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| Messier 4 captured with the Seestar S30 from Giardini Naxos Sicily in June 2025 |
Messier 4 (M4), located in the constellation Scorpius, is a prominent example of a globular star cluster. When described comparatively with other globular clusters, several features stand out:
1. Proximity: Closest Known Globular Cluster
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M4 is the closest globular cluster to Earth, at a distance of about 7,200 light-years.
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In contrast, many other well-known clusters, such as M13 in Hercules (approx. 22,200 light-years) or Omega Centauri (approx. 15,800 light-years), are much farther away.
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This proximity makes M4 easier to study in detail, especially with ground-based telescopes.
2. Size and Brightness: Modest Compared to Giants
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M4 spans about 75 light-years in diameter, which is relatively small for a globular cluster.
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Its apparent magnitude is +5.9, making it visible to the naked eye under dark skies.
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However, compared to Omega Centauri, which is the largest and brightest globular cluster visible from Earth and contains several million stars, M4 is more modest, containing about 100,000 stars.
3. Core Structure: Loosely Concentrated
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M4 has a Class IX concentration on the Shapley–Sawyer scale (I = most concentrated, XII = least).
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This means its core is relatively loose and less dense than those of more tightly packed clusters like M15 (Class IV) or M30 (Class II).
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As a result, M4 does not show the same dramatic central condensation as these denser clusters.
4. Stellar Content: Old but with Peculiarities
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Like other globular clusters, M4 is very old, with an estimated age of about 12.2 billion years.
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Notably, M4 contains white dwarf stars with well-determined cooling ages, which have been used to help constrain the age of the Milky Way.
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It also contains millisecond pulsars and variable stars, similar to many other globular clusters, but its proximity makes these objects easier to resolve.
5. Horizontal Branch Morphology: Red-Dominated
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M4 has a redder horizontal branch than some other clusters, such as M13, which has a bluer and more extended horizontal branch.
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This difference relates to its metallicity (M4 is relatively metal-rich for a globular cluster, with [Fe/H] ≈ –1.1), affecting the evolution and appearance of its stars.
Summary of Comparisons
| Property | M4 | M13 | Omega Centauri | M15 |
|---|---|---|---|---|
| Distance (light-years) | ~7,200 | ~22,200 | ~15,800 | ~33,600 |
| Stars | ~100,000 | ~300,000 | ~10 million | ~500,000 |
| Core Concentration | Loose (Class IX) | Intermediate (Class V) | Dense (Class III) | Very Dense (Class IV) |
| Apparent Magnitude | +5.9 | +5.8 | +3.7 | +6.2 |
| Metallicity [Fe/H] | ~–1.1 | ~–1.5 | ~–1.6 | ~–2.3 |
| Notable Features | Closest; white dwarfs studied | Blue HB; well studied | Largest GC; multiple pops | Very old; dense core |
Final Notes:
Messier 4 stands out for its closeness, modest size, and observational accessibility, making it a crucial target for studies of stellar evolution and Galactic history. While not as massive or dense as some other clusters, its unique proximity allows astronomers to resolve individual stars with greater precision than in nearly any other globular cluster. - Joel Cairo CEO of the Jodrell Plank Observatory
Combined image showing the relative star densities
of M13 (left) and M4 (Right) (not to size scale)
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