Recent work, based on data from the Hubble Space Telescope (HST ) UV Legacy Survey of Galactic Globular Clusters (GCs), has revealed that all the analyzed clusters host two groups of first-(1G) and second-generation (2G) stars. In most GCs, both 1G and 2G stars host substellar populations with different chemical composition.We compare multi-wavelength HST photometry with synthetic spectra to determine for the first time the average helium difference between the 2G and 1G stars in a large sample of 57 GCs and the maximum helium variation within each of them. We find that in all clusters 2G stars are consistent with being enhanced in helium with respect to 1G. The maximum helium variation ranges from less than 0.01 to more than 0.10 in helium mass fraction and correlates with both the cluster mass and the color extension of the horizontal branch (HB). These findings demonstrate that the internal helium variation is one of the main (second) parameters governing the HB morphology.

The split main sequences (MSs) and extended MS turnoffs (eMSTOs) detected in a few young clusters have demonstrated that these stellar systems host multiple populations differing in a number of properties such as rotation and, possibly, age.We analyze Hubble Space Telescope photometry for thirteen clusters with ages between ∼40 and ∼1000 Myrs and of different masses. Our goal is to investigate for the first time the occurrence of multiple populations in a large sample of young clusters.We find that all the clusters exhibit the eMSTO phenomenon and that MS stars more massive than ∼1.6 M ⊙ define a blue and red MS, with the latter hosting the majority of MS stars. The comparison between the observations and isochrones suggests that the blue MSs are made of slow-rotating stars, while the red MSs host stars with rotational velocities close to the breakup value.About half of the bright MS stars in the youngest clusters are H-alpha emitters. These Be stars populate the red MS and the reddest part of the eMSTO thus supporting the idea that the red MS is made of fast rotators.We conclude that the split MS and the eMSTO are a common feature of young clusters in both Magellanic Clouds. The phenomena of a split MS and an eMSTO occur for stars that are more massive than a specific threshold which is independent of the host-cluster mass.As a by-product, we report the serendipitous discovery of a young SMC cluster, GAL-FOR1.

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters (GCs) has investigated GCs and their stellar populations. In previous papers of this series we have introduced a pseudo two-colour diagram, or "chromosome map" (ChM) that maximises the separation between the multiple populations. We have identified two main classes of GCs: Type I, including ∼83% of the objects, and Type II clusters. Both classes host two main groups of stars, referred to in this series as first (1G) and second generation (2G). Type II clusters host more-complex ChMs, exhibiting two or more parallel sequences of 1G and 2G stars. We exploit spectroscopic elemental abundances from literature to assign the chemical composition to the distinct populations as identified on the ChMs of 29 GCs. We find that stars in different regions of the ChM have different composition: 1G stars share the same light-element content as field stars, while 2G stars are enhanced in N, Na and depleted in O. Stars with enhanced Al, as well as stars with depleted Mg populate the extreme regions of the ChM. We investigate the intriguing colour spread among 1G stars observed in many Type I GCs, and find no evidence for internal variations in light elements among these stars, whereas either a ∼ 0.1 dex iron spread or a variation in He among 1G stars remain to be verified. In the attempt of analysing the global properties of the multiple populations phenomenon, we have constructed a universal ChM, which highlights that, though very variegate, the phenomenon has some common pattern among all the analysed GCs. The universal ChM reveals a tight connection with Na abundances, for which we have provided an empirical relation. The additional ChM sequences observed in Type II GCs, are enhanced in metallicity and, in some cases, s-process elements. Omega Centauri can be classified as an extreme Type II GC, with a ChM displaying three main extended "streams", each with its own variations in chemical abundances. One of the most noticeable differences is found between the lower and upper streams, with the latter, associated with higher He, being also shifted towards higher Fe and lower Li abundances. We publicly release the ChMs.