflection nebulosity (like around Merope and Maia), make it an iconic object. The seven brightest stars bear the names of Greek mythological figures (Alcyone, Atlas, Electra, etc.).
Image source: NASA - APOD (new window).
Star Cluster NGC 4755: The Jewel Box
Nicknamed "The Jewel Box" for its stars of contrasting colors (blue, red, and yellow), this open cluster is located in the Southern Cross, 6,400 light-years away. Discovered by Nicolas-Louis de Lacaille in 1752, it hosts the red supergiant star κ Crucis, surrounded by hot blue stars. Its age is estimated at 14 million years.
Image source: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration (new window).
Quintuplet Star Cluster
Near the galactic center (26,000 light-years), this massive cluster contains Wolf-Rayet stars and blue giants. It is named for five bright red stars visible in infrared. Its region is rich in interstellar dust, obscuring part of its visible light.
Image source: NASA, ESA, and Don Figer (STScI) (new window).
Star Cluster M80 (NGC 6093)
Dense globular cluster in Scorpius, M80 contains hundreds of thousands of stars bound by gravity. In 1860, a nova was observed there. Its apparent diameter is 10 arcminutes, but its actual size reaches 95 light-years. Distance: 32,600 light-years.
Image source: NASA, The Hubble Heritage Team, STScI, AURA (new window).
Star Cluster Westerlund-2
The sparkling center of this fireworks display is a vast cluster of thousands of stars called Westerlund 2. The cluster resides in a stellar nursery known as Gum 29, located 20,000 light-years from Earth in the constellation Carina.
Image source: NASA, The Hubble Heritage Team, STScI, AURA (new window).
Star Cluster Pismis 24
In the nebula NGC 6357, Pismis 24 hosts the star Pismis 24-1, initially estimated at 200-300 solar masses (multiple system). The cluster ionizes the surrounding gas, creating a landscape of dark pillars and plasma bubbles.
Image source: NASA, ESA, CSA, STScI; Image Processing: Alyssa Pagan (STScI) (new window).
Star Cluster NGC 602
The blue star cluster called NGC 602 formed when a large part of the gas cloud collapsed under gravity and became very dense. The intense radiation currently produced by these hot, young stars sculpts the inner edge of the gaseous nebula.
Image source: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration (new window).
Star Cluster M25 (IC 4725)
The open cluster shown above, M25, contains thousands of stars and is located about two thousand light-years away. The stars in this cluster all formed together about 90 million years ago. The bright young stars of M25 appear blue.
Image source: Jean-Charles Cuillandre ( CFHT ) & Giovanni Anselmi ( Coelum Astronomia ), Hawaiian Starlight (new window).
Butterfly Cluster (M6, NGC 6405)
Named for its shape resembling wings, M6 is an open cluster in Scorpius, aged 100 million years. Its brightest star, BM Scorpii, is an orange giant. Distance: 1,600 light-years.
Image source: Wikimedia (new window) (Public domain).
Star Cluster M7
Known since antiquity, M7 is an open cluster of 80 stars in Scorpius, at 980 light-years. Its apparent diameter (1.3°) exceeds that of the Moon. Age: 200 million years.
Image source: ESO (new window).
Star Cluster R136
Image of the super star cluster R136, near the center of the Tarantula Nebula (NGC 2070), obtained with the Hubble Space Telescope.
R136 is a star cluster located about 165,000 light-years away in the constellation Dorado, near the center of the Tarantula Nebula in the Large Magellanic Cloud.
Image source: NASA, ESA, F. Paresce (INAF-IASF, Bologna, Italy), R. O'Connell (University of Virginia, Charlottesville) (new window).
Star Cluster M34
Messier 34 (NGC 1039) is an open cluster located in the constellation Perseus, in the Orion Arm of the Milky Way galaxy, within the Local Group of galaxies. M34 is 1,500 light-years from Earth.
Image source: NASA, ESA (new window).
Star Cluster NGC 6934
This bright bundle of stars is a globular cluster, which is a large ball of (usually) a few hundred thousand old stars found on the edges of galaxies. Located 50,000 light-years from Earth, on the outskirts of our Milky Way galaxy, NGC 6934 is home to some of the most distant stars still part of our galactic system — in a sense, it is a distant suburb of the Milky Way's city center.
Image source: ESA/Hubble & NASA (new window).
Star Cluster M17: Omega Nebula
Although often associated with its nebula, M17 hosts an open cluster of 35 massive stars (types O and B) that ionize the surrounding gas. Distance: 5,500 light-years.
Image source: ESO (new window).
Star Cluster NGC 5139
Largest globular cluster in the Milky Way (10 million stars), visible to the naked eye. Its dense core suggests a past as a dwarf galaxy that was absorbed. Age: 12 billion years.
Image source: NASA (new window).
Star Cluster Cygnus OB2
The Milky Way and other galaxies in the universe harbor many young star clusters and associations, each containing hundreds or even thousands of young, hot, massive stars, known as O-type and B-type stars. The Cygnus OB2 star cluster includes more than 60 O-type stars and about a thousand B-type stars.
Image source: NASA/CXC/SAO/J.Drake et al., (new window).
Star Cluster NGC 6362
The NASA/ESA Hubble Space Telescope gives us a spectacular image of the center of the globular cluster NGC6362.
Image source: ESA/Hubble & NASA (new window).
Star Cluster M11
Messier 11 is an open cluster, sometimes likened to a galactic cluster, located some 6,000 light years from Earth in the constellation Scutum.
Image source: ESO (new window).
References
- IAU – Constellations (new window)
- SEDS – Messier 45 (Pleiades) (new window)
- Allison, R. J. et al. – Testing the initial conditions and dynamical evolution of star clusters using Gaia (2012) (new window)
- Hu, K. & Zhou, Q. – Study of Messier 7 Open Cluster with Gaia EDR3 (2022) (new window)
- Nardiello, D. et al. – JWST NIRCam photometry of M 92 (2025) (new window)
- Bouy, H. et al. – The Seven Sisters DANCe. I. Pleiades (2015) (new window)
- Caldwell, S. P. & Dickens, R. J. – Chemical abundances in omega Centauri (1988) (new window)
FAQ: Everything you need to know about Star Clusters
What is a star cluster?
A star cluster is a group of stars bound together by gravity, formed from the same giant molecular cloud. These celestial structures are natural laboratories for astronomers, as they allow the study of the birth, evolution, and death of stars under controlled conditions. Two main categories are distinguished: open clusters and globular clusters.
What is the difference between an open cluster and a globular cluster?
The differences are numerous:
• Age: open clusters are young (a few million to hundreds of millions of years); globular clusters are old (10 to 13 billion years).
• Population: open clusters contain a few dozen to thousands of stars; globular clusters contain hundreds of thousands.
• Location: open clusters are in the galactic disk (spiral arms); globular clusters orbit the galactic center.
• Metallicity: globular clusters are poor in heavy elements (low metallicity), indicating a primordial origin; open clusters are richer in metals.
• Lifespan: open clusters dissipate in a few hundred million years; globular clusters can persist for billions of years.
What are the most famous examples of open and globular clusters?
Among the most famous open clusters:
• The Pleiades (M45): visible to the naked eye, a young cluster (about 100 million years old) in the constellation Taurus.
• The Double Cluster (h and χ Persei): two nearby open clusters in the constellation Perseus.
Among the most famous globular clusters:
• Omega Centauri (NGC 5139): the largest and brightest globular cluster in the Milky Way, visible in the southern hemisphere.
• M13 (Hercules Cluster): a spectacular globular cluster in the constellation Hercules, visible in the northern hemisphere.
How does a star cluster form?
A star cluster is born when a giant molecular cloud (composed of gas and dust) undergoes gravitational compression, often triggered by a shock wave (nearby supernova, galaxy collision). The compression increases the cloud's density, causing it to collapse into protostellar cores. These cores collapse to form stars that remain gravitationally bound for a time, constituting the cluster. In an open cluster, massive stars quickly explode as supernovae, dispersing the cluster. In a globular cluster, sufficient mass allows cohesion to be maintained for billions of years.
Why are clusters so important for astronomy?
Clusters are essential tools for several reasons:
• Uniformity: all stars in a cluster are at the same distance from Earth and share the same age, allowing tests of stellar evolution theories on the Hertzsprung-Russell diagram.
• Dynamics: the structure of clusters reveals gravitational interactions, such as mass segregation (massive stars migrate to the center).
• Galactic chemistry: globular clusters preserve the chemical signature of the first generations of stars, illuminating the progressive enrichment of the Universe in heavy elements.
• Cosmology: globular clusters are among the oldest objects in the galaxy, providing constraints on the age of the Universe.
How are modern telescopes (Gaia, James Webb) revolutionizing the study of clusters?
Modern telescopes bring major advances:
• Gaia (ESA): measures with unprecedented precision the positions, distances, and proper motions of billions of stars, enabling 3D mapping of the structure and dynamics of clusters, detecting stellar streams, and identifying escaped stars.
• James Webb (NASA/ESA/CSA): with its infrared power, it observes star-forming regions within clusters, penetrates dust clouds to study nascent stars, and analyzes the chemical composition of atmospheres and circumstellar environments, revealing details previously inaccessible.
Why do open clusters eventually dissipate?
Open clusters dissipate under the effect of several dynamical processes:
• Galactic tidal forces: the gravitational pull of the galactic disk and giant molecular clouds perturbs the cluster, tearing stars from its periphery.
• Supernovae: massive stars, which initially dominate open clusters, explode as supernovae after a few million years, ejecting gas and dispersing the stars.
• Stellar evaporation: gravitational interactions between stars in the cluster give some of them enough speed to escape.
These processes lead to the progressive dissolution of the cluster in a few hundred million years, with the remaining stars integrating into the general population of the galactic disk.
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