Open frontiers

What we still do not know

The story of the nucleosynthesis of the chemical elements is one of the great success stories of twentieth-century astrophysics and of the first years of the new century. Seventy years after the publication of Burbidge, Burbidge, Fowler and Hoyle in 1957, we know how, where and when practically all the elements of the periodic table were produced: hydrogen-helium fusion in the first Pop III stars, the quiescent burnings from the main sequence to Si-burning in massive stars, slow neutron captures in AGB and massive stars, rapid neutron captures in neutron-star mergers and exotic supernovae, the gamma-process in the shells of core-collapse SNe, explosive nucleosynthesis from SNe Ia. The residual uncertainties are by now quantitative — of order $5$-$30\%$ on specific yields, on the relative mix of different sites — and no longer structural, about the basic mechanisms.

And yet important things remain that we do not know, and they animate active research. Where exactly is gold born in the universe, beyond the confirmed NSM? Why is there less lithium in Pop II stars than Big Bang nucleosynthesis predicts? Do stars of the very first generations — pure, zero-metallicity Pop III — still exist? What do the most exotic explosive sites produce — collapsars, magnetar SNe, pair-instability supernovae? Is there really a third neutron-capture process between s and r, and where does it operate? This concluding chapter reviews the most debated open problems of the field and the way the new instruments of the coming decade — JWST, ELT, Einstein Telescope, Athena, FRIB, FAIR, LUNA-MV, JUNA, ELI-NP — will be able to clarify them quantitatively over the next five to fifteen years.

A useful metric of the completeness of the field is the fraction of nuclides in the cosmic abundance curve for which we know the source to within a factor of 2. The value estimated today is $\sim 95\%$: the residual uncertainties concern above all the p-rich nuclei (chapter 5), the heavy isotopes of the strong s-process (in particular $^{208}\mathrm{Pb}$ at ultra-low metallicity), some rare isotopes of the Fe-Ni peak produced by SNe Ia (Mn, Cr, $^{60}\mathrm{Ni}$), and the neutron-rich isotopes of the first r-process peak ($A \sim 80$-$130$) produced by the weak r-process. The remaining $5\%$ is the object of active research and calls into play new sites (i-process, $\nu$-p, sub-Chandra SNe Ia) or yields revisable at the factor-of-2 level.

The main open problems and the key instruments for their resolution are summarized in the following table:

The cosmological lithium problem

The amount of lithium observed in the oldest, most metal-poor stars — the so-called Spite plateau, $A(\mathrm{Li}) \approx 2{.}2$ in EMP stars with $T_{\mathrm{eff}} > 5800$ K — is about three times lower than the amount predicted by Big Bang nucleosynthesis combined with the Planck cosmological parameters. The BBN+Planck prediction gives $\log\epsilon(\mathrm{Li}) \approx 2{.}7$ for the primordial composition ($^{7}\mathrm{Li}/\mathrm{H} \approx 5 \times 10^{-10}$), while the observed plateau sits stably $0{.}5$ dex lower. The discrepancy has been known for twenty years and has animated an intense debate among stellar, nuclear and cosmological explanations; the consensus emerging over the last decade attributes the deficit to mechanisms of stellar depletion during the main sequence of Pop II stars.

The state of the problem, already discussed in detail in chapter 2 on Big Bang nucleosynthesis, rests on three families of proposed and tested explanations. Stellar depletion in Pop II stars through turbulence-regulated atomic diffusion (Korn et al. 2007 [Korn et al. 2007] in NGC 6397, Mucciarelli et al. 2022 [Mucciarelli et al. 2022] in multiple globular clusters) is today considered the dominant mechanism according to the emerging consensus: the combination of gravitational diffusion, thermal settling and sub-surface turbulent mixing can progressively destroy $^{7}\mathrm{Li}$ in MS stars at sufficient effective temperatures, producing photospheric abundance reductions of $\sim 0{.}4$ dex on Gyr timescales — enough to reconcile the observed plateau with the BBN+Planck prediction within the residual systematic uncertainty. Nuclear errors in the key cross sections of the BBN network ($^{3}\mathrm{He}(\alpha,\gamma)^{7}\mathrm{Be}$, $^{7}\mathrm{Be}(n,p)^{7}\mathrm{Li}$, $^{7}\mathrm{Be}(n,\alpha)^{4}\mathrm{He}$) have been excluded as an explanation by the precision measurements of LUNA at Gran Sasso and n_TOF at CERN, which confirmed the standard rates to within $5\%$ precision. New BBN physics (long-lived decaying neutralinos, time-varying fundamental constants, energy injection from relic particles) is tightly constrained by the independently measured $D/H$ and $Y_p$ ratios and by the Planck CMB constraints, and is today disfavored.

Recent measurements have consolidated the picture. The detection of progressive lithium depletion in NGC 6397 (Korn et al. 2007) showed a trend of $A(\mathrm{Li})$ as a function of $T_{\mathrm{eff}}$ within a single Pop II globular cluster, with $\Delta A(\mathrm{Li}) \approx 0{.}4$ dex on a Gyr scale — consistent with turbulent-diffusion models. The revisitation of $^{6}\mathrm{Li}/^{7}\mathrm{Li}$ in EMP stars by Lind et al. (2013) [Lind et al. 2013] eliminated the so-called “second lithium problem” (an apparent pre-Galactic $^{6}\mathrm{Li}$ excess): the $^{6}\mathrm{Li}$ lines were artifacts of poorly modeled 1D turbulent mixing, and the 3D NLTE analysis reconciles with the standard prediction. The recent LUNA-MV measurement of $^{3}\mathrm{He}(\alpha,\gamma)^{7}\mathrm{Be}$ confirmed the standard rate to within $5\%$. The current state is thus that of a problem almost solved: the residual tension is $\sim 0{.}1$-$0{.}2$ dex, compatible with the systematic uncertainties of turbulent-diffusion models in 3D NLTE stellar atmospheres. A definitive breakthrough could come from precision measurements of $A(\mathrm{Li})$ in Pop II stars with asteroseismology from Kepler, K2 and PLATO (launch expected in late 2026) to constrain $T_{\mathrm{eff}}$ and age independently. The updated methodological review is Fields, Olive, Yeh and Young (2020) [Fields et al. 2020] .

The origin of the r-process after GW170817

GW170817 in 2017 confirmed that neutron-star mergers produce r-elements in significant quantities (chapter 6). The quantitative question remains open: how much of the overall Galactic budget of r-process elements comes from NSM, and how much from alternative sites? The answer is not “all from NSM”: independent observational constraints suggest that NSM cover between $30\%$ and $70\%$ of the Galactic budget, and that the rest must come from exotic core-collapse supernovae whose frequencies and yields are still characterized with factor 2-3 uncertainties.

The current constraints, already discussed in chapter 6, rest on three converging numbers: the Galactic NSM rate ($\sim 10^{-5}$ per year), the mean mass of r-elements per event ($\sim 0{.}02$-$0{.}1\,M_\odot$), and the total Galactic r-process budget ($\sim 10^{4}\,M_\odot$). Combined, they give a cumulative NSM contribution over a Hubble time of $10^{3}$-$10^{4}\,M_\odot$ — sufficient or sub-optimal depending on the assumptions. The missing part is plausibly attributed to three types of alternative sites. Collapsars are supernovae of very massive stars ($M \gtrsim 30\,M_\odot$) with a central BH and accretion, in which neutron-rich ejecta are expelled from the accretion disk; the estimated rate is $\sim 10^{-4}$ per year with r-mass per event $\sim 0{.}01\,M_\odot$, and the picture is consistent with observations of r-enriched EMP stars. MHD-jet SNe are core-collapse SNe with strong magnetic field and rapid rotation, in which a magnetocentrifugal relativistic jet expels neutron-rich ejecta along the polar axis; rate $\sim 10^{-4}$ per year, constrained by long GRBs and orphan GRB afterglows. Magnetar SNe are core-collapse SNe with a central magnetar injecting energy into the ejecta and accelerating an r-process in the wind; the estimated rate is low and the contribution speculative.

The observational constraints that progressively narrow the mix come from three complementary directions. The dispersion of $[\mathrm{Eu/Fe}]$ vs $[\mathrm{Fe/H}]$ in halo EMP stars, with scatter $\pm 1$ dex at $[\mathrm{Fe/H}] < -2$, points to rare and productive r-process events, compatible with NSM or with exotic core-collapse SNe at a rate $\sim 10^{-4}$ per year. The $^{244}\mathrm{Pu}$ detected in deep-sea sediments (Wallner et al. 2015) [Wallner et al. 2015] implies a local r source within a few hundred million years, compatible both with the expected NSM rate and with that of local collapsars. The stars of r-enriched ultra-faint dwarf galaxies (Reticulum II of Ji et al. 2016 [Ji et al. 2016] , Tucana III of Hansen et al. 2017) suggest single-ancestor events, consistent with NSM. Côté et al. [Côté et al. 2018] argue in favor of a significant collapsar contribution ($\gtrsim 50\%$) based on the median NSM delay time ($\sim 1$ Gyr), too long to explain the r enrichment in halo stars with $[\mathrm{Fe/H}] < -3$. Other analyses (Hotokezaka, Wehmeyer, Beniamini) maintain that NSM with short delay times are sufficient. The final resolution will depend on three converging fronts over the next decade: Einstein Telescope (expected $\sim 2035$) will detect $\sim 10^{4}$ BNS per year with robust statistics on the rate, the $M(\mathrm{r})$ per event and the DTD distribution; THESEUS (an ESA M7 candidate) and NewAthena will identify collapsars and jet-SNe through GRB afterglows; high-resolution kilonova spectroscopy with ELT and JWST will identify specific isotopic species capable of discriminating among different sites. The canonical review of the field remains Cowan, Sneden, Lawler et al. (2021) [Cowan et al. 2021] .

The i-process: an intermediate site

Between the s-process (slow, $n_n \sim 10^{7}$ cm$^{-3}$, ordinary AGB stars) and the r-process (rapid, $n_n \sim 10^{24}$ cm$^{-3}$, NSM and explosive events) there theoretically exists an intermediate process, called the i-process (intermediate neutron capture), proposed by Cowan and Rose in 1977 and the object of a theoretical renaissance over the last decade. In recent years some observations have suggested that the i-process is really at work in specific environments — multiple-population globular clusters, EMP stars with heavy-element patterns intermediate between s and r — but the complete picture of the sites and of the contribution to the Galactic budget is still the object of active research.

The i-process operates in conditions $\tau_n \sim \tau_\beta$ with neutron densities $n_n \sim 10^{14}$-$10^{16}$ cm$^{-3}$, intermediate between s and r by many orders of magnitude, integrated neutron exposure $\tau \sim 10$-$100$ mb$^{-1}$, and a path 2-4 neutron units to the right of the valley of stability. The candidate sites identified over the last decade are three. Proton ingestion during the He flash in AGB stars at very low metallicity (Cristallo, Herwig, Pignatari) — at $Z \lesssim Z_\odot/100$ — sees, in some thermal pulses, the AGB convection ingest protons from the overlying H shell, producing $^{13}\mathrm{C}$ via $^{12}\mathrm{C}(p,\gamma)^{13}\mathrm{N}(\beta^{+}\nu)^{13}\mathrm{C}$ in a regime much faster than the classic interpulse pocket, with neutron density $n_n \sim 10^{14}$-$10^{15}$ cm$^{-3}$ and activation of the i-process during the flash. Accretion onto a white dwarf in the He-burning phase in peculiar binary systems is hypothesized but not yet confirmed observationally. Rapid mass accretion onto NS in some peculiar XRB scenarios produces intermediate conditions but with matter not expelled into the interstellar medium.

The observational evidence for an operating i-process includes the CEMP-r/s stars (CEMP simultaneously enriched in r and in s, with $[\mathrm{Ba/Eu}]$ patterns intermediate between pure s and pure r), which constitute about $25\%$ of all CEMP stars and are consistent with an i-process in low-Z AGB mass-transfer binaries. The metal-poor stars of the Sagittarius dSph show similar intermediate heavy-element patterns. Some multiple-population globular clusters, in particular $\omega$ Cen, show internal variations of $[\mathrm{Ba/Eu}]$ compatible with i-process contributions from internal stellar generations. Modern i-process models — NuGrid (Bertolli et al. 2013, Hampel et al. 2016 [Hampel et al. 2016] , 2019), Sakurai-Suzuki (Sakuma et al. 2019 for the i-process in low-metallicity novae), Battino et al. 2022 for the i-process in He-flash stars in RGB-WD systems — reasonably reproduce the observed patterns with a single i-process event. Critical nuclear constraints come from measurements of $(n,\gamma)$ cross sections on neutron-rich nuclei at $A = 130$-$170$ (MIT, n_TOF, KADoNiS) and from beta rates for nuclei far from stability at FRIB and GANIL.

Pop III stars: still visible?

Population III stars are the first stars, formed from metal-free gas. Their direct observation remains one of the open goals of stellar astrophysics. If they were massive, they died more than 13 Gyr ago and contribute only as progenitors of the EMP stars we observe. If any low-mass Pop III stars formed, some could still survive in the Galactic halo today: none has been securely found.

The possibility of surviving Pop III stars depends critically on the fragmentation of primordial clouds. In the absence of metals and dust, the cooling of primordial gas proceeds mainly via H$_2$ molecules at temperatures of $\sim 200$ K, significantly less effective than the metal and dust cooling at $\sim 10$ K that operates in the clouds of the modern Milky Way. The fragmentation of pre-stellar clouds is therefore less effective, and the standard prediction is that the Pop III IMF is top-heavy, with characteristic masses $M_\ast \sim 10$-$300\,M_\odot$ and few low-mass stars able to survive 13 Gyr of evolution. Modern simulations (Bromm, Yoshida, Hirano, Stacy, Susa) confirm this qualitative picture but also show secondary fragmentation in some cases, with production of Pop III stars at $\sim 0{.}1$-$1\,M_\odot$ in non-zero fractions. The minimum mass to survive 13 Gyr at zero-metals composition is $M < 0{.}8\,M_\odot$.

Searches for stars at $[\mathrm{Fe/H}] < -10$ have been under way for decades: extensions of SDSS-SEGUE, LAMOST, Pristine and SkyMapper have catalogued thousands of EMP/UMP candidates. The most metal-poor star known is SMSS 0313-6708 with $[\mathrm{Fe/H}] < -7{.}3$, but with significant carbon enhancement that classifies it as enriched by at least one previous Pop III event, not as pure Pop III. All the most extreme EMP candidates always show traces of carbon or nitrogen, suggesting an origin in matter already minimally enriched by a single progenitor Pop III SN — not pure Pop III. The chemical signature of ultra-metal-poor Pop II stars is thus the most direct indirect probe of Pop III: abundance patterns consistent with a single Pop III core-collapse SN progenitor of $\sim 25$-$60\,M_\odot$ with mixing-fallback, and — crucially — the absence of a PISN signature (no anomalous $[\mathrm{Mg/Ca}]$, no high $[\mathrm{Cr/Mn}]$, no heavy Fe-peak depletion), which implies a Pop III IMF not extending significantly beyond $\sim 100\,M_\odot$, or very rare PISNe.

JWST is conducting active Pop III searches in very high-redshift galaxies. Candidates such as GN-z11 ($z \approx 10$) and JADES-GS-z13-0 ($z \approx 13{.}5$) are undergoing detailed spectroscopic analysis: the confirmation of pure Pop III would require the identification of a complete lack of metals in the emitted spectrum, combined with He II $\lambda 1640$ Å line emission — an indicator of very hot massive stars compatible with Pop III progenitors. Some He II $\lambda 1640$ detections in high-$z$ galaxies (e.g., candidates in SMACS J0723) have been reported, but definitive confirmation is in progress. A complementary route is that of lensed Pop III stars — “Earendel-like objects” programs that use gravitational magnification by galaxy clusters to identify individual stars at $z \sim 6$-$8$ in host galaxies. ELT (science from 2030) will allow precision spectroscopy of EMP/UMP stars in Local Group satellite galaxies and beyond, significantly refining the constraints on Pop III progenitors through the chemical pattern of their immediate descendants.

Experimental frontiers of nuclear physics

The entire picture of stellar nucleosynthesis rests on laboratory measurements of nuclear physics: reaction rates at astrophysical energies, masses of rare isotopes, beta and alpha decay lifetimes, $(n,\gamma)$, $(\gamma,n)$, $(\gamma,\alpha)$ cross sections. The experimental frontiers of the coming decade advance in three complementary directions: measurements at low astrophysical energies in underground laboratories, production and study of nuclei far from stability at radioactive-beam facilities, and measurements of $(\gamma, X)$ reactions with new-generation quasi-monochromatic gamma sources.

The experimental roadmap is dense and well coordinated internationally. LUNA-MV at the Gran Sasso National Laboratories (Italy) is a $3{.}5$ MV underground accelerator operational since 2023, with a scientific program focused on the reactions of quiescent nuclear astrophysics: $^{14}\mathrm{N}(p,\gamma)^{15}\mathrm{O}$ for the CNO-cycle bottleneck, $^{22}\mathrm{Ne}(\alpha,n)^{25}\mathrm{Mg}$ for the weak s-process neutron source, $^{12}\mathrm{C}(\alpha,\gamma)^{16}\mathrm{O}$ in progressive energy steps. JUNA at the Jinping Underground Laboratory (China) is a complementary $400$ kV underground accelerator, operational since 2023, with a scientific program parallel to LUNA on quiescent astrophysical reactions. FRIB at Michigan State University (USA) is the flagship US radioactive-beam facility, operational since 2022, with a program of measurements of masses, beta lifetimes and surrogate $(n,\gamma)$ cross sections for drip-line nuclei relevant to the r-process and the p-process. FAIR at GSI Darmstadt (Germany), under construction with the first physics campaign expected $\sim 2028$, complements FRIB with a specialization in heavier nuclei. ELI-NP in Bucharest (Romania) is the new-generation laser-driven gamma source, with energy up to $\sim 20$ MeV and quasi-monochromatic flux, focused on direct photodisintegration $(\gamma, X)$ measurements on p-rich nuclei. HIGS at the Triangle Universities Nuclear Laboratory (USA) is the complementary quasi-monochromatic gamma source with energy up to $\sim 100$ MeV, focused on photodisintegration and on indirect measurements of astrophysical cross sections.

The individual high-priority reactions for the next five years include $^{12}\mathrm{C}(\alpha,\gamma)^{16}\mathrm{O}$ (the dominant uncertainty for the C/O yield in massive stars, LUNA-MV program + multi-channel R-matrix analysis), $^{22}\mathrm{Ne}(\alpha,n)^{25}\mathrm{Mg}$ (weak s-process neutron source, LUNA-MV), $^{12}\mathrm{C} + {}^{12}\mathrm{C}$ (the key uncertainty for quiescent C-burning in massive stars, STELLA at Frascati and CD-MORE), $^{96}\mathrm{Ru}(\gamma, \alpha)^{92}\mathrm{Mo}$ (crucial for the p-process Mo-Ru deficit, ELI-NP), and $(n,\gamma)$ on drip-line nuclei for the r-process network (n_TOF at CERN, FRIB). The quantitative constraints expected within 5-10 years include $\delta[\mathrm{C/O}]_{\mathrm{massive\ stars}} < 5\%$, $\delta Y_{r\text{-process}} < 20\%$, and $\delta\Phi_{\nu \mathrm{CNO}}^{\mathrm{SSM}} < 5\%$ — significant reductions of the systematic uncertainties that dominate today.

Observational frontiers

In the current decade, a series of revolutionary instruments are entering or will enter into operation and will change the observational landscape of nucleosynthesis. JWST (operational since 2022) has already opened precision infrared spectroscopy of high-redshift galaxies, kilonovae, circumstellar stellar dust and protoplanetary disks, with results that are transforming the study of Pop III progenitors, of primordial galaxy formation, and of the isotopic composition of the circumstellar material of AGB and RSG stars. ELT (Extremely Large Telescope, 39 m, first light expected in 2029 and science operations from 2030) will allow very-high-resolution optical/infrared spectroscopy of individual stars in Local Group satellite galaxies and of Pop II stars in more distant galaxies, redefining the statistics of stellar constraints on chemical evolution. Einstein Telescope in Europe and Cosmic Explorer in the USA, expected $\sim 2035$ as third-generation gravitational-wave detectors, will bring the BNS detection statistics to $10^{3}$-$10^{4}$ per year out to $z \sim 1$-$2$. NewAthena (expected $\sim 2037$) will be the next-generation X-ray observatory, with spectral resolution sufficient to measure abundances in the intracluster medium of galaxy clusters and in young SN remnants with precision an order of magnitude better than today’s. COSI (NASA SMEX, launching 2027) will be the next-generation MeV gamma-ray observatory, with the e-ASTROGAM and AMEGO concepts proposed for the following decade, capable of mapping the Galactic distribution of $^{26}\mathrm{Al}$, $^{60}\mathrm{Fe}$, $^{44}\mathrm{Ti}$, $^{22}\mathrm{Na}$ in SN remnants, in novae, and in massive stars with individual-source precision.

The specific observational programs feeding these goals include: JWST high-$z$ spectroscopy to identify Pop III candidates and measure abundances in galaxies of the reionization epoch (EoR); JWST characterization of grains forming in the atmospheres of AGB and RSG stars as a constraint on condensation models and isotopic yields (chapter 4); ELT spectroscopy of multi-element chemical abundances in dwarf galaxies and halo overdensities for chemical tagging and historical reconstruction of accretion; the BNS statistics of Einstein Telescope to constrain the DTD, the mean $M(\mathrm{r})$ and the correlation with host-galaxy properties; Athena abundances in the ICM of clusters at $z = 0$-$2$ for the evolution of abundances in the intergalactic medium; the MeV $\gamma$ mapping of $^{26}\mathrm{Al}$ with COSI to identify individual sources in the Milky Way (WR, AGB, SNe, novae). The next-generation multi-element surveys — APOGEE-2 and GALAH+ will keep producing $\sim 10^{6}$ stars by 2027, while WEAVE, 4MOST and PFS, entering operation between 2023 and 2026, will reach $\sim 10^{7}$ stars by 2030. The implications for GCE include statistical inference of the star-formation history with $\sim 10^{4}$ stars per population, reliable chemical tagging to reconstruct ancestral accretion events, and precise constraints on the radial metallicity gradient with $\sigma < 0{.}01$ dex/kpc.

What would count as a breakthrough?

Several discoveries would change the field qualitatively. A secure Pop III star — a surviving zero-metallicity low-mass object, or an unambiguously identified high-redshift stellar population — would directly constrain the initial mass function of the first generations. A large sample of kilonova spectra with identified heavy elements would turn r-process site theory into abundance-resolved transient astronomy. A precise solution of the Solar Modeling Problem would align photospheric abundances, helioseismology, opacities and neutrinos. A robust pair-instability supernova signature in a metal-poor star would prove that very massive first stars contributed to early enrichment. A laboratory reduction of the $^{12}\mathrm{C}(\alpha,\gamma)^{16}\mathrm{O}$ uncertainty would propagate through massive-star yields, white-dwarf compositions, and supernova models.

Just as important would be negative results. If no PISN signature appears in much larger EMP samples, the Pop III IMF must be revised. If third-generation gravitational-wave detectors find a merger delay-time distribution too slow to explain early europium, prompt massive-star r-process channels become mandatory. If improved stellar models cannot deplete lithium without also destroying the flatness of the Spite plateau, the cosmological lithium problem reopens.

Conclusion: a living discipline

Seventy years after the 1957 paper of Burbidge, Burbidge, Fowler and Hoyle, stellar nucleosynthesis is a mature discipline. We know how, where and when practically all the elements of the periodic table were produced: the fundamental physics is consolidated, the main mechanisms are identified, the astrophysical sites recognized, and the yields calculable with uncertainties of $5$-$30\%$ for the great majority of nuclides. The residual uncertainties are quantitative — not structural — and concern the fine-tuning of the models, the relative mix of the less-characterized sites, and specific nuclear reactions still difficult to measure in the laboratory.

The quantitative summary of the state of the field as of 2026 is the following. BBN and Pop II foundations: stable, with $\sim 5\%$ uncertainties on the primordial yields and $\sim 0{.}3$ dex residual on the lithium Spite plateau (almost solved via turbulent diffusion). Quiescent burnings: stable, with $10$-$30\%$ nuclear uncertainties on key reactions being reduced through LUNA-MV and JUNA. s-process: stable, with the AGB main component fully understood and the weak component with $\sim 30\%$ residual uncertainties due to rotation and mass loss in massive stars. r-process: in active evolution after 2017, with NSM confirmed as a site ($30$-$70\%$ of the budget) and alternative sites (collapsars, MHD-jet SNe) probable for the rest. p-process: partially understood, with the Mo-Ru deficit attributed to a combination of $\nu$-p in core-collapse SNe + sub-Chandra SNe Ia. Core-collapse SN yields: $20$-$50\%$ uncertainties depending on the code, being clarified with self-consistent 3D simulations. SN Ia yields: progenitor type under debate (Chandra vs sub-Chandra vs DD), with $\sim 30\%$ uncertainties on the specific yields of Mn, Cr, Ni. Pop III stars: never observed directly, active search with JWST and with high-$z$ candidates under analysis.

And yet the field remains alive. GW170817 in 2017 was a discovery that brought an entire chapter to definitive closure — the identification of the r-process site — and opened a completely new era, that of multimessenger astronomy. How many discoveries of that level await us in the next ten to fifteen years, with all the new-generation instruments entering operation between 2025 and 2035? Probably many, and in directions we can today only imagine. The history of the elements is still being written, and it will be for many years to come.

Stellar nucleosynthesis is one of the few examples in modern science in which an almost complete story has been built in a century, stitching together different disciplines — laboratory nuclear physics, stellar evolution, astrophysical spectroscopy, cosmology, isotopic geochemistry, multimessenger astronomy — into a single coherent narrative arc. It is hard to think of another part of astrophysics with a similar degree of intellectual unity and convergence among disciplines. And yet the story is not closed: it has grown in quantitative detail, but the big questions — where exactly are the heaviest elements produced? how much do the first stars contribute? is the Sun normal or anomalous? do surviving Pop III stars exist? — remain partly open. It is this openness — that of the “active problems” — that keeps a discipline alive, and that justifies the continuity of the research effort in laboratory nuclear physics, stellar modeling, astrophysical spectroscopy, hydrodynamic simulation and chemical archaeology over the coming decades.

This chapter concludes the systematic treatment of stellar nucleosynthesis. The book closes with appendices dedicated to the chart of the relevant isotopes (appendix A), to the terminological glossary (appendix B), and to the complete bibliography.