Atomic hydrogen (Hi) is a critical stepping stone in the gas evolution cycle of the interstellar medium (ISM) of the Milky Way. Hi traces both the cold, premolecular state before star formation and the warm, diffuse ISM before and after star formation. This review describes new, sensitive Hi absorption and emission surveys, which, together with high angular and spectral resolution Hi emission data

This article depicts my life and career over the past decades, beginning with my birth in 1927 and ending in my two dreams yet to be realized. This article focuses on my school years during wartime and my work with Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS) from 1951 on—serving as Director of SHAO during 1981–1993 and a Member of CAS since 1980—and shares some

The discovery of pulsars opened a new research field that allows studying a wide range of physics under extreme conditions. More than 3,000 pulsars are currently known, including especially more than 200 of them studied at gamma-ray frequencies. By putting recent insights into the pulsar magnetosphere in a historical context and by comparing them to key observational features at radio and high-energy

The radiation from stars and active galactic nuclei (AGNs) creates photodissociation regions (PDRs) and X-ray-dominated regions (XDRs), where the chemistry or heating is dominated by far-ultraviolet (FUV) radiation or X-ray radiation, respectively. PDRs include a wide range of environments, from the diffuse interstellar medium (ISM) to dense star-forming regions. XDRs are found in the center of galaxies

The observable characteristics and subsequent evolution of young stellar populations is dominated by their massive stars. As our understanding of those massive stars and the factors affecting their evolution improves, so our interpretation of distant, unresolved stellar systems can also advance. As observations increasingly probe the distant Universe, and the rare low-metallicity starbursts nearby

The magnetic field is the main driver of the activity in the solar upper atmosphere, but its measurement is notoriously difficult. In order to determine the magnetic field in the chromosphere, transition region, and corona, we need to measure and interpret the polarization signals that the scattering of anisotropic radiation and the Hanle and Zeeman effects introduce in the emitted spectral line radiation

The cold interstellar medium (ISM) plays a central role in the galaxy evolution process. It is the reservoir that fuels galaxy growth via star formation, the repository of material formed by these stars, and a sensitive tracer of internal and external processes that affect entire galaxies. Consequently, significant efforts have gone into systematic surveys of the cold ISM of the galaxies in the local

Photometric redshifts are essential in studies of both galaxy evolution and cosmology, as they enable analyses of objects too numerous or faint for spectroscopy. The Rubin Observatory, Euclid, and Roman Space Telescope will soon provide a new generation of imaging surveys with unprecedented area coverage, wavelength range, and depth. To take full advantage of these data sets, further progress in photometric

Massive stars have strong stellar winds that direct their evolution through the upper Hertzsprung–Russell diagram and determine the black hole mass function. Furthermore, wind strength dictates the atmospheric structure that sets the ionizing flux. Finally, the wind directly intervenes with the stellar envelope structure, which is decisive for both single-star and binary evolution, affecting predictions

Rocky planets are common around other stars, but their atmospheric properties remain largely unconstrained. Thanks to a wealth of recent planet discoveries and upcoming advances in observing capability, we are poised to characterize the atmospheres of dozens of rocky exoplanets in this decade. The theoretical understanding of rocky exoplanet atmospheres has advanced considerably in the last few years

Spirals in galaxies have long been thought to be caused by gravitational instability in the stellar component of the disk, but discerning the precise mechanism had proved elusive. Tidal interactions, and perhaps bars, may provoke some spiral responses, but spirals in many galaxies must be self-excited. We survey the relevant observational data and aspects of disk dynamical theory. The origin of the

Asteroseismology has grown from its beginnings three decades ago to a mature field teeming with discoveries and applications. This phenomenal growth has been enabled by space photometry with precision 10–100 times better than ground-based observations, with nearly continuous light curves for durations of weeks to years, and by large-scale ground-based surveys spanning years designed to detect all time-variable

The launch of the James Webb Space Telescope (JWST) in late 2021 marks a new start for studies of galaxy formation at high redshift ( z ≳ 6) during the era of cosmic reionization. JWST can capture sensitive, high-resolution images and multiobject spectroscopy in the IR that will transform our view of galaxy formation during the first billion years of cosmic history. This review summarizes our current

In the 1960s, novel and increasingly powerful observational techniques opened up the field of high-energy astrophysics. Cosmology started to become an empirical science, and there was a resurgence in the study of general relativity. Martin Rees became a graduate student at the University of Cambridge during that period and subsequently held postdoc positions in the United States. He was therefore fortunate

The Carrington event in 1859, a solar flare with an associated geomagnetic storm, has served as a prototype of possible superflare occurrence on the Sun. Recent geophysical (14C signatures in tree rings) and precise time-series photometry [the bolometric total solar irradiance (TSI) for the Sun, and the broadband photometry from Kepler and Transiting Exoplanet Survey Satellite, for the stars] have

We survey our understanding of classical novae—nonterminal, thermonuclear eruptions on the surfaces of white dwarfs in binary systems. The recent and unexpected discovery of GeV gamma rays from Galactic novae has highlighted the complexity of novae and their value as laboratories for studying shocks and particle acceleration. We review half a century of nova literature through this new lens, and conclude

A multitude of phenomena—such as the chemical enrichment of the Universe, the mass spectrum of planetary nebulae, white dwarfs and gravitational wave progenitors, the frequency distribution of supernovae, the fate of exoplanets, etc.—are highly regulated by the amounts of mass that stars expel through a powerful wind. For more than half a century, these winds of cool aging stars have been interpreted

In the past few years, significant advances have been made in understanding the distributions of exoplanet populations and the architecture of planetary systems. We review the recent progress of planet statistics, with a focus on the inner ≲1-AU region of planetary systems that has been fairly thoroughly surveyed by the Kepler mission. We also discuss the theoretical implications of these statistical

We review the physics and phenomenology of wave dark matter: a bosonic dark matter candidate lighter than about 30 eV. Such particles have a de Broglie wavelength exceeding the average interparticle separation in a galaxy like the Milky Way and are, thus, well described as a set of classical waves. We outline the particle physics motivations for such particles, including the quantum chromodynamics

We provide a nonspecialist overview of the current state of understanding of the structure and origin of our Solar System's transneptunian region (often called the Kuiper Belt), highlighting perspectives on planetesimal formation, planet migration, and the contextual relationship with protoplanetary disks. We review the dynamical features of the transneptunian populations and their associated differences

We describe the first observations of the same celestial object with gravitational waves and light. ▪  GW170817 was the first detection of a neutron star merger with gravitational waves. ▪  The detection of a spatially coincident weak burst of gamma-rays (GRB 170817A) 1.7 s after the merger constituted the first electromagnetic detection of a gravitational wave source and established a connection between

The spin of a black hole is an important quantity to study, providing a window into the processes by which a black hole was born and grew. Furthermore, spin can be a potent energy source for powering relativistic jets and energetic particle acceleration. In this review, I describe the techniques currently used to detect and measure the spins of black holes. It is shown that: ▪  Two well-understood

Access to microarcsecond astrometry is now routine in the radio, infrared (IR), and optical domains. In particular, the publication of the second data release (Gaia DR2) from the Gaia mission made it possible for every astronomer to work with easily accessible, high-precision astrometry for 1.7 billion sources to twenty-first magnitude over the full sky. ▪  Gaia provides splendid astrometry, but at

The concept of stars being tidally ripped apart and consumed by a massive black hole (MBH) lurking in the center of a galaxy first captivated theorists in the late 1970s. The observational evidence for these rare but illuminating phenomena for probing otherwise dormant MBHs first emerged in archival searches of the soft X-ray ROSAT All-Sky Survey in the 1990s, but has recently accelerated with the

In this autobiographical account, I first describe my family, then childhood and education in India. During 1953–55, I worked in the new field of radio astronomy at the Division of Radiophysics of the Commonwealth Scientific and Industrial Research Organisation in Australia. During 1956–57, I worked at the Radio Astronomy Station of Harvard University at Fort Davis, Texas, where I made observations

Star-forming regions show a rich and varied chemistry, including the presence of complex organic molecules—in both the cold gas distributed on large scales and the hot regions close to young stars ...

Ever deeper and wider lookback surveys have led to a fairly robust outline of the cosmic star formation history, which culminated around z~2 -- a period often nicknamed "cosmic noon." Our knowledge about star-forming galaxies at these epochs has dramatically advanced from increasingly complete population censuses and detailed views of individual galaxies. We highlight some of the key observational

Hydrogen Lyman-α (Lyα) emission has been one of the major observational probes for the high-redshift Universe since the first discoveries of high-z Lyα-emitting galaxies in the late 1990s. Due to t...

The initial mass function (IMF), describing the distribution of birth masses of stars, plays a pivotal role in establishing the observable properties of galaxies. This article reviews the evidence ...

Understanding the properties and physical mechanisms that shape dust attenuation curves in galaxies is one of the fundamental questions of extragalactic astrophysics, with a great practical significance for deriving the physical properties of galaxies, such as the star formation rate and stellar mass. Attenuation curves result from a combination of dust grain properties, dust content, and the spatial

The disks that orbit young stars are the essential conduits and reservoirs of material for star and planet formation. Their structures, meaning the spatial variations of the disk physical conditions, reflect the underlying mechanisms that drive those formation processes. Observations of the solids and gas in these disks, particularly at high resolution, provide fundamental insights on their mass distributions

The corona of the Sun is a unique environment in which magnetohydrodynamic (MHD) waves, one of the fundamental processes of plasma astrophysics, are open to a direct study. There is striking progre...

There is a broad consensus that accretion onto supermassive black holes and consequent jet formation power the observed emission from active galactic nuclei (AGNs). However, there has been less agr...

Characterizing the relationship between stars, gas, and metals in galaxies is a critical component of understanding the cosmic baryon cycle. We compile contemporary censuses of the baryons in colla...

Astronomers have played many roles in their engagement with the larger astronomy education ecosystem. Their activities have served both the formal and informal education communities worldwide, with levels of involvement from the occasional participant to the full-time professional. We discuss these many diverse roles, giving background, context, and perspective on their value in encouraging and improving

We describe ongoing searches for intermediate-mass black holes with M_BH ~ 100-10^5 M_sun. We review a range of search mechanisms, both dynamical and those that rely on accretion signatures. We find that dynamical and accretion signatures alike point to a high fraction of 10^9-10^10 M_sun galaxies hosting black holes with M_BH<10^5 M_sun. In contrast, there are no solid detections of black holes in

The advent of Gaia's 2nd data release in combination with large spectroscopic surveys are revolutionizing our understanding of the Galaxy. Thanks to these and the knowledge accumulated thus far, a more mature picture of the evolution of the early Milky Way is emerging: * Two of the traditional Galactic components, i.e. the stellar halo and the thick disk, appear to be intimately linked: stars with

Over the past decade increasingly robust estimates of the dense molecular gas content in galaxy populations between redshift 0 and the peak of cosmic galaxy/star formation from redshift 1-3 have become available. This rapid progress has been possible due to the advent of powerful ground-based, and space telescopes for combined study of several millimeter to far-IR, line or continuum tracers of the

I review here the spatially-resolved spectroscopic properties of low-redshift star-forming galaxies (and their retired counter-parts), using results from the most recent Integral Field Spectroscopy galaxy surveys. First, I briefly summarise the global spectroscopic properties of these galaxies, discussing the main ionization processes, and the global relations described between the star-formation rates

The existence of $\approx$10^9 Msun supermassive black holes (SMBHs) within the first billion year of the universe has stimulated numerous ideas for the prompt formation and rapid growth of BHs in the early universe. Here we review ways in which the seeds of massive BHs may have first assembled, how they may have subsequently grown as massive as $\approx$10^9 Msun, and how multi-messenger observations

This article is basically a scientific autobiography from a long and very rewarding career, covering childhood, education, theoretical work, observations, instrumentation, and some social activitie...

Stars lose a significant amount of angular momentum between birth and death, implying that efficient processes transporting it from the core to the surface are active. Space asteroseismology delive...

Dear readers: We are sad to report that, soon after submitting her draft manuscript for this prefatory chapter, Nancy Grace Roman passed away on December 25, 2018. This final version of her memoir ...

In situ research on cometary chemistry began when measurements from the Giotto mission at comet 1P/Halley revealed the presence of complex organics in the coma. New telescopes and space missions have provided detailed remote and in situ measurements of the composition of cometary volatiles. Recently, the Rosetta mission to comet 67P/Churyumov-Gerasimenko more than doubled the number of parent species

The solar chromosphere forms a crucial, yet complex and until recently poorly understood, interface between the solar photosphere and the heliosphere. ▪ Advances in high-resolution instrumentation,...

Cosmological observations are beginning to reach a level of precision that allow us to test some of the most fundamental assumptions in our working model of the Universe. One such an assumption is that gravity is governed by the General Theory of Relativity. In this review we discuss how one might go about extending General Relativity and how such extensions can be described in a unified way on large

Over a decade ago, a group of supernova explosions with peak luminosities far exceeding (often by >100) those of normal events, has been identified. These superluminous supernovae (SLSNe) have been a focus of intensive study. I review the accumulated observations and discuss the implications for the physics of these extreme explosions. SLSNe can be classified into hydrogen poor (SLSNe-I) and hydrogen

The corona is a layer of hot plasma that surrounds the Sun, traces out its complex magnetic field, and ultimately expands into interplanetary space as the supersonic solar wind. Although much has b...

There has been an incredibly large investment in obtaining high-resolution stellar spectra for determining chemical abundances of stars. This information is crucial to answer fundamental questions in Astronomy by constraining the formation and evolution scenarios of the Milky Way as well as the stars and planets residing in it. We have just entered a new era, in which chemical abundances of FGK-type

We review the use of emission lines for understanding galaxy evolution, focusing on excitation source, metallicity, ionization parameter, ISM pressure, and electron density. We discuss the physics,...

Star clusters stand at the intersection of much of modern astrophysics: the interstellar medium, gravitational dynamics, stellar evolution, and cosmology. Here we review observations and theoretical models for the formation, evolution, and eventual disruption of star clusters. Current literature suggests a picture of this life cycle with several phases: (1) Clusters form in hierarchically-structured

The lowest luminosity (L < 10^5 L_sun) Milky Way satellite galaxies represent the extreme lower limit of the galaxy luminosity function. These ultra-faint dwarfs are the oldest, most dark matter-dominated, most metal-poor, and least chemically evolved stellar systems known. They therefore provide unique windows into the formation of the first galaxies and the behavior of dark matter on small scales

The recent advancements in far-infrared (far-IR) astronomy brought about by the Herschel, SOFIA, and ALMA observatories have led to technological advancements in millimeterwave and submillimeterwav...

We summarize our understanding of millisecond radio bursts from an extragalactic population of sources. FRBs occur at an extraordinary rate, thousands per day over the entire sky with radiation energy densities at the source about ten billion times larger than those from Galactic pulsars. We survey FRB phenomenology, source models and host galaxies, coherent radiation models, and the role of plasma

The nuclei of most normal galaxies contain supermassive black holes, which can accrete gas through a disk and become active. These Active Galactic Nuclei, AGN, can form jets which are observed on scales from AU to Mpc and from meter wavelengths to TeV gamma ray energies. High resolution radio imaging and multi-wavelength/messenger campaigns are elucidating the conditions under which this happens. Evidence

Exoplanetary science is on the verge of an unprecedented revolution. The thousands of exoplanets discovered over the past decade have most recently been supplemented by discoveries of potentially habitable planets around nearby low-mass stars. Currently, the field is rapidly progressing towards detailed spectroscopic observations to characterise the atmospheres of these planets. While various surveys

I describe here my background and main steps in my studies. Each following step was a basis for the next one without a certain plan. I started my path with the study of kinematical properties of galactic populations, which smoothly transformed into the calculation of population models of galaxies. I had difficulties in satisfactorily modeling galaxies using population data; this led me to the dark