Milky Way Globular Cluster Atlas · The Library

Globular Cluster Knowledge Base

A guide to the physical parameters in our catalog and the most notable globular clusters of the Milky Way — with values cross-checked against the literature.

Parameters

The catalog merges four sources (Harris 1996/2010; Vasiliev & Baumgardt 2021; Baumgardt & Hilker 2018; Bica et al. 2019) plus our own orbit integration. Symbol definitions and units below; the source column gives the primary reference for each field.

Position & distance

SymbolMeaningUnitSource
l, bGalactic longitude and latitude (J2000) — the cluster's direction on the sky in Galactic coordinates.degHarris 1996/2010 ↗
RA, DecEquatorial right ascension and declination (J2000) from Gaia EDR3 astrometry.degVB21 ↗
dHeliocentric distance — how far the cluster is from the Sun.kpcBH18 ↗ / VB21 ↗ / Harris ↗
RgcGalactocentric radius — distance from the Galactic centre (R☉ = 8.275 kpc adopted).kpcderived
X, Y, ZGalactocentric Cartesian coordinates (right-handed; +x toward the Sun from the centre).kpcderived

Photometry & colours

SymbolMeaningUnitSource
VIntegrated apparent V-band magnitude — total brightness as seen from Earth.magHarris 1996/2010 ↗
MVIntegrated absolute V-band magnitude — brightness corrected for distance (distance modulus (m−M)V and extinction applied).magHarris 1996/2010 ↗
(B−V), (U−B), (V−R), (V−I)Integrated colours — proxies for the cluster's stellar population and reddening.magHarris 1996/2010 ↗
E(B−V)Reddening — interstellar extinction in the direction of the cluster, measured from colour excess.magHarris 1996/2010 ↗

Chemistry

SymbolMeaningUnitSource
[Fe/H]Iron abundance relative to the Sun, log scale. Metal-rich clusters (≈ −0.3 to −1) live in the bulge/disc; metal-poor halo clusters reach ≈ −2.5. The distribution is famously bimodal.dexHarris 1996/2010 ↗

Kinematics

SymbolMeaningUnitSource
vrHeliocentric radial velocity — motion along the line of sight.km s⁻¹Harris ↗ / BH18 ↗
μα*, μδProper motion — tangential motion on the sky (μα* = μα cos δ). Precise values from Gaia EDR3 cluster-member averages.mas yr⁻¹VB21 ↗
ϖParallax — geometric distance indicator from Gaia.masVB21 ↗

Structure

SymbolMeaningUnitSource
rhProjected half-light (half-mass) radius — radius enclosing half the cluster light; a measure of compactness.pcBH18 ↗
rcCore radius — where the surface brightness falls to half its central value.pc / arcminHarris
cConcentration — c = log(rt/rc), ratio of tidal to core radius; high c means a strongly concentrated cluster.—Harris
log Tc, log ThLogarithmic core and half-mass relaxation times — the timescale on which stellar encounters erase memory of the initial conditions.yrHarris
μVCentral surface brightness.mag arcsec⁻²Harris

Dynamics

SymbolMeaningUnitSource
MTotal cluster mass (from fitting stellar velocity dispersion profiles; ~×10⁴–10⁶ M☉).M☉BH18 ↗
M/LVV-band mass-to-light ratio — a proxy for the stellar initial mass function and dark remnants.M☉/L☉BH18 ↗
σ0Central (mass-weighted, 1-D) velocity dispersion — random stellar motion in the core; highest in the most massive clusters.km s⁻¹BH18 ↗
vescCentral escape velocity — velocity needed to leave the cluster from its core.km s⁻¹BH18 ↗
log TrhLogarithmic half-mass relaxation time.yrBH18 ↗

Orbit (this work)

SymbolMeaningUnitSource
eOrbital eccentricity — from back-integration in the MWPotential2014 Galactic potential (0 = circular, 1 = radial).—this work
rperi, rapoPericentre and apocentre — closest and farthest Galactocentric distances along the orbit.kpcthis work
zmaxMaximum height above the Galactic plane reached by the orbit.kpcthis work
retroWhether the orbit is retrograde (counter to Galactic rotation).—this work

Observational

SymbolMeaningUnitSource
NstarNumber of Gaia-detected member stars used for the cluster mean astrometry.—VB21 ↗

Records & special objects

Values from our master catalog (distances: BH18/Gaia/Harris; orbits: this work), cross-checked against the literature. Where the literature disagrees, both are given.

Nearest
M4 NGC 6121
d ≈ 2.1 kpc
The closest globular cluster to the Sun (lit.: 1.7–2.2 kpc). A metal-poor, low-concentration cluster in Scorpius, visible in binoculars. Hosts the exoplanet PSR B1620−26 b.
Harris 1996/2010 ↗ · Peterson et al. 1995 ↗
Farthest
AM 1 E 1
d ≈ 119 kpc
The most distant classical Galactic globular cluster, far out in the halo. Faint (V ≈ 15.7) and difficult to observe.
Harris 1996/2010 ↗ · Carraro et al. 2021
Brightest & most massive
ω Centauri NGC 5139
MV = −10.2 · M ≈ 3.5×10⁶ M☉
The most luminous and most massive globular cluster in the Milky Way, with a spread of stellar metallicities. Widely believed to be the stripped core of an accreted dwarf galaxy — the largest surviving remnant of a disrupted satellite.
Harris 1996/2010 ↗ · BH18 ↗ · Bekki & Freeman 2003 ↗
Most metal-poor
M92 NGC 6341 / M15 NGC 7078
[Fe/H] ≈ −2.3 to −2.4
Among the most metal-poor classical clusters ([Fe/H] ≈ −2.3 dex; M15 often quoted at −2.4). Such clusters sample the earliest chemical enrichment of the Galaxy, shortly after the Big Bang nucleosynthesis floor.
Harris 1996/2010 ↗ · Carretta et al. 2009 ↗
Most metal-rich
Liller 1 / NGC 6528 / Terzan 5
[Fe/H] ≈ −0.3 to 0
The most metal-rich clusters are crowded, heavily reddened objects near the Galactic centre. Liller 1 ([Fe/H] ≈ −0.3 in the literature) is embedded in one of the densest stellar fields of the bulge.
Saracino et al. 2015 ↗ · Ferraro et al. 2009 ↗
Closest to Galactic centre
NGC 6522 Baade's Window
Rgc ≈ 0.6 kpc
Sitting in Baade's Window — a rare low-extinction sightline through the bulge — NGC 6522 is the cluster closest to the Galactic centre (Rgc = 0.58 kpc, Minniti et al. 2021), with Liller 1 close behind.
Minniti et al. 2021 ↗ · Harris 1996/2010 ↗
Core-collapsed
M15 · M30 · M70 · NGC 6397 · NGC 6752 · NGC 6624
post-core-collapse
Clusters whose cores have contracted beyond the classical King-model limit, likely through dynamical friction and binary heating. M15 and NGC 6397 are the best-studied examples.
Trager, King & Djorgovski 1995 ↗ · Harris 1996/2010 ↗
Exoplanet host
M4 — PSR B1620−26 b "Methuselah"
planet around pulsar + WD binary
The only planet known in a globular cluster: a ~2.5 MJup planet orbiting a pulsar–white-dwarf binary, ~12.4 kly away in M4. Its age (~12.7 Gyr) makes it one of the oldest known planets.
Sigurdsson et al. 2003 ↗
Largest proper motion
M4 NGC 6121
|μ| ≈ 19 mas yr⁻¹
Its proximity gives M4 the largest measured proper motion among classical globular clusters (μδ ≈ −19 mas yr⁻¹ from Gaia EDR3).
VB21 ↗
Most Gaia members
ω Centauri
≈ 53,000 stars
With its huge mass, ω Cen contributes the largest number of Gaia EDR3 member stars used for cluster astrometry.
VB21 ↗
Most reddened
Liller 1
E(B−V) ≈ 3.0
Lying behind dense bulge dust, Liller 1 suffers among the heaviest reddening of any cluster (E(B−V) ≈ 3 mag), requiring infrared observations.
Harris 1996/2010 ↗ · Saracino et al. 2015 ↗
Orbits (this work)
47 Tuc & ω Cen
e = 0.12 vs 0.69
47 Tuc follows a nearly circular disc-like orbit (e ≈ 0.12, zmax ≈ 3 kpc), while ω Cen has a highly eccentric halo orbit (e ≈ 0.69) — consistent with an accreted origin. Of the 135 clusters with orbits, ~100 are retrograde in our integration.
this work (MWPotential2014)
⚠ Orbit-based "records" (eccentricity, apo/pericentre, zmax) depend on the assumed Galactic potential and the 6-D phase-space data; ~8 clusters are unbound in our integration and are excluded from record lists. Treat orbit values as indicative.

Seminal works & open questions

Globular clusters are laboratories for stellar and Galactic evolution. Below are the field's most influential papers and the questions that remain unanswered.

Seminal works

WorkContributionReference
Baade (1944) Introduced the concept of stellar Population II from observations of M13 and the Andromeda halo — the foundation of stellar-population astronomy. ApJ 100, 137 ↗
King (1966) The King model — a lowered-Maxwellian dynamical model whose surface-brightness profiles fit most clusters and define the concentration parameter c. Still the standard analytic description of cluster structure. AJ 71, 64 ↗
Searle & Zinn (1978) Argued from the absence of an outer-halo abundance gradient that the halo formed by accretion of transient protogalactic fragments — the modern framework for galaxy-assembly studies with clusters. ApJ 225, 357 ↗
Zinn (1985) Systematized the bimodal metallicity distribution of the cluster system, separating disc/bulge from halo populations — the key to mapping the Galaxy's components. ApJ 293, 424 ↗
Carretta et al. (2009) The Na–O anticorrelation survey of 19 clusters: the definitive chemical evidence that clusters host multiple stellar populations (first- and second-generation stars). A&A 505, 117 ↗
Bekki & Freeman (2003) Showed ω Cen can form as the nucleus of an ancient dwarf galaxy stripped by the Milky Way — the template for accreted clusters and the origin of its metallicity spread. MNRAS 346, L11 ↗
Odenkirchen et al. (2003) Detected the Pal 5 tidal stream, demonstrating that clusters are being torn apart by the Milky Way — streams as tracers of Galactic potential and assembly history. AJ 126, 2385 ↗
Gratton, Carretta & Bragaglia (2012) The canonical review of multiple populations: observations (light-element anticorrelations, CNO spreads) and the puzzle of the missing first-generation stars. A&ARv 20, 50 ↗
Bastian & Lardo (2018) The modern observational review of multiple populations, testing each formation model (AGB winds, fast-rotating massive stars, early disc accretion) against photometric and spectroscopic data. ARA&A 56, 83 ↗
Häberle et al. (2024) Strong evidence for an intermediate-mass black hole (~8,200 M☉) at the centre of ω Cen, from seven fast-moving stars in Hubble archival data — a long-sought "missing link" between stellar and supermassive black holes. Nature 631, 521 ↗
Richer et al. (1997) With WFPC2 on HST, isolated 258 white dwarfs in M4 — the first direct imaging of a globular-cluster white-dwarf population, opening the field. 1997ApJ...484..741R ↗
Hansen et al. (2002) Fitted the M4 white-dwarf cooling sequence and derived a cluster age of 12.7 ± 0.7 Gyr — an independent, main-sequence-free lower limit on the age of the Universe. 2002ApJ...574L.155H ↗
Richer et al. (2006) Ultra-deep HST imaging of NGC 6397 reached the faintest main-sequence stars and the white-dwarf luminosity function, constraining the low-mass initial mass function. 2006Sci...313..936R ↗
Hansen et al. (2007) The NGC 6397 cooling sequence in full: cluster distance, the initial–final mass relation, and an independent age — white dwarfs as precision cosmochronometers. 2007ApJ...671..380H ↗
Hansen et al. (2013) Cooling-sequence ages of 47 Tuc vs NGC 6397 differ by ~2 Gyr — the metal-rich cluster is younger, a direct, model-light measurement of the cluster age–metallicity relation. 2013Natur.500...51H ↗
Harris (1996/2010); BH18; VB21 The modern data foundation: the standard parameter catalog, dynamical mass/structural fits, and Gaia EDR3 astrometry — the sources behind this atlas. see References

Open questions

The central mystery
Origin of multiple populations
Why do stars born together in one cluster show huge light-element differences (Na–O, C–N anticorrelations)? The polluting "first generation" is nearly absent today, and no formation model (AGB winds, fast rotators, disc accretion) reproduces all observations.
Bastian & Lardo 2018 ↗ · Gratton et al. 2012 ↗
Missing link
Do clusters host intermediate-mass black holes?
The 2024 ω Cen result (≈8,200 M☉) is the strongest case, but M15 and other candidates remain disputed. IMBHs would connect stellar-mass to supermassive black holes and constrain cluster formation.
Häberle et al. 2024 ↗
Formation channels
How and when did clusters form?
Were clusters born in giant molecular-cloud fragments or as dwarf-galaxy nuclei? The bimodal age/metallicity structure of the halo suggests multiple channels — but the mapping to individual clusters is incomplete.
Searle & Zinn 1978 ↗ · Forbes et al. 2018 ↗
Stellar archaeology
Missing low-mass stars & blue stragglers
Clusters are dynamically depleted of low-mass stars and enriched in blue stragglers and binaries — but the initial mass function, mass-loss history and binary fractions remain poorly constrained.
Meylan & Heggie 1997 ↗
Assembly history
Which clusters were accreted?
Gaia-era proper motions link clusters to streams and dwarf galaxies (ω Cen, NGC 1851, M54/Sagittarius...), but the full census of accreted clusters and their parent satellites is still being assembled.
VB21 ↗ · Malhan et al. 2022 ↗
Cosmology
Clusters as cosmic clocks
At ~12–13 Gyr, clusters set a firm lower limit on the age of the Universe — but systematic uncertainties in distance and stellar models still limit the precision of this cosmological constraint.
VandenBerg et al. 2013 ↗
Open questions are phrased conservatively: several "answers" (e.g. the IMBH in ω Cen) are strong but still debated, and the multiple-population origin remains the field's biggest unsolved problem.

Data & limits

QuantityPrimary sourceNotes
l, b, V, MV, [Fe/H], E(B−V), vr, c, log Tc/h, μVHarris (1996, 2010 revision)Standard reference catalog (147 clusters).
μα*, μδ, ϖ, NstarVasiliev & Baumgardt (2021)Gaia EDR3 cluster-member means (170 clusters).
d, M, M/LV, rh, σ0, vesc, log TrhBaumgardt & Hilker (2018)Dynamical fits to velocity-dispersion profiles.
e, rperi, rapo, zmax, retroThis work3 Gyr back-integration in MWPotential2014 (Bovy 2015), calibrated to vc(R☉) = 238 km/s.

Limitations: distance, proper-motion and radial-velocity uncertainties are not propagated into orbit parameters; the potential is static and axisymmetric (no bar, arms or LMC tide); a few clusters lack Gaia 6-D data and therefore have no orbit.

References

  1. Harris, W. E. (1996). A catalog of parameters for globular clusters on the Milky Way. AJ, 112, 1487.bibcode: 1996AJ....112.1487H (2010 revision) ↗
  2. Vasiliev, E., & Baumgardt, H. (2021). Gaia EDR3 view on galactic globular clusters. MNRAS, 505, 5978.bibcode: 2021MNRAS.505.5978V ↗
  3. Baumgardt, H., & Hilker, M. (2018). A catalogue of masses, structural parameters, and velocity dispersion profiles of 112 Galactic globular clusters. MNRAS, 478, 1520.bibcode: 2018MNRAS.478.1520B ↗
  4. Bica, E., Pavani, D. B., Bonatto, C. J., & Lima, E. F. (2019). A multi-band catalog of 10978 star clusters, associations, and candidates in the Milky Way. AJ, 157, 12.bibcode: 2019AJ....157...12B ↗
  5. Bovy, J. (2015). galpy: A Python library for Galactic dynamics. ApJS, 216, 29 (MWPotential2014).bibcode: 2015ApJS..216...29B ↗
  6. Minniti, D., et al. (2021). Survival in an extreme environment: Which is the closest globular cluster to the Galactic centre? A&A, 647, L4.bibcode: 2021A&A...648A..86M ↗
  7. Saracino, S., et al. (2015). GEMINI/GeMS observations unveil the structure of the heavily obscured globular cluster Liller 1. ApJ, 806, 152.bibcode: 2015ApJ...806..152S ↗
  8. Ferraro, F. R., et al. (2009). The cluster Terzan 5 as a remnant of a primordial building block of the Galactic bulge. Nature, 462, 483.bibcode: 2009Natur.462..483F ↗
  9. Carretta, E., et al. (2009). Na-O anticorrelation and HB. VII. The chemical composition of first and second-generation stars in 19 globular clusters. A&A, 505, 117.bibcode: 2009A&A...505..117C ↗
  10. Bekki, K., & Freeman, K. C. (2003). Formation of ω Centauri from an ancient nucleated dwarf galaxy. MNRAS, 346, L11.bibcode: 2003MNRAS.346L..11B ↗
  11. Sigurdsson, S., et al. (2003). A young white dwarf companion to pulsar B1620−26: evidence for early planet formation. Science, 301, 193.bibcode: 2003Sci...301..193S ↗
  12. Trager, S. C., King, I. R., & Djorgovski, S. (1995). Catalogue of Galactic globular-cluster surface-brightness profiles. AJ, 109, 218.bibcode: 1995AJ....109..218T ↗
  13. Peterson, R. C., Rees, R. F., & Cudworth, K. M. (1995). The distance to M4. ApJ, 443, 124.bibcode: 1995ApJ...443..124P ↗
  14. Baade, W. (1944). The resolution of Messier 32, NGC 205, and the central region of the Andromeda nebula. ApJ, 100, 137.bibcode: 1944ApJ...100..137B ↗
  15. King, I. R. (1966). The structure of star clusters. III. Some simple dynamical models. AJ, 71, 64.bibcode: 1966AJ.....71...64K ↗
  16. Searle, L., & Zinn, R. (1978). Compositions of halo clusters and the formation of the galactic halo. ApJ, 225, 357.bibcode: 1978ApJ...225..357S ↗
  17. Zinn, R. (1985). The globular cluster system of the Galaxy. IV. The halo and disk subsystems. ApJ, 293, 424.bibcode: 1985ApJ...293..424Z ↗
  18. Gratton, R. G., Carretta, E., & Bragaglia, A. (2012). Multiple populations in globular clusters. A&ARv, 20, 50.bibcode: 2012A&ARv..20...50G ↗
  19. Bastian, N., & Lardo, C. (2018). Multiple stellar populations in globular clusters. ARA&A, 56, 83.bibcode: 2018ARA&A..56...83B ↗
  20. Odenkirchen, M., et al. (2003). The extended tails of Palomar 5: A 10° arc of tidal debris. AJ, 126, 2385.bibcode: 2003AJ....126.2385O ↗
  21. Häberle, M., et al. (2024). Fast-moving stars around an intermediate-mass black hole in ω Centauri. Nature, 631, 521.bibcode: 2024Natur.631..285H ↗
  22. Meylan, G., & Heggie, D. C. (1997). Internal dynamics of globular clusters. A&ARv, 8, 1.bibcode: 1997A&ARv....8....1M ↗
  23. Malhan, K., et al. (2022). The global dynamical atlas of the Milky Way mergers. ApJ, 926, 107.bibcode: 2022ApJ...926..107M ↗