This article reviews today's open issues, frontiers of research, and advances in our understanding of the heliosphere throughout the Sun's last 10 million years. Today's heliosphere, the cocoon formed by the solar wind as it moves through the Galaxy, engulfs all the planets extending in the nose direction to ∼120 astronomical units (AU). ▪ The Sun moves at 19 pc/Myr and has traversed many different structures in the interstellar medium that affect the heliosphere, at times collapsing it to sub-astronomical-unit scales. These periods of collapse introduce climate and radiation changes in Earth's environment. ▪ The frequency of such encounters could be as often as every couple of million years, making it a major external disturbance to the development of life on Earth. ▪ Stepwise shifts in past global climate, seen in deep-sea sediment cores, indicate intervals of more rapid cooling at 13–14 Mya, 6–7 Mya, and 2–3 Mya, for which the driving mechanisms are subject to ongoing debate. Such cooling events might be triggered by heliosphere collapse.
The advent of routine operations with the Atacama Large Millimeter/submillimeter Array (ALMA) in the past decade has led to a revolution in the direct study of the interstellar medium (ISM) in “normal” high-redshift galaxies in the rest-frame far-infrared (FIR). This review summarizes the observational literature on z > 6.5 sources observed with ALMA and NOEMA (NOrthern Extended Millimeter Array). The main findings are as follows: ▪ Cool gas consistently scales with star formation in a wide range of galaxy environments, similar to local relations, suggesting that fundamental processes of star formation have stayed relatively constant over 13 billion years of cosmic time. ▪ Significant metal enrichment is present in galaxies just ∼300–400 million years after the Big Bang. ▪ There is a trend to a higher obscured fraction of star formation with stellar mass and star-formation rate (SFR) already in place at z ∽ 7, with measurements of the cosmic SFR density showing that >10% of the SFR density is obscured at this epoch. ▪ The estimated dust masses compared to the stellar mass suggest that rapid dust enrichment occurs, likely from supernovae with little dust destruction and/or rapid growth in the ISM, which is in agreement with maximal predictions from models. ▪ In individual sources, cold disks are already in existence, with their gas disks being more extended and smoother than their observed stellar counterparts.
Understanding the formation and evolution of the Milky Way and its constituents relies on making measurements of data that are faithful representations of the underlying physical system. Data-driven spectral models function in several capacities in this context: generating spectra, extracting encoded information, and connecting that information to theory. I review how data-driven methods have changed the spectroscopic landscape and assess their future: ▪ A core utility of data-driven models is that they deliver precise stellar measurements at scale and with modest computational cost, providing consistent parameters and abundances across surveys, including for low-resolution and low signal-to-noise spectra. This circumvents prior limitations and unlocks the statistical power of large datasets. ▪ Entirely new avenues of study have been opened up by inferred spectroscopic ages, distances, and evolutionary states, whereas residual analyses have uncovered nonstellar signals and allow a novel discovery space. ▪ Biases can be introduced or inherited by data-driven models; identifying and accounting for them is important for accurate interpretation. ▪ Data-driven models do not replace theoretical models; together, they enable the interpretation and parameterization of spectra. ▪ A key benefit of data-driven frameworks, alongside public releases of survey data, is democratization of spectroscopic analysis. ▪ Future facilities and computational tools will expand data-driven methods and their utility. Although this review focuses on the stellar temperature range of 3,000–7,000 K, the methods are general.
We review our current understanding on the physical processes that govern angular momentum transport and evolution of protoplanetary disks. Extremely rich in physics, these processes are intimately connected to disk gas dynamics, with profound implications for planet formation. We organize them into a three-level hierarchical framework: (1) The coupling of gas with magnetic fields and radiation sets the microphysical foundation for understanding protoplanetary disk dynamics. Key ingredients include non-ideal magnetohydrodynamic effects (requiring ionization chemistry), along with heating and cooling processes. The disk can be divided into three radial sectors governed by distinct microphysics. (2) Protoplanetary disks host diverse gas dynamical processes, including hydrodynamic, magnetic and gravitational instabilities, along with thermally and magnetically-driven disk winds. Many of these individual processes are reasonably well understood, while others still require detailed investigation. (3) Protoplanetary disks are highly complex ecosystems where multiple processes interact. It is recognized that the bulk disk exhibits weak turbulence, with magnetically-driven wind likely serving as the primary transport mechanism. However, our knowledge remains highly limited regarding the disk's innermost region, early stages, long-term evolution, and environmental effects.
The shutdown of star formation - quenching - marks a pivotal transition in the lives of massive galaxies, which dominate the present-day stellar mass density. This review synthesizes our current understanding of the mechanisms that trigger and maintain quiescence. We discuss the nuances of how quiescent systems are identified across cosmic time and summarize the evolving physical properties of the growing massive population, including their stellar populations, chemical enrichment histories, and gas and dust reservoirs, highlighting several key results: (1) Quiescent galaxies can be identified with empirical color selections, but evolving specific star formation rate thresholds offer a more robust physical distinction from star-forming systems. (2) The earliest massive quiescent stellar populations show rapid formation histories and high metallicities, with enhanced α-elemental abundances often distinct from local analogs. (3) Nascent studies of gas and dust in quiescent galaxies reveal diverse multiphase reservoirs and outflows, pointing to fast ejective and slow regulatory modes of galaxy quenching. (4) In situ processes establish galaxy central density, while assembly continues via (minor) mergers post-quenching, reshaping all massive galaxies and disrupting rotation in most cases. We distill observations into two broad modes by which massive galaxies form and quench: one involves a rapid, early shutdown driven by supermassive black hole outflows on short timescales; the other proceeds gradually through gas exhaustion, virial heating, or preventative feedback, each leaving distinct observational signatures. Together, these pathways offer a testable framework for modeling the formation and evolution of massive galaxies, which will be informed by future studies of their stars, gas, dust, and dynamics.
Lithium plays a unique role in astrophysics, as it is a powerful diagnostic for the physics and evolution of low-mass stars, Galactic archaeology, and cosmology. We review the Li observations in stars at different phases of their evolution, the strengths and the limitations of the current theoretical stellar models to explain the Li abundance data, our understanding of the Li sources and of the evolution of Li through- out the Galactic history. Key takeaways from the current state of the research in the field are: 1) Stellar evolution models accounting for fundamental transport processes of chemical species and angular momentum hold the promise of providing a common stellar Li depletion explanation to the Li abundance patterns observed in all Galactic stellar populations, including the dip and the plateau(s). 2) Novae are most probably the main source of Li in the Galaxy, on observational (but not yet theoretically established) grounds. 3) Radial migration of stars in the Galactic disk holds the key to understand many aspects of the Li evolution in the Milky Way.
Carbon is an essential element for a habitable world. Inner (r < 3 au) disk planetary carbon compositions are strongly influenced by supply and survival of carbonaceous solids. Here we trace the journey of carbon from the interstellar medium to the processes leading to planet formation. The review highlights the following central aspects: -Organics forming in evolved star envelopes are supplemented by aromatic molecules forming in the dense ISM to represent the seeds of (hydro)carbon supply through pervasive pebble drift to rocky planets and sub-Neptune cores. -Within the protoplanetary disk the sharp gradient in the C/Si content of Solar System bodies and mineral geochemistry outlines a tale of carbon loss from pebbles to within planetesimals and planets, and from planetary atmospheres. -Within two planet formation paradigms (pebble and planetesimal accretion) a range of planetary carbon content is possible that is strongly influenced by early (< 0.5 Myr) formation of a pressure bump that titrates drift. Overall, it is unlikely that the carbon architecture of our Solar System applies to all systems. In the absence of giant planets, carbon-rich rocky worlds and sub-Neptunes may be common. We outline observations that support their presence and discuss habitability of terrestrial worlds.
Long-period variables (LPVs) are evolved red giant and supergiant stars whose pulsations provide unique insights into late stages of stellar evolution and serve as essential tools in modern astrophysics. Their period-luminosity and period-age relations make them valuable distance and age indicators, while their light curve morphology, amplitudes, and multiperiodicity reveal the underlying physics of stellar interiors and mass-loss. In this review, we provide an overview of the current status of LPV studies, focusing on their observational properties and applications, including: - Modern classification of LPVs into Miras, semiregular variables (SRVs), and OGLE small-amplitude red giants (OSARGs), which occupy multiple period-luminosity sequences associated with different pulsation modes, chemical compositions, and evolutionary stages - Mira variables as reliable distance indicators across diverse stellar environments and their increasing role as standard candles - The increasing role of SRVs and OSARGs - Long secondary period (LSP) variables as potential tracers of exoplanets Together with advances in theoretical modeling, these developments establish LPVs as valuable tracers of Galactic structure, stellar populations, and the extragalactic distance scale.
Evidence of a gravitational wave (GW) signal has emerged in pulsar timing array (PTA) data, opening a new window into the nanoHz GW Universe. We explore the physics of GW signals potentially explaining the data, with a primary focus on GW backgrounds (GWBs), considering both astrophysical and cosmological origins. We describe how: (i) An astrophysical nanoHz GWB emerges as the superposition of individual signals from inspiralling massive black-hole binaries (MBHBs); (ii) Environment coupling, eccentricity, and sparse sampling, affect the MBHB signal spectrum and statistical properties, causing great uncertainty in theoretical predictions, but simultaneously offering a handle to discriminate a potential astrophysical origin; (iii) PTA data offers unprecedented opportunities to constrain high-energy physics beyond the standard model, by probing early Universe GWBs, originated during or after inflation; (iv) Different early Universe GWBs, typically created by non-linear and out-of-equilibrium dynamics, can explain the PTA data, as e.g. from inflation scenarios, first order phase transitions, or topological defects; (v) The PTA detection of GWs opens a new window to explore the Universe, with profound implications for astrophysics and particle physics, probing e.g. the equation of state of the early Universe, the origin of the cosmological perturbations, the nature of the dark matter, or whether exotic objects like primordial black holes or cosmic strings exist.
Space-based time-domain telescopes such as CoRoT, Kepler/K2 and TESS have profoundly impacted astrophysics over the past two decades. Continuous light curves with high cadence and high photometric precision are now available for millions of sources within our galaxy and beyond. In addition to revolutionizing exoplanet science, the data have enabled breakthroughs ranging from the solar system to stellar interiors, the transient universe, and active galaxies. The key summary points of this review are: (1) Stellar astrophysics has been transformed by the ability to probe the internal structures of stars, test the physics of stellar convection, connect stellar rotation and magnetic activity, and reveal complex variability in young stars. (2) Ages of stellar populations probe the formation history of our Milky Way, and binary star variability enables the detection of "dark" galactic populations such as solar-mass black holes and neutron stars. (3) Early-time observations of explosive transients provide new insights into the progenitors of supernovae, while the quasi-periodic variability of galaxies probes the physics of accretion processes onto supermassive black holes and the tidal disruption of stars. (4) Observations of solar system objects reveal asteroid compositions through their rotation periods and amplitudes, constrain the cloud structure of ice giants, and allow the discovery of new objects in the outer solar system. (5) Open data policies and software have contributed to remarkable scientific productivity and enabled discoveries by citizen scientists, including new exoplanets and exotic variability in mature Sun-like stars.
Precise measurements of a star's radial velocity (RV) made using extremely stable, high resolution, optical or near infrared spectrographs can be used to determine the masses and orbital parameters of gravitationally-bound extra-solar planets (exoplanets). Indeed, RV surveys and follow up efforts have provided the vast majority of published exoplanet mass measurements and in doing so have enabled studies into exoplanet interior and atmospheric compositions. Here we review the current state of the RV field, with particular attention paid to: -The evolution of precise RV methodologies over the past two decades -Modern RV spectrograph designs that can be calibrated to a stability level of better than 50 cm/s over timescales of years -RV data reduction and post-processing techniques that minimize the impact of instrument systematics and stellar variability -Techniques for detecting exoplanets in RV data and disentangling planetary signals from stellar variability
Deep learning has generated diverse perspectives in astronomy, with ongoing discussions between proponents and skeptics motivating this review. We examine how neural networks complement classical statistics, extending our data analytical toolkit for modern surveys. Astronomy offers unique opportunities through encoding physical symmetries, conservation laws, and differential equations directly into architectures, creating models that generalize beyond training data. Yet challenges persist as unlabeled observations number in billions while confirmed examples with known properties remain scarce and expensive. This review demonstrates how deep learning incorporates domain knowledge through architectural design, with built-in assumptions guiding models toward physically meaningful solutions. We evaluate where these methods offer genuine advances versus claims requiring careful scrutiny. - Neural architectures overcome trade-offs between scalability, expressivity, and data efficiency by encoding physical symmetries and conservation laws into network structure, enabling learning from limited labeled data. - Simulation-based inference and anomaly detection extract information from complex, non-Gaussian distributions where analytical likelihoods fail, enabling field-level cosmological analysis and systematic discovery of rare phenomena. - Multi-scale neural modeling bridges resolution gaps in astronomical simulations, learning effective subgrid physics from expensive high-fidelity runs to enhance large-volume calculations where direct computation remains prohibitive. - Emerging paradigms-reinforcement learning for telescope operations, foundation models learning from minimal examples, and large language model agents for research automation-show promise though are still developing in astronomical applications.
Stellar mergers are responsible for a great variety of astrophysical phenomena. They form blue straggler stars, give rise to spectacular transients, and produce some of the most massive stars in the Universe. Here, we focus on mergers from binary evolution and stellar collisions but do not cover mergers involving compact objects. We review how mergers come about, explain the physics and outcomes of the merger process, discuss the evolution and ultimate fates of merged stars, and relate to observations. ▪ Mergers of main sequence stars often fully rejuvenate and have interior structures similar to genuine single stars. ▪ Contrarily, mergers involving post–main sequence stars can have interior structures that cannot be achieved by single-star evolution. Some of these merger products may become long-lived blue supergiants and even end their evolution as such stars. They could thus explode in SN 1987A-like events, lead to interacting and superluminous supernovae, or collapse into very massive black holes. Such black holes may even populate the pair-instability-supernova black-hole mass gap. ▪ Strong magnetic fields are produced in stellar mergers. Merged stars may thus be at the origin of some magnetic OBA stars and their descendants, highly magnetic white dwarfs and neutron stars. ▪ Initially, stellar merger products rotate rapidly, but there are several mechanisms that can quickly spin them down. Hence, merged stars may be rather slow rotators for most of their evolution.
High-precision and long-duration light curves from space telescopes have revolutionized the fields of asteroseismology and binary star systems. In particular, the number of pulsating systems in eclipsing binaries has drastically increased thanks to space-based observations covering almost the entire sky. When combined with multiepoch spectroscopy, this not only allows us to measure model-independent dynamical masses and radii for thousands of eclipsing binary systems but also facilitates the powerful synergy of binarity and asteroseismology. Moreover, asteroseismology of pre- and postinteraction binary stars allows the physics of binary evolution, including tides, mass transfer, and even mergers, to be constrained. ▪ Eclipsing binaries are among the best laboratories for testing stellar structure and evolution theory because we are able to measure their masses and radii independently of models. ▪ Combining binary and asteroseismic modeling yields precise constraints on the physical processes at work within stellar interiors, such as rotation and mixing. ▪ Pulsating binaries are challenging to study, given the plethora of different techniques and physical processes that need to be considered depending on their orbital and physical properties. ▪ The impact of tides on the pulsational, stellar structure, and orbital properties of a binary system can be tested through tidal asteroseismology.
Ultrahigh-energy cosmic rays (UHECRs) are charged particles with energies between ∼1018eV and ∼3 × 1020eV ∼ 50J. They exhibit fundamental physics at energies inaccessible to terrestrial accelerators; challenge experimental physics; and connect strongly to astronomical observations through electromagnetic, neutrino, and even gravitational wave channels. Much theoretical and observational progress has occurred in the 60 years since the discovery of UHECRs to determine their nature and identify their sources: ▪ The highest-energy UHECRs appear to be heavy nuclei with rigidity extending up to ∼10 EV. ▪ A significant (6.9σ) dipole anisotropy has been measured, but our poor understanding of Galactic magnetic fields makes it hard to interpret. ▪ The UHECR luminosity density is ∼1044 erg Mpc−3 year−1, which constrains explanations of their origin. ▪ The most promising acceleration mechanisms involve diffusive shock acceleration and unipolar induction. ▪ The most promising sources include intergalactic accretion shocks and relativistic jets from stellar-mass or supermassive black holes. We explore the prospects for using the highest-energy events, combined with multimessenger astronomy, to help us solve the riddle of UHECRs.
The scope of this literature review is observations of the products of first-stage evolution for binaries having components with M < 2 M_⊙. A taxonomy for these products comprises dwarfs ("blue stragglers"), giants ("yellow stragglers"), subdwarf B stars, and giant-like stars ("sub-subgiants" and "red stragglers"). This literature review is organized according to this taxonomy within three distinct environments: open star clusters, globular star clusters, and the Galactic field. This literature review is the Supplemental Material for Blue Stragglers and Friends: Initial Evolutionary Pathways in Close Low-Mass Binaries (Mathieu Pols, 2025, ARAA, 63:467-512, doi: www.annualreviews.org/content/journals/10.1146/annurev-astro-071221-054402). It is intended to support and expand on Section 3 of the main text of this Annual Review of Astronomy and Astrophysics, where an integrated perspective on the common and contrasting astrophysical properties of these binary evolution products is provided. Figures used in the main text to highlight key observational results are referenced in this literature review. The closing date of this review is January 17, 2025, with some citations subsequently updated.
In this review, we show how combining dynamical and stellar population models with integral field spectroscopic data of nearby galaxies enables uncovering their assembly history. ▪ We discuss the advantages and limitations of various dynamical modelling approaches, focusing on measuring the mass distributions of nearby galaxies, including central black holes and dark matter halos. ▪ We highlight the use of Schwarzschild's orbit-superposition method to robustly decompose galaxies into dynamically distinct components and derive their intrinsic properties. ▪ We cover the application of single stellar population models to interpret observations of unresolved stars in nearby galaxies. ▪ We outline how combining dynamical and stellar population models can reveal the fossil records of galaxy assembly, from the origin of inner galaxy structures, to the buildup of disks, to the recovery of past galaxy mergers. We close by demonstrating how these models of nearby galaxies provide a bridge between studies of resolved stars in the local Universe and high-redshift galaxy observations. Together with direct coupling to state-of-the-art cosmological simulations, extragalactic archaeology promises key insights into galaxy formation and evolution.
The classic model of the Local Group (LG) is that of two dominant constituents, the Milky Way and M31, first separating and then detaching from the Hubble flow, leading to a nearly radial approaching orbit. This simple model has been confronted by new measurements of the 3D M31 kinematics, by cosmological simulations, and by theoretical understanding of the impact of massive substructures such as the Large Magellanic Cloud. This article explores the consequences of new observations and theory on the determination of the mass and dynamics of the LG. The M31 tangential velocity measurement and contribution from the cosmological constant both increase the implied timing mass of the LG to be ∼ 5 × 10^12 M_⊙. Timing mass estimates for the LG tend to be larger than the sum of the Milky Way and M31 halo masses, and larger than independent LG mass estimators. Precision future kinematics have the potential to explore the origin of this difference, shed light on dark matter in the LG, the origin of its angular momentum, and possibly even local values of cosmological parameters.
The escape of Lyman continuum (LyC) radiation from early galaxies transformed the intergalactic medium (IGM) and is intimately connected to the fueling and feedback processes that regulate galaxy evolution. IGM attenuation interferes with high-redshift LyC observations, but growing samples of LyC observations at z<0.1 are revealing the properties of LyC-emitting galaxies. Along with multi-wavelength observations of nearby LyC-emitting candidates, cosmological simulations, and simulations of LyC escape from star-forming clouds, recent studies are providing insights into the physics of LyC escape and the possible characteristics of the galaxies that reionized the universe. Here, I review progress in LyC detections, the inferred indirect signatures of LyC escape and their application to high redshift, and our current understanding of the physical conditions that lead to high LyC escape. These findings include: LyC-emitting populations are diverse, and multiple factors correlate with LyC escape, particularly neutral gas absorption, dust attenuation, nebular ionization, and concentrated star formation. Radiative feedback plays a critical role in the youngest starbursts with the highest LyC escape fractions, but mechanical feedback may also contribute. Further research is needed to clarify the timing and role of different feedback mechanisms and to connect local LyC-production sites with the broader interstellar medium. Indirect LyC diagnostics show promise, but we need to understand whether and how the properties of LyC-emitting galaxies evolve from low to high redshift.
Gas giant planets, if present, are the most massive objects in a planetary system and play a pivotal role in shaping its overall architecture. The formation of these planets has constantly been a central issue in planetary science. Increasing evidence from spacecraft explorations of Jupiter and Saturn, as well as telescope observations of exoplanets, has provided new constraints on the formation process of gas giant planets. The classic challenge of explaining formation timescales still remains a significant issue, while new constraints on planetary interiors have introduced additional complexities. Recent shifts away from the single-size planetesimal hypothesis, nevertheless, show promise in resolving these problems. Additionally, various discoveries regarding exoplanets have led to theoretical improvements, while the discovery of numerous super-Earths and sub-Neptunes has posed new challenges in understanding gas accretion. This review synthesizes the latest theoretical advancements, discussing resolved issues and emerging challenges in giant planet formation.
