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. This is the first time so many faint, small clusters have been found through the Galaxy’s dust. Image source: ESO/J. Borissova.
This article presents star clusters, groups of stars bound by gravity, formed from the same molecular cloud. Two categories are distinguished: open clusters, young (a few million to hundreds of millions of years), containing a few dozen to a few thousand stars, scattered in the galactic disk (e.g., Pleiades); and globular clusters, dense and ancient (10 to 13 billion years), containing hundreds of thousands of stars, orbiting the galactic center (e.g., Omega Centauri). Open clusters gradually dissipate under tidal forces, while globular clusters persist, preserving the chemical signature of the first generations of stars. Their study offers natural laboratories for testing stellar evolution, gravitational dynamics, and galactic chemistry.
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 a few thousand stars), and located in the galactic disk, often in spiral arms where gas is abundant. Their lives are brief: 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 ancient 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. Telescopes like Gaia and James Webb are revolutionizing their study.
A star cluster is a group of stars bound together by gravity, formed from the same giant molecular cloud. These celestial structures provide astronomers with natural laboratories to study the birth, evolution, and death of stars. There are two main categories: open clusters and globular clusters, each telling a different story about our galaxy.
Open clusters are young groups containing from a few dozen to a few thousand stars, scattered across 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 ancient stars (10 to 13 billion years old), orbiting the centers of galaxies like satellites. Their low metallicity reveals their primitive origin, often linked to the formation of the Milky Way itself (e.g., Omega Centauri).
A cluster is born when gravitational forces compress a cloud of gas and dust, triggering star formation within it. In open clusters, massive stars (O and B types) initially dominate, but their short lifespans (a few million years) lead 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, better resist these disruptions and can persist for billions of years. Their study provides clues about the age and early chemistry of the Universe.
Clusters are time capsules:
Telescopes like Gaia (ESA) or James Webb (NASA/ESA/CSA) are revolutionizing their study by precisely measuring their movements, compositions, and even atmospheres.
Image description: The Pleiades cluster, also known as M45, is an open cluster in the constellation Taurus, visible to the naked eye. Comprising about 3,000 stars, it is only 100 million years old and located 444 light-years from Earth. Its blue stars, enveloped in reflection nebulae (such as 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: ESO.
Image description: Nicknamed "The Jewel Box" for its stars' 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/Hubble.
Image description: Close to the galactic center (26,000 light-years away), this massive cluster contains Wolf-Rayet stars and blue giants. Its name comes from five bright red stars visible in infrared. The region is rich in interstellar dust, obscuring some of its visible light. Image source: NASA/CXC.
Image description: A dense globular cluster in Scorpius, M80 contains hundreds of thousands of stars bound by gravity. In 1860, a nova was observed within it. Its apparent diameter is 10 arcminutes, but its actual size spans 95 light-years. Distance: 32,600 light-years. Image source: Hubble Heritage Team.
Image description: A young cluster (1-2 million years old) in Carina, hosting some of the Milky Way’s most massive stars, such as WR 20a (a binary system of 82 and 83 solar masses). Stellar winds sculpt complex gaseous structures, visible in X-rays. Image source: NASA/CXC.
Image description: Within the NGC 6357 nebula, Pismis 24 hosts the star Pismis 24-1, initially estimated at 200-300 solar masses but later revealed as a multiple system. The cluster ionizes surrounding gas, creating dark pillars and plasma bubbles. Image source: ESO.
Image description: Located in the Small Magellanic Cloud, this young cluster (5 million years old) is surrounded by emission nebulae. Its massive stars carve a cavity in the ambient gas, revealing filamentary structures in false color (Hubble). Image source: NASA/ESA/Hubble.
Image description: An open cluster in Sagittarius, 2,000 light-years away, containing the Cepheid variable star U Sagittarii. Its age is estimated at 90 million years, with about sixty confirmed stars. Image source: ESO.
Image description: Named for its wing-like shape, M6 is an open cluster in Scorpius, 100 million years old. Its brightest star, BM Scorpii, is an orange giant. Distance: 1,600 light-years. Image source: ESO.
Image description: Known since antiquity, M7 is an open cluster of 80 stars in Scorpius, 980 light-years away. Its apparent diameter (1.3°) exceeds that of the Moon. Age: 200 million years. Image source: ESO.
Image description: At the heart of 30 Doradus in the Large Magellanic Cloud, RMC 136 hosts R136a1, the most massive known star (250 solar masses). The cluster, 2 million years old, is a laboratory for studying extreme star formation. Image source: ESA/Hubble.
Image description: An asterism (not a true gravitationally bound cluster) in Camelopardalis, composed of about twenty stars aligned over 2.5°. Discovered by amateur astronomer Lucian Kemble, it points toward the open cluster NGC 1502. Image source: NASA APOD.
Image description: In Perseus, M34 is an open cluster of 100 stars, 200 million years old. Its distance (1,500 light-years) and magnitude (5.5) make it visible with binoculars. Image source: ESO.
Image description: A globular cluster in Delphinus, 50,000 light-years away. Loosely concentrated, it contains Population II stars (metal-poor). Diameter: 120 light-years. Image source: Hubble.
Image description: Though often associated with its nebula, M17 hosts an open cluster of 35 massive stars (O and B types) that ionize the surrounding gas. Distance: 5,500 light-years. Image source: ESO.
Image description: The 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 absorbed by the Milky Way. Age: 12 billion years. Image source: NASA/ESA/Hubble.
Image description: A massive stellar association in Cygnus, containing 65 O-type stars and thousands of young stars. Its distance (4,700 light-years) and obscuration make it a prime target for infrared study. Image source: NASA/CXC.
Image description: In Auriga, M38 (left) and M36 (right) are two open clusters located 4,200 and 4,100 light-years away, respectively. M38, cross-shaped, contains a yellow giant, while M36 is younger (25 million years) and more compact. Image source: NASA APOD.
Image description: In Gemini, M35 is an open cluster of 2,500 stars, 150 million years old. Its apparent neighbor, NGC 2158, is an older and more distant cluster. Image source: ESO.
Image description: A globular cluster in Canes Venatici, containing 500,000 stars and 274 variable stars. Distance: 33,900 light-years. Age: 11.4 billion years. Image source: Hubble.
Image description: One of the oldest globular clusters (13 billion years), in Serpens. It hosts 105 variable stars and a very dense stellar population at its core. Image source: ESO.
Image description: Close to the galactic center (25,000 light-years away), this young cluster (2-4 million years old) contains stars 100 times more massive than the Sun. Its extreme environment makes it a laboratory for studying star formation in dense regions. Image source: NASA/CXC.
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.
The differences are numerous:
• Age: open clusters are young (a few million to hundreds of millions of years); globular clusters are ancient (10 to 13 billion years).
• Population: open clusters contain a few dozen to a few thousand 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 their 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.
Among the most famous open clusters:
• The Pleiades (M45): visible to the naked eye, a young cluster (about 100 million years) 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.
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). 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 maintains cohesion for billions of years.
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 testing of stellar evolution theories on the Hertzsprung-Russell diagram.
• Dynamics: cluster structure 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.
Modern telescopes provide major advances:
• Gaia (ESA): measures with unprecedented precision the positions, distances, and proper motions of billions of stars, enabling 3D mapping of cluster structure and dynamics, detection of stellar streams, and identification of escaped stars.
• James Webb (NASA/ESA/CSA): with its infrared power, it observes star-forming regions within clusters, penetrates dust clouds to study newborn stars, and analyzes the chemical composition of atmospheres and circumstellar environments, revealing previously inaccessible details.
Open clusters dissipate due to several dynamic processes:
• Galactic tidal forces: gravitational attraction from the galactic disk and giant molecular clouds perturbs the cluster, stripping stars from its periphery.
• Supernovae: massive stars, which initially dominate open clusters, explode as supernovae after a few million years, ejecting gas and dispersing stars.
• Stellar evaporation: gravitational interactions between cluster stars give some of them sufficient velocity 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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