Split Main Sequence in Clusters

• Split Main Sequence is a feature in color–magnitude diagrams where the main sequence divides into distinct loci due to differences in stellar rotation and chemical composition.
• In young clusters, slow/non-rotating stars form a blue main sequence while faster rotators create a red sequence, as confirmed by photometry and spectroscopy.
• In globular clusters, split main sequences trace chemically distinct populations with variations in helium and light elements, highlighting complex evolutionary histories.

A split main sequence is a color–magnitude-diagram morphology in which the main sequence divides into two or more loci rather than following a single narrow isochrone. In young open and massive clusters, the phenomenon is generally interpreted as the photometric imprint of a broad, and in some cases bimodal, distribution of stellar rotation rates, with a blue main sequence (bMS) of slow or non-rotating stars and a red main sequence (rMS) of more rapidly rotating stars. In old globular clusters, by contrast, split or multiple main sequences trace chemically distinct stellar populations with different helium and light-element abundances rather than a rotation dichotomy (Cordoni et al., 2018, Milone et al., 2011).

1. Phenomenology across stellar-cluster regimes

In young clusters, split or broadened main sequences are now recognized as common. Photometric analyses of Galactic open clusters and Magellanic Cloud clusters show that clusters younger than about $700$ Myr display broadened or split main sequences, while extended main-sequence turnoffs (eMSTOs) persist to older ages, up to about $2.5$ Gyr in the cited surveys (Cordoni et al., 2018). A large HST survey of thirteen young clusters likewise found that all surveyed clusters exhibit eMSTOs and that all except the oldest one show a split main sequence, with the red sequence hosting the majority of upper-main-sequence stars (Milone et al., 2018).

The morphology is not uniform with stellar mass. In the HST survey, the split main sequence is observed only for stars more massive than about $1.6\,\mathcal{M}_\odot$, and the two sequences merge below the “MS kink,” associated in that summary with the onset of convective envelopes near $T_{\rm eff}\sim7500$ K (Milone et al., 2018). Individual clusters show the same broad pattern with cluster-specific details: NGC 1755 has a blue-to-red main-sequence division of about $25\%$ to $75\%$, NGC 1866 has a blue sequence containing about one-third of the main-sequence stars, and NGC 2422 provides a Galactic example of a bifurcated main sequence at an age of about $90$ Myr (Milone et al., 2016, Milone et al., 2016, He et al., 2022).

Spatial trends are present but are not universal. NGC 1866 shows red-main-sequence stars that are more centrally concentrated than blue-main-sequence stars, whereas the ratio of blue to red main-sequence stars in NGC 1850 shows no significant radial change in one analysis (Milone et al., 2016, Correnti et al., 2016). This suggests that split-main-sequence morphology is a robust photometric feature, but that its dynamical imprint can be cluster-dependent.

Cluster regime	Characteristic morphology	Dominant interpretation
Young open and massive clusters	Upper main sequence split into bMS and rMS; often accompanied by eMSTO	Rotation-rate dichotomy
Old globular clusters	Double or multiple main sequences, sometimes persisting to very low masses	Multiple stellar populations with different He and light elements

2. Rotation-driven split main sequences in young clusters

The rotation interpretation rests on both stellar-evolution modeling and direct spectroscopy. Rotation changes stellar structure, makes stars oblate, and produces gravity darkening; the resulting photometric effect is that rapid rotators appear redder, while slow or non-rotators remain bluer (Wang et al., 2022). High-precision photometry and spectroscopy therefore map a bMS–rMS bifurcation onto a slow–fast rotational dichotomy.

A key refinement is that the red main sequence does not necessarily require near-critical rotation. New MESA grids together with SYCLIST comparisons show that initial slow rotation, about $0$–$35\%$ of the linear Keplerian rotation velocity, and intermediate rotation, about $50$–$2.5$0, are adequate to explain the photometric split; the paper argues that earlier claims of ubiquitous near-critical rotation partly reflected differing model conventions for “critical” velocity (Wang et al., 2022). This point is supported observationally in NGC 1850, where spectroscopy of 2,184 stars shows a clear correlation between $2.5$1 and color at fixed magnitude: the average $2.5$2 values are about $2.5$3 for the bMS and about $2.5$4 for the rMS, corresponding to about $2.5$5–$2.5$6 and $2.5$7–$2.5$8 of the critical rotation velocity, respectively (Kamann et al., 2022).

Be stars supply an additional empirical anchor. In the young-cluster survey, about half of the bright main-sequence stars in the youngest clusters are H$2.5$9 emitters, and these Be stars populate the red main sequence and the reddest part of the eMSTO, supporting the interpretation of the rMS as the fast-rotating component (Milone et al., 2018). In NGC 1850, the spectroscopic sample contains about 200 fast rotating Be stars, and shell features indicate that $1.6\,\mathcal{M}_\odot$0 of them are observed through their decretion disks; the same study notes that shell stars can significantly alter MSTO morphology (Kamann et al., 2022).

A recurrent misconception is that split main sequences in young clusters are simply age spreads in disguise. The cluster-by-cluster modeling summarized in the dataset does not support an age-only explanation for the split itself. In NGC 1755, age, helium, or metallicity differences alone do not reproduce the observed bifurcation, whereas two populations with different rotation do reproduce the split main sequence, albeit with some residual tension around the eMSTO (Milone et al., 2016).

3. Split main sequences in globular clusters: helium and light-element populations

In globular clusters, the same photometric term denotes a different physical phenomenon. NGC 6397 exhibits a double main sequence in high-precision multi-band HST photometry, with $1.6\,\mathcal{M}_\odot$1 of the stars on one sequence and $1.6\,\mathcal{M}_\odot$2 on the other. The two sequences are interpreted as a primordial population with field-like composition and a second generation with enhanced sodium and nitrogen, depleted carbon and oxygen, and a slightly enhanced helium abundance of $1.6\,\mathcal{M}_\odot$3 (Milone et al., 2011).

The filter dependence is diagnostic. In NGC 6397, the second sequence is anomalously faint in $1.6\,\mathcal{M}_\odot$4, and modeling with BaSTI isochrones plus ATLAS12 and SYNTHE synthetic spectra indicates that helium variation alone cannot reproduce the full set of color separations. The best match requires both helium and CNO differences, with the F336W band carrying particular leverage because it samples NH absorption and is therefore sensitive to nitrogen enhancement (Milone et al., 2011).

NGC 2808 provides a more complex case. Above the main-sequence bend, HST WFC3/IR imaging recovers the previously known red, middle, and blue main sequences; near the bend they merge, and below it the main sequence splits again into two components containing $1.6\,\mathcal{M}_\odot$5 and $1.6\,\mathcal{M}_\odot$6 of the stars (Milone et al., 2012). The more populous faint infrared sequence is associated with the first stellar generation and primordial helium, whereas the less populous faint sequence is helium-rich and poor in carbon and oxygen. Because the split persists down to very low-mass, fully convective stars, the paper argues that the abundance anticorrelations are primordial rather than produced by evolutionary mixing (Milone et al., 2012).

A distinct and explicitly nonstandard proposal links multiple main sequences in $1.6\,\mathcal{M}_\odot$7 Centauri and NGC 2808 to inhomogeneous Big Bang nucleosynthesis. In that scenario, helium enhancement arises in regions with very high baryon-to-photon ratio, and the model predicts enhancement of heavy elements with mass number around $1.6\,\mathcal{M}_\odot$8 in blue-main-sequence stars (Moriya et al., 2010). This proposal belongs to the history of interpretations, but the observational summaries in the dataset emphasize helium and light-element population differences as the working explanation for the globular-cluster cases.

4. Observational diagnostics and modeling frameworks

The split-main-sequence literature is methodologically heterogeneous because the phenomenon is probed in both photometry and spectroscopy. In young clusters, HST ultraviolet–optical CMDs such as $1.6\,\mathcal{M}_\odot$9 versus $T_{\rm eff}\sim7500$0, Gaia photometry, and Gaia astrometry are used to isolate cluster members and define the blue and red sequences, while artificial-star tests, field subtraction, and differential-reddening corrections are used to demonstrate that the bifurcation is not a photometric artifact (Milone et al., 2016, Cordoni et al., 2018). In globular clusters, ultraviolet and blue passbands are especially powerful because NH, H$T_{\rm eff}\sim7500$1O, and related opacity effects make chemical subpopulations separate strongly in selected color baselines (Milone et al., 2011, Milone et al., 2012).

Direct rotational measurements provide the critical link between CMD morphology and stellar spin. MUSE spectroscopy in NGC 1850 resolves $T_{\rm eff}\sim7500$2 along the split main sequence and confirms the expected color–rotation correlation (Kamann et al., 2022). The NGC 2422 study, using spectra from the Canada-France-Hawaii Telescope and the Southern African Large Telescope, found that $T_{\rm eff}\sim7500$3 is only weakly correlated with CMD position because of contamination from rMS stars with low projected rotational velocities; the same work suggests that some of these slowly rotating rMS stars may hide a binary companion (He et al., 2022).

Model comparisons likewise vary with regime. Young-cluster analyses in the dataset employ Geneva rotating models, SYCLIST, MESA, and MIST to translate rotation distributions into CMD morphology (Milone et al., 2016, Wang et al., 2022). Globular-cluster analyses use BaSTI isochrones and synthetic spectra from ATLAS12 and SYNTHE to connect abundance patterns to observed colors (Milone et al., 2011). The choice of model family is not a technical detail alone: one of the central conclusions of the recent rotation literature is that the inferred spin distribution depends on how “critical” rotation is defined, and direct comparisons between MESA and SYCLIST were required to show that moderate, not necessarily extreme, rotation can recover the observed split (Wang et al., 2022).

5. Competing explanations for the slow-rotator component

The unresolved problem in young clusters is not whether rotation matters, but what produces the slow-rotator population. Several channels have been proposed, and recent work has narrowed the viable parameter space.

Binary interaction remains a major candidate. A recent HST study of NGC 1818, NGC 1850, and NGC 2164 found that the fraction of binaries among blue-main-sequence stars exceeds that among red-main-sequence stars by factors of about $T_{\rm eff}\sim7500$4, $T_{\rm eff}\sim7500$5, and $T_{\rm eff}\sim7500$6, respectively. In the authors’ discussion, this supports a tidal-braking scenario in which blue-main-sequence stars are preferentially members of binaries that have experienced rotational braking (Muratore et al., 2024). Earlier work on NGC 1856 also proposed binary synchronization as a possible origin of the slowly or non-rotating component, specifically for binaries with orbital periods from $T_{\rm eff}\sim7500$7 to $T_{\rm eff}\sim7500$8 days (D'Antona et al., 2015).

Other binary-based scenarios are now more strongly constrained. High-performance $T_{\rm eff}\sim7500$9-body simulations tailored to NGC 1856 found that tidally locked binaries at the relevant age are mostly high mass-ratio systems and populate a much redder sequence than the blue main sequence; the same simulations found that blue straggler stars produced by binary interactions occupy the blue side of the CMD but are far too few to explain the observed bMS fraction (Wang et al., 2023). A direct observational test of the merger hypothesis in three young massive LMC clusters reached the same broad conclusion from a different angle: bright blue stars identified as blue-straggler-like objects have rotational-velocity distributions that differ significantly from those measured on the bMS, so stellar mergers do not play a significant role in forming the split main sequence or the bimodal rotational distribution (Bastian et al., 9 Sep 2025).

A magnetic-braking channel analogous to that of Ap stars has also been tested directly. The 2026 Gaia XP study examined eight young open clusters and asked whether the slow-rotator component should carry the Ap-like $25\%$0 flux depression. The diagnostic was defined as

$25\%$1

with lower $25\%$2 corresponding to a stronger Ap-like signature. A mixture model,

$25\%$3

yielded best-fit Ap-like fractions of $25\%$4 for all eMS stars, $25\%$5 for the bMS, and $25\%$6 for the rMS when all clusters were combined. Because these values are far below the typical bMS fraction of about $25\%$7–$25\%$8, the study disfavors Ap-like magnetic braking as the dominant explanation for the extended or split main sequence, while explicitly leaving open magnetic channels that do not produce a clear Ap-like $25\%$9 signature (Li, 5 Jul 2026).

Pre-main-sequence angular-momentum regulation remains plausible. The same 2026 study notes empirical support for star–disc coupling or disc-locking, citing the result that disc-bearing protostars spin more slowly, and presents this as an alternative channel not ruled out by the lack of an Ap-like signature (Li, 5 Jul 2026). By contrast, variable convective-core overshooting can broaden the MSTO but, according to the overshooting study in the dataset, cannot by itself produce a significant split main sequence at $75\%$0; rapid rotation is still required for a pronounced bifurcation (Yang et al., 2017).

6. Synthesis, ambiguities, and open problems

The current picture is internally differentiated. In young clusters, the split main sequence is best treated as a rotational phenomenon, with spectroscopy, photometry, and stellar-evolution calculations all pointing to a slow- versus fast-rotator dichotomy (Kamann et al., 2022, Wang et al., 2022). In old globular clusters, split and multiple main sequences belong to the broader multiple-population problem, where helium and light-element abundance patterns dominate the interpretation (Milone et al., 2011, Milone et al., 2012). The same photometric label therefore covers at least two distinct astrophysical regimes.

Several ambiguities remain. Rotation explains the split main sequence more cleanly than the eMSTO in some systems: for NGC 1850 and NGC 1866, the cited analyses argue that rotation reproduces the main-sequence split well but does not fully reproduce the MSTO morphology without additional ingredients such as an age spread or other effects (Correnti et al., 2016, Milone et al., 2016). This suggests that split main sequence and eMSTO, though often coexisting, are not necessarily identical diagnostics.

The origin of the slow-rotator population is the central unresolved issue. Recent evidence disfavors two specific dominant channels—binary mergers and Ap-like magnetic braking—yet does not close the case for binaries, pre-main-sequence disc physics, or non-Ap magnetic effects (Bastian et al., 9 Sep 2025, Li, 5 Jul 2026). A plausible implication is that the observed bMS may be composite rather than monolithic, with different clusters or mass ranges weighting different braking channels differently.

The next discriminants are already clear from the cited work. Surface-abundance measurements have been proposed as a way to distinguish single-star and binary channels for Be-star formation (Wang et al., 2022). Time-domain radial-velocity monitoring is identified as crucial for testing whether apparently slow rotators on the rMS are binaries in disguise (He et al., 2022). Direct spectropolarimetric measurements of magnetic fields in main-sequence stars of young clusters are explicitly called for to assess magnetic channels that do not carry a classical Ap-like $75\%$1 depression (Li, 5 Jul 2026). As these tests mature, the split main sequence is likely to remain a central empirical interface between stellar rotation, multiplicity, magnetism, and cluster formation history.