Explore the vast range of titles available.

The Milky Way is a barred spiral galaxy, with physical properties inferred from various tracers informed by the extrapolation of structures seen in other galaxies. However, the distances of these tracers are measured indirectly and are model-dependent. We constructed a map of the Milky Way in three dimensions, based on the positions and distances of thousands of classical Cepheid variable stars. This map shows the structure of our Galaxy’s young stellar population and allows us to constrain the warped shape of the Milky Way’s disk. A simple model of star formation in the spiral arms reproduces the observed distribution of Cepheids.

Local Group galaxies, particularly the Large and Small Magellanic Clouds, have historically played and continue to play a unique role in studies of the period–luminosity (PL), period–luminosity–color (PLC), and period–Wesenheit (PW) relations, not just for pulsating stars. In recent years, significant efforts have been devoted to calibrate the PL, PLC, and PW relationships at different wavelengths, including studies of the influence of metallicity and nonlinearities on the accuracy of measured distances. However, the PL diagram has many more astrophysical applications. It serves as a vital tool for classifying different types of pulsating stars and can even facilitate the discovery of new classes of variable stars. Moreover, it aids in distinguishing among various modes of pulsation, facilitates the identification of pulsating stars that are members of binary systems, and enables studies of the three-dimensional structures of neighboring galaxies. In this contribution, I present the latest results on the PL, PLC, and PW relations obeyed by various types of variable stars in Local Group galaxies – from δ Scuti stars to Mira variabless and from close binary systems to the mysterious long secondary periods exhibited by red giant and supergiant stars.

In an analysis of a large sample of microlensing events, a few suggest the existence of Earth-mass free-floating planets, but only the expected number of Jupiter-mass free-floating objects were detected. No large population of unbound or wide-orbit Jupiter-mass planets Theories of planet formation predict that there should be a population of free-floating planets with masses ranging from less than to several times the mass of Earth. However, the theories do not predict a substantial population of unbound Jupiter-mass planets, the existence of which was surprisingly inferred from the results of a previous analysis of microlensing events. Przemek Mróz et al . have now analysed a much larger sample of microlensing events and place an upper limit, at two standard deviations, on the population of free-floating Jupiter-mass planets that is almost a factor of ten lower than the previous claim, thereby essentially ruling out the existence of this particular population. They do, however, see a few very short events, which, on the basis of the theories of planet formation, indicate the existence of free-floating Earth-mass planets. Planet formation theories predict that some planets may be ejected from their parent systems as result of dynamical interactions and other processes 1 , 2 , 3 . Unbound planets can also be formed through gravitational collapse, in a way similar to that in which stars form 4 . A handful of free-floating planetary-mass objects have been discovered by infrared surveys of young stellar clusters and star-forming regions 5 , 6 as well as wide-field surveys 7 , but these studies are incomplete 8 , 9 , 10 for objects below five Jupiter masses. Gravitational microlensing is the only method capable of exploring the entire population of free-floating planets down to Mars-mass objects, because the microlensing signal does not depend on the brightness of the lensing object. A characteristic timescale of microlensing events depends on the mass of the lens: the less massive the lens, the shorter the microlensing event. A previous analysis 11 of 474 microlensing events found an excess of ten very short events (1–2 days)—more than known stellar populations would suggest—indicating the existence of a large population of unbound or wide-orbit Jupiter-mass planets (reported to be almost twice as common as main-sequence stars). These results, however, do not match predictions of planet-formation theories 3 , 12 and surveys of young clusters 8 , 9 , 10 . Here we analyse a sample of microlensing events six times larger than that of ref. 11 discovered during the years 2010–15. Although our survey has very high sensitivity (detection efficiency) to short-timescale (1–2 days) microlensing events, we found no excess of events with timescales in this range, with a 95 per cent upper limit on the frequency of Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star. We detected a few possible ultrashort-timescale events (with timescales of less than half a day), which may indicate the existence of Earth-mass and super-Earth-mass free-floating planets, as predicted by planet-formation theories 3 , 12 .

The gravitational wave detectors have shown a population of massive black holes that do not resemble those observed in the Milky Way 1 – 3 and whose origin is debated 4 – 6 . According to a possible explanation, these black holes may have formed from density fluctuations in the early Universe (primordial black holes) 7 – 9 , and they should comprise several to 100% of dark matter to explain the observed black hole merger rates 10 – 12 . If these black holes existed in the Milky Way dark matter halo, they would cause long-timescale gravitational microlensing events lasting years 13 . The previous experiments were not sufficiently sensitive to such events 14 – 17 . Here we present the results of the search for long-timescale microlensing events among the light curves of nearly 80 million stars located in the Large Magellanic Cloud that were monitored for 20 years by the Optical Gravitational Lensing Experiment survey 18 . We did not find any events with timescales longer than 1 year, whereas all shorter events detected may be explained by known stellar populations. We find that compact objects in the mass range from 1.8 × 10 −4 M ⊙ to 6.3 M ⊙ cannot make up more than 1% of dark matter, and those in the mass range from 1.3 × 10 −5 M ⊙ to 860  M ⊙ cannot make up more than 10% of dark matter. Thus, primordial black holes in this mass range cannot simultaneously explain a substantial fraction of dark matter and gravitational wave events. The results of the search for long-timescale microlensing events among the light curves of nearly 80 million stars located in the Large Magellanic Cloud indicate that there are no massive black holes in the Milky Way halo.

Long-term pre- and post-eruption observations of the classical nova V1213 Centauri reveal that its progenitor was a dwarf nova and that the mass-transfer rate increased considerably as a result of the nova explosion. Pre- and post-explosion of a classical nova What do novae look like during the millennia between eruptions? The classical nova V1213 Cen (Nova Centauri 2009) was first detected in May 2009. Now Przemek Mróz et al . report long-term observations of V1213 Cen from before the explosion to 2016, including serendipitous Very Large Telescope observations from April 2005. The star behaved as a low-mass-transfer dwarf nova in the years before the eruption. The post-nova is two orders of magnitude brighter than the pre-nova at minimum light with no trace of dwarf nova behaviour, implying that the mass-transfer rate increased considerably as a result of the nova explosion. V1213 Cen is now slowly fading and is likely to remain bright for a few decades before—according to hibernation theory predictions—again transforming into a dwarf nova. Cataclysmic variable stars—novae, dwarf novae, and nova-likes—are close binary systems consisting of a white dwarf star (the primary) that is accreting matter from a low-mass companion star (the secondary) 1 . From time to time such systems undergo large-amplitude brightenings. The most spectacular eruptions, with a ten-thousandfold increase in brightness, occur in classical novae and are caused by a thermonuclear runaway on the surface of the white dwarf 2 . Such eruptions are thought to recur on timescales of ten thousand to a million years 3 . In between, the system’s properties depend primarily on the mass-transfer rate: if it is lower than a billionth of a solar mass per year, the accretion becomes unstable and the matter is dumped onto the white dwarf during quasi-periodic dwarf nova outbursts 4 . The hibernation hypothesis 5 predicts that nova eruptions strongly affect the mass-transfer rate in the binary, keeping it high for centuries after the event 6 . Subsequently, the mass-transfer rate should significantly decrease for a thousand to a million years, starting the hibernation phase. After that the nova awakes again—with accretion returning to the pre-eruption level and leading to a new nova explosion. The hibernation model predicts cyclical evolution of cataclysmic variables through phases of high and low mass-transfer. The theory gained some support from the discovery of ancient nova shells around the dwarf novae Z Camelopardalis 7 and AT Cancri 8 , but direct evidence for considerable mass-transfer changes prior, during and after nova eruptions has not hitherto been found. Here we report long-term observations of the classical nova V1213 Cen (Nova Centauri 2009) covering its pre- and post-eruption phases and precisely documenting its evolution. Within the six years before the explosion, the system revealed dwarf nova outbursts indicative of a low mass-transfer rate. The post-nova is two orders of magnitude brighter than the pre-nova at minimum light with no trace of dwarf nova behaviour, implying that the mass-transfer rate increased considerably as a result of the nova explosion.

The era of large-scale photometric variability surveys began a quarter of a century ago, when three microlensing projects - EROS, MACHO, and OGLE - started their operation. These surveys initiated a revolution in the field of variable stars and in the next years they inspired many new observational projects. Large-scale optical surveys multiplied the number of variable stars known in the Universe. The huge, homogeneous and complete catalogs of pulsating stars, such as Cepheids, RR Lyrae stars, or long-period variables, offer an unprecedented opportunity to calibrate and test the accuracy of various distance indicators, to trace the three-dimensional structure of the Milky Way and other galaxies, to discover exotic types of intrinsically variable stars, or to study previously unknown features and behaviors of pulsators. We present historical and recent findings on various types of pulsating stars obtained from the optical large-scale surveys, with particular emphasis on the OGLE project which currently offers the largest photometric database among surveys for stellar variability.

Stellar-mass black holes are the final remnants of stars born with more than 15 solar masses. Billions are expected to reside in the Local Group, yet only a few are known, mostly detected through X-rays emitted as they accrete material from a companion star. Here, we report on VFTS 243: a massive X-ray-faint binary in the Large Magellanic Cloud. With an orbital period of 10.4 d, it comprises an O-type star of 25 solar masses and an unseen companion of at least nine solar masses. Our spectral analysis excludes a non-degenerate companion at a 5 σ confidence level. The minimum companion mass implies that it is a black hole. No other X-ray-quiet black hole is unambiguously known outside our Galaxy. The (near-)circular orbit and kinematics of VFTS 243 imply that the collapse of the progenitor into a black hole was associated with little or no ejected material or black-hole kick. Identifying such unique binaries substantially impacts the predicted rates of gravitational-wave detections and properties of core-collapse supernovae across the cosmos. An inactive black hole has been found in the Large Magellanic Cloud, bound into a binary star system. Having experienced a negligible ‘kick’ during formation, the existence of this black hole has strong implications for black hole-–black hole mergers.

Regular intrinsic brightness variations observed in many stars are caused by pulsations. These pulsations provide information on the global and structural parameters of the star. The pulsation periods range from seconds to years, depending on the compactness of the star and properties of the matter that forms its outer layers. Here, we report the discovery of more than a dozen previously unknown short-period variable stars: blue large-amplitude pulsators. These objects show very regular brightness variations with periods in the range of 20–40 min and amplitudes of 0.2–0.4 mag in the optical passbands. The phased light curves have a characteristic sawtooth shape, similar to the shape of classical Cepheids and RR Lyrae-type stars pulsating in the fundamental mode. The objects are significantly bluer than main-sequence stars observed in the same fields, which indicates that all of them are hot stars. Follow-up spectroscopy confirms a high surface temperature of about 30,000 K. Temperature and colour changes over the cycle prove the pulsational nature of the variables. However, large-amplitude pulsations at such short periods are not observed in any known type of stars, including hot objects. Long-term photometric observations show that the variable stars are very stable over time. Derived rates of period change are of the order of 10 −7 per year and, in most cases, they are positive. According to pulsation theory, such large-amplitude oscillations may occur in evolved low-mass stars that have inflated helium-enriched envelopes. The evolutionary path that could lead to such stellar configurations remains unknown. A previously unidentified class of variable stars has been found in OGLE survey data, characterized by periodic brightness variations on ~30-min timescales, amplitudes of ~0.3 mag and temperatures of ~30,000 K. They are potentially evolved low-mass stars.

We used the OGLE data to search for binary central stars of planetary nebulae (CSPNe) in the Large Magellanic Cloud (LMC). Nine binary CSPNe with periods between 0.24 and 23.6 days were discovered. The obtained fraction of binary CSPNe for the LMC PNe is without correcting for incompleteness.

The evolution of granulation is an important mechanism of the light variations of red supergiants (RSGs). Based on pure and complete samples of RSGs in the Magellanic Clouds, the mechanisms and characteristics of the granulation of RSGs are investigated based on time-series data. As predicted by the basic physical process of granulation and previous works, there are tight relations between granulation and stellar parameters of RSGs (i.e., the scaling relations). The scaling relations of RSGs provide a new method to infer stellar parameters by using the characteristic timescale and amplitude of granulations. Some faint sources deviate from the scaling relations, which may be due to the difference in the properties of the granulation of the RSGs before and after the blue loop or contamination by Mira variables. However, both of these possibilities suggest that the scaling relations of granulation is different among different types of stars.