Annual Review of Astronomy and Astrophysics - Volume 62, 2024

Human interest in the possibility of other worlds in the Universe has existed for over two millennia. In recent centuries, this question has been translated into the following terms: Are there planetary systems linked to stars other than the Sun?

Developments in astronomical instrumentation have transformed this philosophical dream into a new, vibrant chapter in astronomy. This article describes my journey that started over 40 years ago with the exploration of the dynamics of our Galaxy, that brought astonishing scientific progress to which my collaborators and I have contributed, and eventually led to the amazing discovery of the plurality of worlds.

Massive stars play a major role in the evolution of their host galaxies and serve as important probes of the distant Universe. It has been established that the majority of massive stars reside in close binaries and interact with their companion stars during their lifetimes. Such interactions drastically alter their life cycles and complicate our understanding of their evolution, but are also responsible for the production of interesting and exotic interaction products.

• ▪  Extensive observation campaigns with well-understood detection sensitivities have enabled the conversion of observed properties into intrinsic characteristics, facilitating a direct comparison to theory.
• ▪  Studies of large samples of massive stars in our Galaxy and the Magellanic Clouds have unveiled new types of interaction products, providing critical constraints on the mass transfer phase and the formation of compact objects.
• ▪  The direct detection of gravitational waves has revolutionized the study of stellar mass compact objects, providing a new window to study massive star evolution. Their formation processes are, however, still unclear. The known sample of compact object mergers will increase by orders of magnitude in the coming decade, which is vastly outgrowing the number of stellar-mass compact objects detected through electromagnetic radiation.

Stars are among the most fundamental structures of our Universe. They comprise most of the baryonic and luminous mass of galaxies; synthesize heavy elements; and inject mass, momentum, and energy into the interstellar medium. They are also home to the planets. Because stellar properties are primarily decided by their mass, the so-called stellar initial mass function (IMF) is critical to the structuring of our Universe. We review the various physical processes and theories that have been put forward as well as the numerical simulations that have been carried out to explain the origin of the stellar IMF. Key messages from this review include the following:

• ▪  Gravity and turbulence most likely determine the power-law, high-mass part of the IMF.
• ▪  Depending of the Mach number and the density distribution, several regimes are possible, including ΓIMF ≃ 0, −0.8, −1, or −1.3, where dN/d log M ∝ M^ΓIMF. These regimes are likely universal; however, the transition between these regimes is not.
• ▪  Protostellar jets can play a regulating influence on the IMF by injecting momentum into collapsing clumps and unbinding gas.
• ▪  The peak of the IMF may be a consequence of dust opacity and molecular hydrogen physics at the origin of the first hydrostatic core. This depends weakly on large-scale environmental conditions such as radiation, magnetic field, turbulence, or metallicity. This likely constitutes one reason for the relative universality of the IMF.

Dwarf irregular (dIrr) galaxies are among the most common type of galaxy in the Universe. They typically have gas-rich, low-surface-brightness, metal-poor, and relatively thick disks. Here, we summarize the current state of our knowledge of the interstellar medium (ISM), including atomic, molecular, and ionized gas, along with their dust properties and metals. We also discuss star-formation feedback, gas accretion, and mergers with other dwarfs that connect the ISM to the circumgalactic and intergalactic media. We highlight one of the most persistent mysteries: the nature of pervasive gas that is yet undetected as either molecular or cold hydrogen, the “dark gas.” Some highlights include the following:

• ▪  Significant quantities of Hi are in far-outer gas disks.
• ▪  Cold Hi in dIrrs would be molecular in the Milky Way, making the chemical properties of star-forming clouds significantly different.
• ▪  Stellar feedback has a much larger impact in dIrrs than in spiral galaxies.
• ▪  The escape fraction of ionizing photons is significant, making dIrrs a plausible source for reionization in the early Universe.
• ▪  Observations suggest a significantly higher abundance of hydrogen (H2 or cold Hi) associated with CO in star-forming regions than that traced by the CO alone.

Over the past decade, advancement of observational capabilities, specifically the Atacama Large Millimeter/submillimeter Array (ALMA) and Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instruments, alongside theoretical innovations like pebble accretion, have reshaped our understanding of planet formation and the physics of protoplanetary disks. Despite this progress, mysteries persist along the winded path of micrometer-sized dust, from the interstellar medium, through transport and growth in the protoplanetary disk, to becoming gravitationally bound bodies. This review outlines our current knowledge of dust evolution in circumstellar disks, yielding the following insights:

• ▪  Theoretical and laboratory studies have accurately predicted the growth of dust particles to sizes that are susceptible to accumulation through transport processes like radial drift and settling.
• ▪  Critical uncertainties in that process remain the level of turbulence, the threshold collision velocities at which dust growth stalls, and the evolution of dust porosity.
• ▪  Symmetric and asymmetric substructures are widespread. Dust traps appear to be solving several long-standing issues in planet formation models, and they are observationally consistent with being sites of active planetesimal formation.
• ▪  In some instances, planets have been identified as the causes behind substructures. This underlines the need to study earlier stages of disks to understand how planets can form so rapidly.

In the future, better probes of the physical conditions in optically thick regions, including densities, turbulence strength, kinematics, and particle properties, will be essential for unraveling the physical processes at play.

The envelopes and disks that surround protostars reflect the initial conditions of star and planet formation and govern the assembly of stellar masses. Characterizing these structures requires observations that span the near-IR to centimeter wavelengths. Consequently, the past two decades have seen progress driven by numerous advances in observational facilities across this spectrum, including the Spitzer Space Telescope, Herschel Space Observatory, the Atacama Large Millimeter/submillimeter Array, and a host of other ground-based interferometers and single-dish radio telescopes.

• ▪  Nearly all protostars have well-formed circumstellar disks that are likely to be rotationally supported; the ability to detect a disk around a protostar is more a question of spatial resolution rather than whether or not a disk is present.
• ▪  The disks around protostars have inherently higher millimeter/submillimeter luminosities as compared to disks around more-evolved pre-main-sequence stars, though there may be systematic variations between star-forming regions.
• ▪  The envelopes around protostars are inherently asymmetric, and streamers emphasize that mass flow through the envelopes to the disks may not be homogeneous.
• ▪  The current mass distribution of protostars may be impacted by selection bias given that it is skewed toward solar-mass protostars, which is inconsistent with the stellar initial mass function.

Ice mantles play a crucial role in shaping the astrochemical inventory of molecules during star and planet formation. Small-scale molecular processes have a profound impact on large-scale astronomical evolution. The areas of solid-state laboratory astrophysics and computational chemistry involve the study of these processes. We review laboratory efforts in ice spectroscopy, methodological advances and challenges, and laboratory and computational studies of ice physics and ice chemistry. We place the last of these in context with ice evolution from clouds to disks. Three takeaway messages from this review are:

• ▪  Laboratory and computational studies allow interpretation of astronomical ice spectra in terms of identification, ice morphology, and local environmental conditions as well as the formation of the involved chemical compounds.
• ▪  A detailed understanding of the underlying processes is needed to build reliable astrochemical models to make predictions about abundances in space.
• ▪  The relative importance of the different ice processes studied in the laboratory and computationally changes during the process of star and planet formation.

• ▪  The Hubble parameter, H0, is not an univocally defined quantity: It relates redshifts to distances in the near Universe, but it is also a key parameter of the ΛCDM standard cosmological model. As such, H0 affects several physical processes at different cosmic epochs and multiple observables. We have counted more than a dozen H0s that are expected to agree if (a) there are no significant systematics in the data and their interpretation and (b) the adopted cosmological model is correct.
• ▪  With few exceptions (proverbially confirming the rule), these determinations do not agree at high statistical significance; their values cluster around two camps: the low (68 km s¹ Mpc¹) and high (73 km s¹ Mpc¹) camps. It appears to be a matter of anchors. The shape of the Universe expansion history agrees with the model; it is the normalizations that disagree.
• ▪  Beyond systematics in the data/analysis, if the model is incorrect, there are only two viable ways to “fix” it: by changing the early time (z ≳ 1,100) physics and, thus, the early time normalization or by a global modification, possibly touching the model's fundamental assumptions (e.g., homogeneity, isotropy, gravity). None of these three options has the consensus of the community.
• ▪  The research community has been actively looking for deviations from ΛCDM for two decades; the one we might have found makes us wish we could put the genie back in the bottle.

The mantra “know thy star, know thy planet” has proven to be very important for many aspects of exoplanet science. Here I review how stellar abundances inform our understanding of planet composition and, thus, formation and evolution. In particular, I discuss how:

• ▪  The strongest star–planet connection is still the giant planet–metallicity correlation, the strength of which may indicate a break point between the formation of planets versus brown dwarfs.
• ▪  We do not have very good constraints on the lower metallicity limit for planet formation, although new statistics from TESS are helping, and it appears that, at low [Fe/H], α elements can substitute for iron as seeds for planet formation.
• ▪  The depletion of refractory versus volatile elements in stellar photospheres (particularly the Sun) was initially suggested as a sign of small planet formation but is challenging to interpret, and small differences in binary star compositions can be attributed mostly to processes other than planet formation.
• ▪  We can and should go beyond comparisons of the carbon-to-oxygen ratio in giant planets and their host stars, incorporating other volatile and refractory species to better constrain planet formation pathways.
• ▪  There appears to be a positive correlation between small planet bulk density and host star metallicity, but exactly how closely small planet refractory compositions match those of their host stars—and their true diversity—is still uncertain.

Observations that resolve nearby galaxies into individual regions across multiple phases of the gas–star formation–feedback “matter cycle” have provided a sharp new view of molecular clouds, star-formation efficiencies, timescales for region evolution, and stellar feedback. We synthesize these results, covering aspects relevant to the interpretation of observables, and conclude the following:

• ▪  The observed cloud-scale molecular gas surface density, line width, and internal pressure all reflect the large-scale galactic environment while also appearing mostly consistent with properties of a turbulent medium strongly affected by self-gravity.
• ▪  Cloud-scale data allow for statistical inference of both evolutionary and physical timescales. These suggest a period of cloud collapse on the order of the free-fall or turbulent crossing time (∼10–30 Myr) followed by forming massive stars and subsequent rapid (≲5 Myr) gas clearing after the onset of star formation. The star-formation efficiency per free-fall time is well determined over thousands of individual regions at εff ≈ 0.5−0.3^+0.7%.
• ▪  The role of stellar feedback is now measured using multiple observational approaches. The net yield is constrained by the requirement to support the vertical weight of the galaxy disk. Meanwhile, the short gas-clearing timescales suggest a large role for presupernova feedback in cloud disruption. This leaves the supernovae free to exert a large influence on the larger galaxy, including stirring turbulence, launching galactic-scale winds, and carving superbubbles.

This review covers the techniques, observations, and inferences of solar flare spectroscopy. It is not a spectroscopist's view of solar flares but rather a solar flare physicist's view of spectroscopy. Spectroscopy is carried out across the electromagnetic spectrum, but this review emphasizes the optical to soft X-ray part of the spectrum and discusses results from spectroscopy applied to the preflare, impulsive, and gradual phases, as well as a few highlights from modeling.

• ▪  The main spectroscopic signatures of the preflare phase are line broadening in optically thin ultraviolet to soft X-ray lines and small Doppler shifts in active region filaments that are becoming unstable.
• ▪  In the impulsive phase, fast upflows of heated plasma into the corona and slow downflows of cooler chromospheric plasma take place at the sites of strong chromospheric energy deposition.
• ▪  Radiation-hydrodynamic modeling of optically thick spectral lines gives a picture of an impulsive-phase chromosphere with a dense, heated layer deep in the atmosphere and an overlying, downward moving condensation that is partially optically thin.
• ▪  Gradual-phase observations show us the heated coronal plasma cooling and draining but also provide evidence for ongoing slow energy input and slow upflows in other locations.
• ▪  Interesting hints of non-Maxwellian and nonequilibrium plasmas have been found, along with possible evidence of plasma turbulence from line broadening.

The chemical compositions of stars encode the history of the universe and are thus fundamental for advancing our knowledge of astrophysics and cosmology. However, measurements of elemental abundance ratios, and our interpretations of them, strongly depend on the physical assumptions that dictate the generation of synthetic stellar spectra. Three-dimensional radiation-hydrodynamic (3D RHD) box-in-a-star simulations of stellar atmospheres offer a more realistic representation of surface convection occurring in late-type stars than do traditional one-dimensional (1D) hydrostatic models. As evident from a multitude of observational tests, the coupling of 3D RHD models with line formation in nonlocal thermodynamic equilibrium (non-LTE) today provides a solid foundation for abundance analysis for many elements. This review describes the ongoing and transformational work to advance the state of the art and replace 1D LTE spectrum synthesis with its 3D non-LTE counterpart. In summary:

• ▪  3D and non-LTE effects are intricately coupled, and consistent modeling thereof is necessary for high-precision abundances; such modeling is currently feasible for individual elements in large surveys. Mean 3D (〈3D〉) models are not adequate as substitutes.
• ▪  The solar abundance debate is presently dominated by choices and systematic uncertainties that are not specific to 3D non-LTE modeling.
• ▪  3D non-LTE abundance corrections have a profound impact on our understanding of FGK-type stars, exoplanets, and the nucleosynthetic origins of the elements.

Galactic winds shape the stellar, gas, and metal content of galaxies. To quantify their impact, we must understand their physics. We review potential wind-driving mechanisms and observed wind properties, with a focus on the warm ionized and hot X-ray-emitting gas. Energy and momentum injection by supernovae (SNe), cosmic rays, radiation pressure, and magnetic fields are considered in the light of observations:

• ▪  Emission and absorption line measurements of cool/warm gas provide our best physical diagnostics of galactic outflows.
• ▪  The critical unsolved problem is how to accelerate cool gas to the high velocities observed. Although conclusive evidence for no one mechanism exists, the momentum, energy, and mass-loading budgets observed compare well with theory.
• ▪  A model in which star formation provides a force ∼L/c, where L is the bolometric luminosity, and cool gas is pushed out of the galaxy's gravitational potential, compares well with available data. The wind power is ∼0.1 of that provided by SNe.
• ▪  The very hot X-ray-emitting phase may be a (or the) prime mover. Momentum and energy exchange between the hot and cooler phases is critical to the gas dynamics.
• ▪  Gaps in our observational knowledge include the hot gas kinematics and the size and structure of the outflows probed with UV absorption lines.

Simulations are needed to more fully understand mixing, cloud–radiation, cloud–cosmic ray, andcloud–hot wind interactions, the collective effects of star clusters, and both distributed andclustered SNe. Observational works should seek secondary correlations in the wind data thatprovide evidence for specific mechanisms and compare spectroscopy with the column density–velocity results from theory.

M dwarfs dominate the stellar population, accounting for three of every four stars, the nearest of which is Proxima Centauri, the closest destination beyond our Solar System. These cool stars span large ranges in luminosities (one ten-thousandth to 6% L⊙) and temperatures (2,100–3,900 K) and have spectra dominated by absorption bands of titanium oxide (TiO) and, for the latest spectral types, vanadium oxide (VO). They have masses that span 0.075 to 0.61 M⊙, a factor of eight, which is comparable with a spread in masses for dwarf types mid-B through K. Unlike these more massive stars, in the age of the Universe no M dwarfs have evolved in any significant way. M dwarf systems are multiple roughly one-quarter of the time, with the closest binaries found in orbits that have been circularized via tides for orbital periods of about one week. Unlike any other type of main sequence star, there is a gap in the distribution of M dwarfs near masses of 0.35 M⊙ that pinpoints the separation of partially and fully convective stars, yet both types of M dwarfs are often active, showing both Hα in emission and flares. Many planets are found orbiting M dwarfs, and most of them are terrestrial or neptunian in size, rather than jovian, yet much more work remains to be done to characterize the exoplanet population. Overall, the Solar Neighborhood is dominated by M dwarfs that are likely orbited by many small, as yet unseen, planets—some of which may harbor life very near to that in our Solar System:

• ▪  M dwarfs account for three of every four stars.
• ▪  M dwarf counts increase all the way to the end of the main sequence.
• ▪  M dwarfs are partially radiative at high masses and fully convective at low masses.