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In the era of precision cosmology, it is essential to determine the Hubble constant empirically with an accuracy of one per cent or better 1 . At present, the uncertainty on this constant is dominated by the uncertainty in the calibration of the Cepheid period–luminosity relationship 2 , 3 (also known as the Leavitt law). The Large Magellanic Cloud has traditionally served as the best galaxy with which to calibrate Cepheid period–luminosity relations, and as a result has become the best anchor point for the cosmic distance scale 4 , 5 . Eclipsing binary systems composed of late-type stars offer the most precise and accurate way to measure the distance to the Large Magellanic Cloud. Currently the limit of the precision attainable with this technique is about two per cent, and is set by the precision of the existing calibrations of the surface brightness–colour relation 5 , 6 . Here we report a calibration of the surface brightness–colour relation with a precision of 0.8 per cent. We use this calibration to determine a geometrical distance to the Large Magellanic Cloud that is precise to 1 per cent based on 20 eclipsing binary systems. The final distance is 49.59 ± 0.09 (statistical) ± 0.54 (systematic) kiloparsecs. A new calibration of the surface brightness–colour relation of eclipsing binary stars gives a distance to the Large Magellanic Cloud that is precise to one per cent.

Ground-based and satellite observations show that the black hole in the ultraluminous X-ray source P13 has a mass of less than 15 times that of the Sun and displays the properties that typically distinguish ultraluminous X-ray sources from other stellar-mass black holes. What drives ultraluminous X-ray sources? Ultraluminous X-ray sources (ULXs) are non-nuclear point sources that are widely believed to contain either intermediate mass black holes or smaller, stellar mass black holes accreting from a binary companion. The study of ULXs provides information about black hole formation and/or modes of high Eddington rate accretion. Two papers in this issue of Nature describe pulsating ULXs with unusual properties. Christian Motch et al . find that source P13 in the galaxy NGC 7793 is in a ∼64 day period binary system. By modelling the strong optical and UV modulations arising from X-ray heating of the B9Ia donor star, they constrain the black hole mass to be less than 15 solar masses. Matteo Bachetti et al . observe a source in the galaxy M82 that, the pulsation data imply, harbours a neutron star rather than a black hole, raising doubts over the assumption that black holes power the most luminous X-ray binaries. Most ultraluminous X-ray sources 1 have a typical set of properties not seen in Galactic stellar-mass black holes. They have luminosities of more than 3 × 10 39  ergs per second, unusually soft X-ray components (with a typical temperature of less than about 0.3 kiloelectronvolts) and a characteristic downturn 2 , 3 in their spectra above about 5 kiloelectronvolts. Such puzzling properties have been interpreted either as evidence of intermediate-mass black holes 4 , 5 or as emission from stellar-mass black holes accreting above their Eddington limit 6 , 7 , analogous to some Galactic black holes at peak luminosity 8 , 9 . Recently, a very soft X-ray spectrum was observed in a rare and transient stellar-mass black hole 10 . Here we report that the X-ray source P13 in the galaxy NGC 7793 11 is in a binary system with a period of about 64 days and exhibits all three canonical properties of ultraluminous sources. By modelling the strong optical and ultraviolet modulations arising from X-ray heating of the B9Ia donor star, we constrain the black hole mass to be less than 15 solar masses. Our results demonstrate that in P13, soft thermal emission and spectral curvature are indeed signatures of supercritical accretion. By analogy, ultraluminous X-ray sources with similar X-ray spectra and luminosities of up to a few times 10 40  ergs per second can be explained by supercritical accretion onto massive stellar-mass black holes.

Observations of eight long-period, late-type eclipsing-binary systems composed of cool, giant stars are used to determine a distance to the Large Magellanic Cloud accurate to 2.2 per cent, providing a base for a determination of the Hubble constant to an accuracy of 3 per cent. Accurate distance to our nearest-neighbour galaxy The physical properties of stars in eclipsing binary systems can be accurately determined thanks to the intimate interactions between the two bodies, and by monitoring the fluctuating light from such systems it is possible to obtain accurate extragalactic distance measurement. This technique has now been used to determine the most accurate distance estimate yet for the Large Magellanic Cloud (LMC), our nearest-neighbour galaxy. The data from eight long-period, late-type eclipsing systems particularly suitable for this calibration technique suggest that the LMC is around 49.97 kiloparsecs from us, to an accuracy of 2.2%. The distance to the LMC is a key element in determining the Hubble constant, an important measure of the rate of expansion of the Universe. In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better 1 , 2 . At present, its uncertainty is dominated by the uncertainty in the distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale 2 , 3 . Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately 4 , 5 . The eclipsing-binary method was previously applied to the LMC 6 , 7 , but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future.

The first stars and their immediate successors should be found today in the central regions (bulges) of galaxies; old, metal-poor stars have now been found in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than that of the Sun without noticeable carbon enhancement, making it possibly the oldest known star in the Galaxy. The search for the earliest stars Galaxy formation models predict that the first stars born after the Big Bang and their immediate descendants should now be found preferentially near the centres (or bulges) of galaxies. Louise Howes et al . report observations of stars in the Milky Way bulge with some of the properties expected for these early stars formed during the 'cosmic dawn'. The authors identify more than 500 stars with an iron abundance less than 1/100th of the solar value, including one star with an iron abundance more than 10,000 times lower than solar value — without the carbon enhancement seen in stars in the outer regions of the Galaxy. Their chemical compositions are in general similar to typical halo stars of the same metallicity, although intriguing differences exist, including lower abundances of carbon. The first stars are predicted to have formed within 200 million years after the Big Bang 1 , initiating the cosmic dawn. A true first star has not yet been discovered, although stars 2 , 3 , 4 with tiny amounts of elements heavier than helium (‘metals’) have been found in the outer regions (‘halo’) of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions (‘bulges’) of galaxies, because they formed in the largest over-densities that grew gravitationally with time 5 , 6 . The Milky Way bulge underwent a rapid chemical enrichment during the first 1–2 billion years 7 , leading to a dearth of early, metal-poor stars 8 , 9 . Here we report observations of extremely metal-poor stars in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than the solar value without noticeable carbon enhancement. We confirm that most of the metal-poor bulge stars are on tight orbits around the Galactic Centre, rather than being halo stars passing through the bulge, as expected for stars formed at redshifts greater than 15. Their chemical compositions are in general similar to typical halo stars of the same metallicity although intriguing differences exist, including lower abundances of carbon.

Cepheid variable mass Cepheid variable stars have been important in the development of modern astrophysics through their use in establishing cosmic distances, but despite extensive research they retain some of their mysteries. One is the mass discrepancy problem, the fact that the masses of classical Cepheid supergiants calculated from pulsation theory (it is pulsation that causes their luminosity to vary) are smaller than the masses calculated from stellar evolution models. The ideal system in which to make an accurate mass determination would be a well-detached double-lined eclipsing binary in which one of the components was a classical Cepheid. Pietrzynski et al . report the discovery of just such a system in the Large Magellanic Cloud. The resultant mass determination, to a precision of one per cent, is in agreement with the mass as predicted by pulsation theory. Masses of pulsating classical Cepheid supergiants derived from stellar pulsation theory are smaller than the masses derived from stellar evolution theory. An independent determination for a classical Cepheid in a binary system is needed to determine which is correct. These authors report the discovery of a classical Cepheid in the Large Magellanic Cloud. They determine the mass to a precision of one per cent and show that it agrees with its pulsation mass. Stellar pulsation theory provides a means of determining the masses of pulsating classical Cepheid supergiants—it is the pulsation that causes their luminosity to vary. Such pulsational masses are found to be smaller than the masses derived from stellar evolution theory: this is the Cepheid mass discrepancy problem 1 , 2 , for which a solution is missing 3 , 4 , 5 . An independent, accurate dynamical mass determination for a classical Cepheid variable star (as opposed to type-II Cepheids, low-mass stars with a very different evolutionary history) in a binary system is needed in order to determine which is correct. The accuracy of previous efforts to establish a dynamical Cepheid mass from Galactic single-lined non-eclipsing binaries was typically about 15–30% (refs 6 , 7 ), which is not good enough to resolve the mass discrepancy problem. In spite of many observational efforts 8 , 9 , no firm detection of a classical Cepheid in an eclipsing double-lined binary has hitherto been reported. Here we report the discovery of a classical Cepheid in a well detached, double-lined eclipsing binary in the Large Magellanic Cloud. We determine the mass to a precision of 1% and show that it agrees with its pulsation mass, providing strong evidence that pulsation theory correctly and precisely predicts the masses of classical Cepheids.

Using gravitational microlensing, we detected a cold terrestrial planet orbiting one member of a binary star system. The planet has low mass (twice Earth's) and lies projected at ∼0.8 astronomical units (AU) from its host star, about the distance between Earth and the Sun. However, the planet's temperature is much lower, <60 Kelvin, because the host star is only 0.10 to 0.15 solar masses and therefore more than 400 times less luminous than the Sun. The host itself orbits a slightly more massive companion with projected separation of 10 to 15 AU. This detection is consistent with such systems being very common. Straightforward modification of current microlensing search strategies could increase sensitivity to planets in binary systems. With more detections, such binary-star planetary systems could constrain models of planet formation and evolution.

Lonely planets guide Gravitational microlensing observations in the direction of the Galactic Bulge have come up with a surprising result: the discovery of ten previously unknown extrasolar planets that are not bound to host stars. These seemingly free-ranging Jupiter-mass objects could be in very distant orbits around host stars, but no hosts could be detected within a distance of 10 astronomical units from the free-floating planets. It seems possible, therefore, that planet scattering is a routine part of the planet formation process. Since 1995, more than 500 exoplanets have been detected using different techniques 1 , 2 , of which 12 were detected with gravitational microlensing 3 , 4 . Most of these are gravitationally bound to their host stars. There is some evidence of free-floating planetary-mass objects in young star-forming regions 5 , 6 , 7 , 8 , but these objects are limited to massive objects of 3 to 15 Jupiter masses with large uncertainties in photometric mass estimates and their abundance. Here, we report the discovery of a population of unbound or distant Jupiter-mass objects, which are almost twice ( ) as common as main-sequence stars, based on two years of gravitational microlensing survey observations towards the Galactic Bulge. These planetary-mass objects have no host stars that can be detected within about ten astronomical units by gravitational microlensing. However, a comparison with constraints from direct imaging 9 suggests that most of these planetary-mass objects are not bound to any host star. An abrupt change in the mass function at about one Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.

The pulsating star OGLE-BLG-RRLYR-02792 is known to be a member of an eclipsing binary system, and its mass is now determined to be only 0.26 times that of the Sun, meaning that it cannot be a classical RR Lyrae pulsator. A new class of variable star: not RR Lyrae Astronomers use pulsating variable stars of the RR Lyrae type as indicators of the ages of galaxies, and as tools to measure distances to nearby galaxies. So the news that one of these stars had apparently been found as part of an eclipsing binary system was welcome: it meant that the mass of one of these pulsators, previously available only from models, could be unambiguously determined. But the story is not that simple. Pietrzyński et al . have now determined that the star in question, known as RRLYR-02792, has a mass 0.26 times that of the Sun. This means that it is not a classical RR Lyrae star. Instead, it seems to be a pulsator with observational properties temporarily similar to those of classical RR Lyrae stars, but with different stellar parameters and a different evolutionary history as part of a close binary. The authors estimate that 0.2% of samples of RR Lyrae variables may by contaminated by systems similar to this one, so distances previously measured using RR Lyrae stars should not be significantly affected by the presence of these binaries. RR Lyrae pulsating stars have been extensively used as tracers of old stellar populations for the purpose of determining the ages of galaxies, and as tools to measure distances to nearby galaxies 1 , 2 , 3 . There was accordingly considerable interest when the RR Lyrae star OGLE-BLG-RRLYR-02792 (referred to here as RRLYR-02792) was found to be a member of an eclipsing binary system 4 , because the mass of the pulsator (hitherto constrained only by models) could be unambiguously determined. Here we report that RRLYR-02792 has a mass of 0.26 solar masses ( ) and therefore cannot be a classical RR Lyrae star. Using models, we find that its properties are best explained by the evolution of a close binary system that started with and stars orbiting each other with an initial period of 2.9 days. Mass exchange over 5.4 billion years produced the observed system, which is now in a very short-lived phase where the physical properties of the pulsator happen to place it in the same instability strip of the Hertzsprung–Russell diagram as that occupied by RR Lyrae stars. We estimate that only 0.2 per cent of RR Lyrae stars may be contaminated by systems similar to this one, which implies that distances measured with RR Lyrae stars should not be significantly affected by these binary interlopers.

A statistical analysis of microlensing data from 2002–07 reveals that stars in the Milky Way are orbited by planets as a rule, rather than an exception. Planets common in the Milky Way Most of the extrasolar planets known so far were discovered using methods biased towards planets that are relatively close to their parent stars, and in this population about 17–30% of solar-like stars host a planet. A rather different picture emerges from an analysis of gravitational microlensing data collected between 2002 and 2007. This method probes planets that are farther away from their stars. The data reveal that it is the rule, rather than the exception, for stars in our Galaxy to host one planet or more. 'Super-Earths' are the most abundant type, being associated with around 62% of stars; 52% host cool Neptune-like planets; and 17% host 'Jupiters'. Most known extrasolar planets (exoplanets) have been discovered using the radial velocity 1 , 2 or transit 3 methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17–30% (refs 4 , 5 ) of solar-like stars host a planet. Gravitational microlensing 6 , 7 , 8 , 9 , on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing 10 . These planets are at least as numerous as the stars in the Milky Way 10 . Here we report a statistical analysis of microlensing data (gathered in 2002–07) that reveals the fraction of bound planets 0.5–10  au (Sun–Earth distance) from their stars. We find that of stars host Jupiter-mass planets (0.3–10  M J , where M J = 318  M ⊕ and M ⊕ is Earth’s mass). Cool Neptunes (10–30  M ⊕ ) and super-Earths (5–10  M ⊕ ) are even more common: their respective abundances per star are and . We conclude that stars are orbited by planets as a rule, rather than the exception.

We present multiwavelength broadband photometry and V V , I I time-resolved photometry for two variable bright stars in the SMC, OGLE004336.91-732637.7 (SMC-SC3) and OGLE004633.76-731204.3 (SMC-SC4). The light curves span 12 yr and show long-term periodicities (SMC-SC3) and modulated eclipses (SMC-SC4) that are discussed in terms of wide-orbit intermediate-mass interacting binaries and associated envelopes. SMC-SC3 shows a primary period of 238.1 days along with a complicated waveform suggesting ellipsoidal variablity influenced by an eccentric orbit. This star also shows a secondary variability with an unstable periodicity that has a mean value of 15.3 days. We suggest this could be associated with nonradial pulsations.