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Messier 4
Messier 4
Messier 4
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Messier 4
Globular star cluster Messier 4
Observation data (J2000 epoch)
ClassIX[1]
ConstellationScorpius
Right ascension16h 23m 35.22s[2]
Declination−26° 31′ 32.7″[2]
Distance6.033 kly (1.850 kpc)[3]
Apparent magnitude (V)5.6[4]
Apparent dimensions (V)26′.0
Physical characteristics
Mass8.4×104[5] M☉
Radius35 light-years [citation needed]
Metallicity[Fe/H] = −1.07[6] dex
Estimated age(12.2 ± 0.2) Gyr[7]
Notable featuresClosest globular cluster
Other designationsNGC 6121[8]
See also: Globular cluster, List of globular clusters

Messier 4 or M4 (also known as NGC 6121 or the Spider Globular Cluster) is a globular cluster in the constellation of Scorpius. It was discovered by Philippe Loys de Chéseaux in 1745 and catalogued by Charles Messier in 1764.[9] It was the first globular cluster in which individual stars were resolved.[9]

Visibility

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M4 is conspicuous in even the smallest of telescopes as a fuzzy ball of light. It appears about the same size as the Moon in the sky. It is one of the easiest globular clusters to find, being located only 1.3 degrees west of the bright star Antares, with both objects being visible in a wide-field telescope. Modestly sized telescopes will begin to resolve individual stars, of which the brightest in M4 are of apparent magnitude 10.8.[9]

Characteristics

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M4 is a rather loosely concentrated cluster of class IX and measures 75 light-years across. It features a characteristic "bar" structure across its core, visible to moderate sized telescopes. The structure consists of 11th-magnitude stars and is approximately 2.5′ long and was first noted by William Herschel in 1783. At least 43 variable stars have been observed within M4.[9]

M4 is approximately 6,000 light-years away,[10] making it the closest globular cluster to the Solar System. It has an estimated age of 12.2 billion years.[7]

In astronomy, the abundance of elements other than hydrogen and helium is called the metallicity, and it is usually denoted by the abundance ratio of iron to hydrogen as compared to the Sun. For this cluster, the measured abundance of iron is equal to [Fe/H] = −1.07±0.1. This value is the logarithm of the ratio of iron to hydrogen relative to the same ratio in the Sun. Thus the cluster has an abundance of iron equal to 8.5% of the iron abundance in the Sun. This strongly suggests this cluster hosts two distinct stellar populations, differing by age. Thus the cluster probably saw two main cycles or phases of star formation.[6]

The space velocity components are (U, V, W) = (−57±3, −193±22, −8±5) km/s. This confirms an orbit around the Milky Way of a period of (116±3) million years with eccentricity 0.80±0.03: during periapsis it comes within (0.6 ± 0.1) kpc from the galactic core, while at apoapsis it travels out to (5.9±0.3) kpc. The inclination is at (an angle of) 23°±6° from the galactic plane, thus it reaches as much as (1.5±0.4) kpc above the disk.[11] When passing through the disk, this cluster does so at less than 5 kpc from the galactic nucleus. The cluster undergoes tidal shock during each passage, which can cause the repeated shedding of stars. Thus the cluster may have been much more massive.[6]

Notable stars

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Photographs by the Hubble Space Telescope in 1995 found white dwarf stars in M4 that are among the oldest known stars in our galaxy; aged 13 billion years. One has been found to be a binary star with a pulsar companion, PSR B1620−26 and a planet orbiting it with a mass of 2.5 times that of Jupiter (MJ).[12] One star in Messier 4 was also found to have much more of the rare light element lithium than expected.[13]

CX-1 Is located in M4. It is known as a possible millisecond pulsar/neutron star binary. It orbits in 6.31 hours.[14]

Spinthariscope analogy

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The view of Messier 4 through a good telescope was likened by Robert Burnham Jr. to that of hyperkinetic luminous alpha particles seen in a spinthariscope.[15]

Central black hole

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Simulation of stellar motions in Messier 4
Simulation of stellar motions in Messier 4, where astronomers suspect that an intermediate-mass black hole could be present.[5][16] If confirmed, the black hole would be in the center of the cluster, and would have a sphere of influence (black hole) limited by the red circle.

In 2023, an analysis of Hubble Space Telescope and European Space Agency's Gaia spacecraft data from Messier 4 revealed an excess mass of roughly 800 solar masses in the center of this cluster, which appears to not be extended. This could thus be considered as kinematic evidence for an intermediate-mass black hole[5][16] (even if an unusually compact cluster of compact objects like white dwarfs, neutron stars or stellar-mass black holes cannot be completely discounted).

References

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See also

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Messier 4

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Messier 4 (M4, also known as NGC 6121) is a situated in the southern constellation , approximately 6,000 light-years (1.9 kpc) from Earth. It is the closest known to the Solar System and comprises a dense collection of several hundred thousand ancient stars gravitationally bound together in a spherical formation. Discovered independently by Swiss astronomer Philippe Loys de Chéseaux in 1745–1746 and French astronomer in 1764, it was the fourth object cataloged in Messier's famous list of deep-sky objects. With an apparent visual magnitude of 5.6, Messier 4 is visible to the under clear, dark skies and appears as a fuzzy patch about 1.3 degrees west of the bright red star (Alpha Scorpii). The cluster spans an apparent diameter of 36 arcminutes on the sky, corresponding to a physical diameter of roughly 75 light-years, and is classified as a loosely concentrated (concentration class IX) due to its relatively sparse core compared to more tightly packed examples. Its is 16h 23m 35s and -26° 31' (J2000.0), placing it ideally for observation from the during late spring and summer. Messier 4 is renowned for hosting some of the oldest known stars, with ages estimated at 12–13 billion years, providing crucial data for calibrating the age of the at around 13–14 billion years. The cluster contains at least 116 confirmed variable stars, including 49 RR Lyrae variables and 23 eclipsing binaries, which have been used to refine measurements through period-luminosity relations. A standout feature is the PSR B1620-26 (also known as PSR J1623-2631), the first such discovered in a in 1987, with a period of 11 milliseconds and forming part of a rare triple system with a companion and a sub--mass approximately 2.5 times the mass of and over 13 billion years old. Observations suggest the presence of an at the cluster's core, with an estimated mass of about 800 solar masses, inferred from the dynamics of in its central region. imaging has resolved intricate details, revealing a central "bar" structure and a high proportion of white dwarfs—up to 40,000—highlighting the cluster's role as a key for studying in dense environments. Studies using EDR3 data refine its distance to around 1.88 kpc (approximately 6,000 light-years), confirming its proximity and making it a prime target for multi-wavelength observations.

Discovery and Observation

Discovery and Cataloging

Messier 4 was first discovered by the Swiss astronomer Philippe Loys de Chéseaux in 1745–1746 during his observations of southern sky objects, where he cataloged it as the 19th entry in his list of nebulae and noted its appearance as a hazy patch without resolved stellar components. This discovery was later incorporated into Nicolas-Louis de Lacaille's 1755 catalog of southern objects as Lacaille I.9, further documenting its position near the bright star in . Independently rediscovered by French astronomer on May 8, 1764, the object was added as the fourth entry in his renowned catalog of nebulae and star clusters, earning the designation M4. Messier described it as "A cluster of very small stars; with a small it can be seen in the form of a ," marking it as the only globular he could partially resolve into individual stars using his 3.5-inch refractor at the time. This cataloging effort was part of Messier's broader project to compile a list of deep-sky objects to aid hunters in distinguishing them from cometary apparitions. The object's nature as a became more apparent through subsequent observations in the late , with British astronomer independently observing it on May 22, 1783, and classifying it as a "very compressed and rich " (Class VI.10 in his system), noting a central bar of 11th-magnitude stars approximately 2.5 arcminutes long. Herschel's larger reflecting telescopes allowed for greater resolution, confirming its stellar composition and contributing to the early understanding of as distinct from true nebulae. By the 19th century, astronomers such as had further sketched and described M4's dense, spherical arrangement of stars, solidifying its status as one of the earliest to be systematically studied and recognized as a resolved stellar aggregate rather than a nebulous entity.

Visibility and Appearance

Messier 4 is situated 1.3° west of the prominent (Alpha Scorpii) in the constellation , making it relatively easy to locate with basic star charts. Its equatorial coordinates for the J2000 epoch are 16h 23m 35.22s and declination −26° 31′ 32.7″. As the nearest to , it offers a favorable position for observation from mid-southern latitudes. With an apparent visual magnitude of 5.6, Messier 4 is invisible to the under typical light-polluted conditions but may appear as a faint, fuzzy patch comparable in angular size to the (about 36 arcminutes across) from pristine . In , it resembles a compact, unresolved glow, often tinged slightly orange due to interstellar dust absorption along the . Small telescopes (under 4 inches) reveal it as a brighter, more defined ball of light, though still unresolved into distinct stars. Resolving individual stars requires telescopes of 4–6 inches in aperture, where the brightest members at magnitude 10.8–11 begin to emerge against the dense backdrop. In larger instruments, a striking linear "bar" of 11th-magnitude stars, stretching about 2.5 arcminutes in length at a position angle of 12°, becomes prominent across the cluster's central region, adding to its distinctive elongated appearance. The overall view through a good evokes a scintillating swarm, likened by astronomer Robert Burnham Jr. to the hyperkinetic flashes of alpha particles in a . Messier 4 is optimally visible during June evenings from the , when culminates high overhead, though it remains observable from northern latitudes during summer months before midnight. Seasonal windows extend from late May through August in the south, with the cluster rising earlier in the evening as the season progresses.

Physical Characteristics

Distance, Size, and Age

Messier 4 lies at a distance of 6,033 light-years (1.850 ± 0.02 kpc) from the , rendering it the nearest to . This measurement, derived from a combination of Early Data Release 3 astrometry, photometry, and literature data, positions the cluster approximately 6.45 ± 0.01 kpc from the and near the interface between the galactic disk and halo. The cluster spans an apparent diameter of 36 arcminutes on the sky, equivalent to a physical of about 63 light-years given its proximity. Messier 4 has a total mass of 8.4 × 10⁴ solar masses and is classified as concentration class IX on the Shapley-Sawyer scale, reflecting its loosely concentrated core with a core radius of roughly 0.45 pc and a half-mass radius of 3.95 pc. Estimates place the age of Messier 4 at 12.2 ± 0.2 billion years, obtained through main-sequence fitting to color-magnitude diagrams and analysis of cooling sequences, which provide independent constraints on the cluster's formation timeline. These methods highlight the cluster's status as one of the oldest stellar systems in the , predating the Solar System by nearly three times.

and Orbital Parameters

Messier 4 exhibits a of [Fe/H] = −1.07, indicating an iron abundance approximately 8.5% that of the Sun, a value characteristic of ancient halo globular clusters formed in the early . This low metal content reflects the cluster's origin from primordial gas with limited enrichment from previous stellar generations, aligning with its estimated age of around 12 billion years. The cluster follows a highly eccentric orbit around the , with an of 116 ± 3 million years. Its eccentricity measures 0.80 ± 0.03, leading to a perigalacticon distance of roughly 0.62 kpc and an apogalacticon of about 6.52 kpc from . Additionally, the is inclined at 23° ± 6° relative to the , positioning Messier 4 as a member of the inner halo population. This eccentric trajectory brings the cluster into close proximity with the Galactic disk during perigalacticon passages, subjecting it to strong tidal forces from the and the Galaxy's . Such interactions promote ongoing mass loss through stellar stripping and evaporation, contributing to the cluster's observed low present-day mass of approximately 8.4 × 10^4 solar masses despite its ancient formation. These dynamics highlight Messier 4's role in tracing the Milky Way's halo assembly and evolution.

Stellar Population

Composition and Variables

Messier 4 contains an estimated 100,000 stars, the majority of which are low-mass main-sequence stars with masses between approximately 0.09 and 0.65 solar masses, alongside a population of evolved red giants. The stellar population is dominated by K- and M-type dwarfs on the lower main sequence and features two distinct populations with slightly varying metallicities, reflecting the cluster's ancient, metal-poor nature with an overall metallicity of [Fe/H] = -1.16. The horizontal branch exhibits a predominantly blue morphology, consistent with the helium core flash that occurs in low-mass stars during the red giant branch phase, leading to a well-defined sequence of hot helium-burning stars. Approximately 95 variable stars have been confirmed within Messier 4 (from 116 suspects), including a significant number of RR Lyrae variables that serve as standard candles for distance calibration due to their well-known periods and luminosities. Of these, 47 are RR Lyrae stars—approximately 36 of the fundamental mode (RRab) and 11 of the first overtone (RRc)—with pulsation periods ranging from 0.23 to 0.63 days; their light curves display characteristic asymmetric shapes with rapid rises and slower declines typical of radial pulsations in horizontal branch stars. The cluster has a projected half-light radius of 4.33 arcminutes and a core radius of 1.16 arcminutes, corresponding to a relatively low central density and sparse core typical of concentration class IX globular clusters. Resolved observations indicate a binary fraction of about 15% among the , higher than in many other globular clusters, as determined from photometric and astrometric monitoring of brightness variations and proper motions.

Notable Objects

One of the most remarkable objects in Messier 4 is the triple system PSR B1620−26, comprising a millisecond pulsar and a low-mass white dwarf in a close orbit with a period of approximately 191 days, orbited by a circumbinary gas giant exoplanet designated PSR B1620−26 b. The planet, nicknamed Methuselah, has a mass of about 2.5 Jupiter masses and completes its highly eccentric orbit around the binary pair every roughly 100 years at a semi-major axis of 23 AU, surviving in the dense cluster environment through dynamical capture rather than in situ formation. This system, located just outside the cluster core, provides key insights into planetary survival in globular clusters and dates to over 12 billion years old, comparable to the cluster's age. A standout stellar anomaly in Messier 4 is a lithium-rich on the cluster's turn-off region, exhibiting an unusually high abundance of log ε(Li) ≈ 2.8, far exceeding the typical depletion expected from standard models for such ancient, metal-poor stars. This detection challenges theories of lithium preservation or production, suggesting possible external pollution from prior stellar generations or internal mixing processes not accounted for in canonical models. Among the cluster's X-ray emitting sources, CX-1 stands out as a variable Chandra-detected object with a of about 8 × 10^{31} erg s^{-1} in the 0.5–2.5 keV band, likely a cataclysmic variable or low-mass involving a accretor. Its hard power-law spectrum (photon index ≈ 1.0) and optical counterpart showing Hα excess indicate ongoing , consistent with a compact binary nature, though no precise has been confirmed. The brightest resolved stars in Messier 4 are red giants along the cluster's , reaching apparent visual magnitudes of 11.0 to 12.0, with several exhibiting variability as part of the cluster's approximately 95 confirmed variables, including early-designated examples like V1 from classical surveys. Post-2020 astrometric and photometric analyses have revealed an elevated fraction of binary systems among these giants, highlighting unusual dynamical interactions in the cluster's core, though no additional confirmed exoplanets beyond have been identified.

Scientific Studies

Stellar Evolution and White Dwarfs

Messier 4 hosts a substantial population of white dwarfs, the end products of low- to intermediate-mass , which have been identified primarily through deep (HST) imaging campaigns initiated in 1995. Early HST observations resolved the first extensive cooling sequence of white dwarfs in a globular cluster, revealing dozens of these faint objects in a small field, with subsequent deeper exposures identifying over 270 white dwarfs down to visual magnitudes of approximately V = 30. These white dwarfs, remnants of stars with initial masses up to about 8 solar masses, provide a snapshot of the cluster's ancient , as their progenitors have long since evolved off the . The cooling sequences of these , traced through their luminosity function, exhibit a sharp rise in number counts at faint magnitudes (I > 25.5), characteristic of a single-burst with minimal ongoing . HST's high resolution has been crucial for isolating these objects at apparent magnitudes fainter than 28, enabling detailed modeling of their cooling tracks and confirming the cluster's age at 12.2 ± 0.5 billion years through comparisons with standard white dwarf evolutionary models. This age aligns with isochrone fits to the main-sequence turnoff and underscores the uniformity of in this ancient environment. Due to Messier 4's advanced age, observations of its surviving low-mass main-sequence stars and white dwarf progenitors offer unique insights into the low-mass end of the initial mass function (IMF), where higher-mass stars have evolved away. HST photometry of the lower main sequence reveals a shallow IMF slope of α ≈ 0.75 (where dN/dM ∝ M^{-α}) down to masses of ~0.09 solar masses, indicating a slow rise in the number of low-mass stars without a clear turnover in this range. Theoretical models suggest a potential IMF turnover around 0.1–0.2 solar masses, influenced by the cluster's dynamics and binary interactions, which preferentially probe this regime in old systems like Messier 4. A 2025 JWST study of the cooling sequence in Messier 4, using imaging, confirmed the presence of collision-induced absorption effects and observed a broader color dispersion at faint magnitudes, possibly due to excess. This analysis refined the age estimate and provides deeper insights into atmospheres in dense environments. Overall, the population in Messier 4 implies formation occurred within the first billion years after the , providing constraints on early efficiency and chemical enrichment.

Central Black Hole

A 2023 study utilizing data from the and mission revealed anomalies in the velocity dispersion of stars near of Messier 4, indicating the presence of an excess dark mass concentrated at the cluster's . This analysis employed Bayesian modeling to derive the mass profile, assuming isotropic motions in and tangential (β ≈ -0.4 ± 0.1) in the outskirts, which robustly pointed to a beyond what is expected from visible stars alone. The inferred dark mass is approximately 800 ± 300 solar masses (M⊙M_\odot), distributed within a compact radius of about 0.016–0.034 pc (roughly 3,300–7,000 ), suggesting either an (IMBH) or a dense cluster of unresolved stellar remnants. This scale implies a point-like or highly concentrated object, as Hubble's high-precision over 12 years rules out a more extended distribution of ordinary or a stable cluster of stars and smaller black holes due to dynamical instabilities. No direct electromagnetic signatures, such as emission from accretion, have been detected for this object, consistent with a quiescent IMBH or a non-accreting remnant . Alternative interpretations include an extended population of stellar remnants, such as white dwarfs, neutron stars, or stellar-mass black holes, though these face challenges in explaining the observed without rapid segregation or disruption. The remains open, as the data do not conclusively distinguish between an IMBH and a remnant cluster, highlighting the need for higher-resolution dynamical probes. The study's authors propose additional observations to refine the mass profile and nature of this central feature.

References

  1. https://academic.oup.com/mnras/article/505/4/5957/6283731
  2. https://science.nasa.gov/asset/hubble/globular-cluster-messier-4-m4/
  3. http://www.messier.seds.org/m/m004.html
  4. https://www.universetoday.com/articles/messier-4
  5. http://www.astro.utoronto.ca/~cclement/cat/C1620m264
  6. https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-4/
  7. https://www.deepskycorner.ch/obj/m4.en.php
  8. http://www.messier.seds.org/xtra/history/m-cat.html
  9. https://simbad.u-strasbg.fr/simbad/sim-id?Ident=Messier+4
  10. https://science.[nasa](/page/NASA).gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-4/
  11. https://.u-strasbg.fr/simbad/sim-id?Ident=Messier+4
  12. https://www.messier-objects.com/messier-4/
  13. https://archive.org/details/burnhams-celestial-handbook-volume-3
  14. https://earthsky.org/clusters-nebulae-galaxies/find-m4-a-globular-cluster-by-the-scorpions-heart/
  15. https://ui.adsabs.harvard.edu/abs/2021MNRAS.505.5957B/abstract
  16. https://people.smp.uq.edu.au/HolgerBaumgardt/globular/parameter.html
  17. https://astropixels.com/globularclusters/M4-01.html
  18. https://ui.adsabs.harvard.edu/abs/2018MNRAS.478.1520B/abstract
  19. https://academic.oup.com/mnras/article/478/2/1520/4990650
  20. https://iopscience.iop.org/article/10.1086/342528
  21. https://arxiv.org/abs/astro-ph/0401443
  22. https://iopscience.iop.org/article/10.1088/0004-637X/748/1/62
  23. https://physics.mcmaster.ca/~harris/mwgc.dat
  24. https://people.smp.uq.edu.au/HolgerBaumgardt/globular/orbits.html
  25. https://arxiv.org/pdf/astro-ph/0205086.pdf
  26. https://ui.adsabs.harvard.edu/abs/2012ApJ...748...62V/abstract
  27. https://www.astro.utoronto.ca/~cclement/cat/C1620m264
  28. https://www.cfa.harvard.edu/news/binary-stars-globular-cluster-messier-4
  29. https://arxiv.org/abs/astro-ph/0309502
  30. https://science.nasa.gov/missions/hubble/oldest-known-planet-identified/
  31. https://www.aanda.org/articles/aa/full_html/2012/03/aa17709-11/aa17709-11.html
  32. https://doi.org/10.1086/424832
  33. https://doi.org/10.1086/342528
  34. https://doi.org/10.1002/asna.20240125
  35. https://doi.org/10.1086/342529
  36. https://doi.org/10.1093/mnras/stad1068
  37. https://science.nasa.gov/missions/hubble/nasas-hubble-hunts-for-intermediate-sized-black-hole-close-to-home/
  38. https://www.sci.news/astronomy/messier-4-intermediate-mass-black-hole-11947.html
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