Astronomy, astrophysics and cosmology have changed out of all recognition over the last 100 years. The IAU has provided an essential means of fostering international collaboration in these disciplines including times of international tension. Developments will be highlighted which have profoundly changed our understanding and insight into the workings of our Universe.

Of all the sciences, astronomy is by far the most border-less in its activities, and the most advanced in its concepts of collaborating across borders. In their dealings and in their needs, today’s teams are mature enough to ignore gender differences and ethnic differences, and across the past 50+ years of IAU membership which I personally can chalk up, the IAU personnel, Commissions, and other bodies have come to reflect more nearly the same – albeit small – gender ratios as found in its member institutions. In the IAU there has always been space for the individual, and if one recalls the early contributions to the IAU by major players like Edith M¨uller, Giusa Cayrel, Anne Underhill and Charlotte Moore, I think it can be said that astronomy was, and knew it was, better off by giving such people the latitude that they deserved as scientists, rather than because they were women. When a meeting in Baltimore in 1992 was called to discuss “Women in Astronomy”, the pressure came from the younger generations, who feared that the low percentages of tenured women in astronomy would be allowed to continue unnoticed, so they created the Baltimore Charter to draw attention to what certainly appeared to some as discrimination. Even though there could be no quick fixes to the situation, and the winds of change have been more like zephyrs than the cleansing gales that some hoped for, the percentage of women now rising through the ranks is definitely on the increase, and is witnessing growing ethnic diversity. Those are a matter of pride for the IAU, and must be highlighted in this its Centenary Year.

The South African Astronomical Observatory (SAAO), formerly known as the Royal Observatory, Cape of Good Hope, will be 200 years old in 2020. Also, South Africa (SA), formerly a British colony known as the Cape of Good Hope, will celebrate her 100-year anniversary as an International Astronomical Union (IAU) member in 2020, following the IAU centenary in 2019 that this IAU Symposium 349 celebrates. In light of all this, particularly in anticipation of the 200-year anniversary of SAAO in 2020, the SA National Research Foundation (NRF) has developed a Roadmap for the History of Astronomy in South Africa. As part of this we are conducting an oral history of astronomers to complement the historical celebrations of the institutions and science relating to astronomy in SA, supported by the SA NRF. Primarily drawing on literature and setting the scene for this work, here we present a snippet of the on-going oral histories, to glean the role of the IAU in astronomy in South Africa and show the potential of the oral histories to inform and complement written history.

OAD, the Office of Astronomy for Development, one of the most significant innovations within the IAU, was created at the XXVII General Assembly in Rio de Janeiro in 2009 and opened in 2011. The new office brought together and strengthened several activities of the IAU aimed at helping astronomers in developing or isolated countries to keep in touch with their colleagues elsewhere and up-to-date with the developments in our science. Those activities were mediated through the old commission structure by Commission 38 (Exchange of Astronomers) and Commission 46 (Astronomy Education and Development) which oversaw the International Schools of Young Astronomers (ISYA), the Visiting Lecturer Programme (VLP) and Teaching for Astronomy Development (TAD). In addition, Jorge Sahade, during his term as IAU President (1985–1988), formed the Working Group for the Promotion and Development of Astronomy, as a sub-committee of the Executive Committee, and asked the present writer, then a Vice-President, to act as chair. That Working Group (later renamed the Working Group for the Worldwide Development of Astronomy, WGWWDA) operated within the context of the already existing services of the IAU and in cooperation with the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS). In this paper, the writer gives an account of the activities of the WGWWDA both during and between General Assemblies, until the year 2000, shortly after which he relinquished responsibility for them.

Getting a better understanding of the evolution and nucleosynthetic yields of the most metal-poor stars (Z ≲ 10−5) is critical because they are part of the big picture of the history of the primitive universe. Yet many of the remaining unknowns of stellar evolution lie in the birth, life, and death of these objects. We review stellar evolution of intermediate-mass Z ≤ 10−5 models existing in the literature, with a particular focus on the problem of their final fates. We emphasise the importance of the mixing episodes between the stellar envelope and the nuclearly processed core, which occur after stars exhaust their central He (second dredge-up and dredge-out episodes). The depth and efficiency of these episodes are critical to determine the mass limits for the formation of electron-capture SNe. Our knowledge of these phenomena is not complete because they are strongly affected by the choice of input physics. These uncertainties affect stars in all mass and metallicity ranges. However, difficulties in calibration pose additional challenges in the case of the most metal-poor stars. We also consider the alternative SN I1/2 channel to form SNe out of the most metal-poor intermediate-mass objects. In this case, it is critical to understand the thermally pulsing Asymptotic Giant Branch evolution until the late stages. Efficient second dredge-up and, later, third dredge-up episodes could be able to pollute stellar envelopes enough for the stars to undergo thermal pulses in a way very similar to that of higher initial Z objects. Inefficient second and/or third dredge-up may leave an almost pristine envelope, unable to sustain strong stellar winds. This may allow the H-exhausted core to grow to the Chandrasekhar mass before the envelope is completely lost, and thus let the star explode as an SN I1/2. After reviewing the information available on these two possible channels for the formation of SNe, we discuss existing nucleosynthetic yields of stars of metallicity Z ≤ 10−5 and present an example of nucleosynthetic calculations for a thermally pulsing Super-Asymptotic Giant Branch star of Z = 10−5. We compare theoretical predictions with observations of the lowest [Fe/H] objects detected. The review closes by discussing current open questions as well as possible fruitful avenues for future research.

It is shown that upon combining GALEX far-ultraviolet and Johnson B magnitudes a resultant FUV–B colour can be obtained that for red giant stars of luminosity classes III and II correlates well with chromospheric emission in the cores of the Mg iih and k lines. Giant stars throughout the colour range 0.8 ≤ B – V ≤ 1.6 exhibit such a phenomenon. The main result of this paper is to show that GALEX far-ultraviolet photometry can provide information about the degree of chromospheric activity among red giant stars, and as such may offer a tool for surveying the evolution of chromospheric activity from the main sequence into the red giant phases of stellar evolution.

Breakthrough Listen is a 10-yr initiative to search for signatures of technologies created by extraterrestrial civilisations at radio and optical wavelengths. Here, we detail the digital data recording system deployed for Breakthrough Listen observations at the 64-m aperture CSIRO Parkes Telescope in New South Wales, Australia. The recording system currently implements two modes: a dual-polarisation, 1.125-GHz bandwidth mode for single-beam observations, and a 26-input, 308-MHz bandwidth mode for the 21-cm multibeam receiver. The system is also designed to support a 3-GHz single-beam mode for the forthcoming Parkes ultra-wideband feed. In this paper, we present details of the system architecture, provide an overview of hardware and software, and present initial performance results.

We use Gemini Multi-Object Spectrograph integral Field Unit observations of the inner 285 × 400 pc2 region of the Seyfert 2 galaxy NGC 5643 to map the [S iii]λ9069 emission line flux distribution and kinematics, as well as the stellar kinematics, derived by fitting the Ca iiλλλ8498,8542,8662 triplet, at a spatial resolution of 45 pc. The stellar velocity field shows regular rotation, with a projected velocity of 100 km s−1 and kinematic major axis along a position angle of –36°. A ring of low stellar velocity dispersion values (∼70 km s−1), attributed to young/intermediate age stellar populations, is seen surrounding the nucleus with a radius of 50 pc. We found that the [S iii] flux distribution shows an elongated structure along the east–west direction and its kinematics is dominated by outflows within a bi-cone at an ionised gas outflow rate of 0.3 M⊙ yr−1. In addition, velocity slices across the [S iii]λ9069 emission line reveal a kinematic component attributed to rotation of gas in the plane of the galaxy.

The birth of stars and the formation of galaxies are cornerstones of modern astrophysics. While much is known about how galaxies globally and their stars individually form and evolve, one fundamental property that affects both remains elusive. This is problematic because this key property, the birth mass distribution of stars, referred to as the stellar initial mass function, is a key tracer of the physics of star formation that underpins almost all of the unknowns in galaxy and stellar evolution. It is perhaps the greatest source of systematic uncertainty in star and galaxy evolution. The past decade has seen a growing variety of methods for measuring or inferring the initial mass function. This range of approaches and evolving definitions of the quantity being measured has in turn led to conflicting conclusions regarding whether or not the initial mass function is universal. Here I review this growing wealth of approaches, and highlight the importance of considering potential initial mass function variations, reinforcing the need to carefully quantify the scope and uncertainties of measurements. I present a new framework to aid the discussion of the initial mass function and promote clarity in the further development of this fundamental field.

Merger trees harvested from cosmological N-body simulations encode the assembly histories of dark matter halos over cosmic time and are a fundamental component of semi-analytical models of galaxy formation. The ability to compare the tools used to construct merger trees, namely halo finders and tree building algorithms, in an unbiased and systematic manner is critical to assess the quality of merger trees. In this paper, we present the dendrogram, a novel method to visualise merger trees, which provides a comprehensive characterisation of a halo’s assembly history—tracking subhalo orbits, halo merger events, and the general evolution of halo properties. We show the usefulness of the dendrogram as a diagnostic tool of merger trees by comparing halo assembly simulation analysed with three different halo finders—VELOCIraptor, AHF, and Rockstar—and their associated tree builders. Based on our analysis of the resulting dendrograms, we highlight how they have been used to motivate improvements to VELOCIraptor. The dendrogram software is publicly available online, at: https://github.com/rhyspoulton/MergerTree-Dendrograms.

It has recently been shown that the abundance of cold neutral gas may follow a similar evolution as the star formation history. This is physically motivated, since stars form out of this component of the neutral gas and if the case, would resolve the long-standing issue that there is a clear disparity between the total abundance of neutral gas and star-forming activity over the history of the Universe. Radio-band 21-cm absorption traces the cold gas and comparison with the Lyman-α absorption, which traces all of the gas, provides a measure of the cold gas fraction, or the spin temperature, Tspin. The recent study has shown that the spin temperature (degenerate with the ratio of the absorber/emitter extent) appears to be anti-correlated with the star formation density, ψ*, with 1/Tspin undergoing a similar steep evolution as ψ* over redshifts of 0 ≲ z ≲ 3, whereas the total neutral hydrogen exhibits little evolution. Above z ∼ 3, where ψ* shows a steep decline with redshift, there are insufficient 21-cm data to determine whether 1/Tspin continues to follow ψ*. Knowing this is paramount in ascertaining whether the cold neutral gas does trace the star formation over the Universe’s history. We explore the feasibility of resolving this with 21-cm observations of the largest contemporary sample of reliable damped Lyman-α absorption systems and conclude that, while today’s largest radio interferometers can reach the required sensitivity at z ≲ 3.5, the Square Kilometre Array is required to probe higher redshifts.

The purpose of this paper is to determine the origin of the photometric variations of 48 Lib using the data from the STEREO and to investigate their relations with the disk structure. The photometric data comprise a period of five years from 2007 to 2011. The spectroscopic data covering the same time interval are provided from the BeSS database. The Hα lines are examined by measuring their equivalent widths and line intensities. Hα variations are then compared with those displayed by the photometric data. From the photometry, high-precision results (10−5 c d−1 in frequency and 10−4 mag in amplitude) are obtained. It is detected that the star has shown 24 frequencies, mainly clustered around the peaks at 2.48896(1) and 5.08150(2) c d−1. The analysis reveals that the photometric frequencies are not due to pulsation, but caused by the rotation, and that the remaining frequencies arise from transient activities on or just above the photosphere. Also, it is shown that the spectroscopic data exhibit a significant Hα variability, and that the Hα line variation depends on the variation of frequency and amplitude, something which has been often proposed in the literature but has never before been demonstrated observationally. This proves that the disk structure and photometric variations are related.

We describe the motivation and design details of the ‘Phase II’ upgrade of the Murchison Widefield Array radio telescope. The expansion doubles to 256 the number of antenna tiles deployed in the array. The new antenna tiles enhance the capabilities of the Murchison Widefield Array in several key science areas. Seventy-two of the new tiles are deployed in a regular configuration near the existing array core. These new tiles enhance the surface brightness sensitivity of the array and will improve the ability of the Murchison Widefield Array to estimate the slope of the Epoch of Reionisation power spectrum by a factor of ∼3.5. The remaining 56 tiles are deployed on long baselines, doubling the maximum baseline of the array and improving the array u, v coverage. The improved imaging capabilities will provide an order of magnitude improvement in the noise floor of Murchison Widefield Array continuum images. The upgrade retains all of the features that have underpinned the Murchison Widefield Array’s success (large field of view, snapshot image quality, and pointing agility) and boosts the scientific potential with enhanced imaging capabilities and by enabling new calibration strategies.

Type Ic supernovae can be classified as normal supernovae Ic, type Ic super-luminous supernovae, X-ray flash-connected supernovae, broad-line supernovae Ic, and gamma-ray burst-connected supernovae. Here we suggest an inner connection for all kinds of supernovae Ic which is based on whether a pair of jets are successfully launched: a normal supernovae Ic is a normal core collapsar without jets launched; a gamma-ray burst-associated supernovae Ic is a core collapsar with relativistic jets launched and successfully breaking out the envelope of the progenitor; an X-ray flash-associated supernovae Ic is a core collapsar with jets launched but can only develop a relativistic shock breakout; a broad-line supernovae Ic is an off-axis gamma-ray burst or an X-ray flash-associated supernova; and a type Ic super-luminous supernovae is close to the X-ray flash-connected supernovae Ic, but the shock breakout is not relativistic and most of the jet energy is deposited into the supernova component. Based on the luminosity-distance diagram, we derived the luminosity function of all different types of supernovae Ic as a whole. We also show that the normal supernovae Ic and gamma-ray burst-connected supernovae Ic have similar accumulative distributions.