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Type Ia supernovae are important cosmological distance indicators. Each of these bright supernovae supposedly results from the thermonuclear explosion of a white dwarf star that, after accreting material from a companion star, exceeds some mass limit, but the true nature of the progenitor star system remains controversial. Here we report the spectroscopic detection of circumstellar material in a normal type Ia supernova explosion. The expansion velocities, densities, and dimensions of the circumstellar envelope indicate that this material was ejected from the progenitor system. In particular, the relatively low expansion velocities suggest that the white dwarf was accreting material from a companion star that was in the red-giant phase at the time of the explosion.

Type Ia supernovae: maintaining the standard The status of type Ia supernovae as cosmological 'standard candles' relies upon the assumption that they are very similar to one another and form a uniform class of objects. Recently, however, observational differences between type Ia supernovae have emerged. Maeda et al . propose a new model that takes account of recent theoretical and observational data and explains the observed spectral diversity as a consequence of random directions from which a theoretically proposed asymmetric explosion is viewed. On this basis, the spectral evolution diversity is no longer a concern in the continued use of type Ia supernovae as standard candles. Type Ia supernovae form a class of cosmological 'standard candles', a property that led to the discovery of an accelerating Universe, but recent investigations have revealed that they are more complicated in nature. Here the authors report that their observed spectral diversity is a consequence of the random directions from which their theoretically asymmetrical explosions are viewed, and that this diversity is therefore no longer a concern in using them as standard candles. Type Ia supernovae form an observationally uniform class of stellar explosions, in that more luminous objects have smaller decline-rates 1 . This one-parameter behaviour allows type Ia supernovae to be calibrated as cosmological ‘standard candles’, and led to the discovery of an accelerating Universe 2 , 3 . Recent investigations, however, have revealed that the true nature of type Ia supernovae is more complicated. Theoretically, it has been suggested 4 , 5 , 6 , 7 , 8 that the initial thermonuclear sparks are ignited at an offset from the centre of the white-dwarf progenitor, possibly as a result of convection before the explosion 4 . Observationally, the diversity seen in the spectral evolution of type Ia supernovae beyond the luminosity–decline-rate relation is an unresolved issue 9 , 10 . Here we report that the spectral diversity is a consequence of random directions from which an asymmetric explosion is viewed. Our findings suggest that the spectral evolution diversity is no longer a concern when using type Ia supernovae as cosmological standard candles. Furthermore, this indicates that ignition at an offset from the centre is a generic feature of type Ia supernovae.

A supernova with a past Theory suggests that stars with initial masses greater than 25–30 times that of the Sun end their stellar lives as Wolf–Rayet stars, becoming hydrogen-deficient by rapidly losing mass through strong stellar winds. Any subsequent supernova explosion should produce ejecta of low kinetic energy and faint optical luminosity, with a small mass fraction of radioactive nickel. Until now no core-collapse supernovae fitting this description have been detected. But SN 2008ha, discovered on 7 November 2008, appears to fit the bill. A detailed photometric and spectroscopic study shows SN 2008ha to be the faintest and lowest-luminosity hydrogen-deficient supernova known. This discovery raises the possibility that other similar events have been observed previously — SN 2002cx is one candidate — but were classified as 'peculiar thermonuclear supernovae'. Theory suggests that stars with initial masses greater than 25–30 solar masses end up as Wolf-Rayet stars, which are deficient in hydrogen in their outer layers; subsequent supernova explosions should produce ejecta of low kinetic energy, a faint optical luminosity and a small mass fraction of radioactive nickel, but no weak, hydrogen-deficient, core-collapse supernovae have hitherto been see. Now, SN 2008ha is reported to be a faint hydrogen-poor supernova. The final fate of massive stars depends on many factors. Theory suggests that some with initial masses greater than 25 to 30 solar masses end up as Wolf–Rayet stars, which are deficient in hydrogen in their outer layers because of mass loss through strong stellar winds. The most massive of these stars have cores which may form a black hole and theory predicts that the resulting explosion of some of them produces ejecta of low kinetic energy, a faint optical luminosity and a small mass fraction of radioactive nickel 1 , 2 , 3 . An alternative origin for low-energy supernovae is the collapse of the oxygen–neon core of a star of 7–9 solar masses 4 , 5 . No weak, hydrogen-deficient, core-collapse supernovae have hitherto been seen. Here we report that SN 2008ha is a faint hydrogen-poor supernova. We propose that other similar events have been observed but have been misclassified as peculiar thermonuclear supernovae (sometimes labelled SN 2002cx-like events 6 ). This discovery could link these faint supernovae to some long-duration γ-ray bursts, because extremely faint, hydrogen-stripped core-collapse supernovae have been proposed to produce such long γ-ray bursts, the afterglows of which do not show evidence of associated supernovae 7 , 8 , 9 .

Dry run for a supernova When a bright optical transient was discovered in galaxy UGC 4904 in October 2004 the signs were that it was big enough and bright enough to be a supernova. Further work suggested that it was not quite on that scale; but almost exactly two years after its discovery it seems to have exploded in a big way. Supernova SN 2006jc is in exactly the same place in the sky as the earlier optical transient. This is the first time that such a double outburst has been observed. One possibility is that the initial transient was an outburst from a Wolf-Rayet star, a very hot massive star losing mass rapidly. Or the system might be a binary containing a luminous blue variable star that erupted in 2004, followed two years later by a companion Wolf-Rayet star exploding as SN 2006jc. The peculiar Type Ib supernova SN 2006jc is spatially coincident with a bright optical transient that occurred in 2004. An outburst (similar to that of a luminous blue variable star) of a Wolf–Rayet star could be invoked for the transient, but this would be the first observational evidence of such a phenomenon. Alternatively a massive binary system composed of an LBV which erupted in 2004, and a Wolf–Rayet star exploding as SN 2006jc, could explain the observations. The death of massive stars produces a variety of supernovae, which are linked to the structure of the exploding stars 1 , 2 . The detection of several precursor stars of type II supernovae has been reported (see, for example, ref. 3 ), but we do not yet have direct information on the progenitors of the hydrogen-deficient type Ib and Ic supernovae. Here we report that the peculiar type Ib supernova SN 2006jc is spatially coincident with a bright optical transient 4 that occurred in 2004. Spectroscopic and photometric monitoring of the supernova leads us to suggest that the progenitor was a carbon-oxygen Wolf–Rayet star embedded within a helium-rich circumstellar medium. There are different possible explanations for this pre-explosion transient. It appears similar to the giant outbursts of luminous blue variable stars (LBVs) of 60–100 solar masses 5 , but the progenitor of SN 2006jc was helium- and hydrogen-deficient (unlike LBVs). An LBV-like outburst of a Wolf–Rayet star could be invoked, but this would be the first observational evidence of such a phenomenon. Alternatively, a massive binary system composed of an LBV that erupted in 2004, and a Wolf–Rayet star exploding as SN 2006jc, could explain the observations.

When a star passes within the tidal radius of a supermassive black hole, it will be torn apart 1 . For a star with the mass of the Sun ( M ⊙ ) and a non-spinning black hole with a mass <10 8 M ⊙ , the tidal radius lies outside the black hole event horizon 2 and the disruption results in a luminous flare 3–6 . Here we report observations over a period of ten months of a transient, hitherto interpreted 7 as a superluminous supernova 8 . Our data show that the transient rebrightened substantially in the ultraviolet and that the spectrum went through three different spectroscopic phases without ever becoming nebular. Our observations are more consistent with a tidal disruption event than a superluminous supernova because of the temperature evolution 6 , the presence of highly ionized CNO gas in the line of sight 9 and our improved localization of the transient in the nucleus of a passive galaxy, where the presence of massive stars is highly unlikely 10,11 . While the supermassive black hole has a mass >10 8 M ⊙ 12,13 , a star with the same mass as the Sun could be disrupted outside the event horizon if the black hole were spinning rapidly 14 . The rapid spin and high black hole mass can explain the high luminosity of this event. Transient object ASASSN-15lh was previously cast as the most luminous supernova ever discovered. Now, however, there is convincing evidence that its flare was a tidal disruption event: a rapidly-spinning black hole tearing apart a neighbouring star.

Type II supernovae are the final stage of massive stars (above 8  M ⊙ ) which retain part of their hydrogen-rich envelope at the moment of explosion. They typically eject up to 15 M ⊙ of material, with peak magnitudes of −17.5 mag and energies in the order of 10 51 erg, which can be explained by neutrino-driven explosions and neutron star formation. Here, we present our study of OGLE-2014-SN-073, one of the brightest type II supernovae ever discovered, with an unusually broad lightcurve combined with high ejecta velocities. From our hydrodynamical modelling, we infer a remarkable ejecta mass of 60 - 16 + 42 M ⊙ and a relatively high explosion energy of 12 .4 - 5 .9 + 13 .0 × 1 0 51 erg. We show that this object belongs, along with a very small number of other hydrogen-rich supernovae, to an energy regime that is not explained by standard core-collapse neutrino-driven explosions. We compare the quantities inferred by the hydrodynamical modelling with the expectations of various exploding scenarios and attempt to explain the high energy and luminosity released. We find some qualitative similarities with pair-instability supernovae, although the prompt injection of energy by a magnetar seems to be a viable alternative explanation for such an extreme event. The authors present a spectrophotometric and hydrodynamical study of supernova OGLE-2014-SN-073, which had remarkably high inferred ejecta mass and energy, potentially higher than can be explained with canonical core-collapse neutrino-driven explosions.

In the version of this Article originally published the Fig. 6 y axis label read 'Mej' but should have read 'MNi'. This has now been corrected.

Nature Astronomy 1, 0002 (2016); published 12 December 2016; corrected 22 December 2016. In the version of this Letter originally published the estimated energy radiated by ASASSN-15lh up to 25 May 2016 was incorrect and should have read 1.88 ± 0.19 × 1052 erg.

Observations and modelling of an optical transient counterpart to a gravitational-wave event and γ-ray burst reveal that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a source of heavy elements. When neutron stars collide Merging neutron stars are potential sources of gravitational waves and have long been predicted to produce jets of material as part of a low-luminosity transient known as a 'kilonova'. There is growing evidence that neutron-star mergers also give rise to short, hard gamma-ray bursts. A group of papers in this issue report observations of a transient associated with the gravitational-wave event GW170817—a signature of two neutron stars merging and a gamma-ray flash—that was detected in August 2017. The observed gamma-ray, X-ray, optical and infrared radiation signatures support the predictions of an outflow of matter from double neutron-star mergers and present a clear origin for gamma-ray bursts. Previous predictions differ over whether the jet material would combine to form light or heavy elements. These papers now show that the early part of the outflow was associated with lighter elements whereas the later observations can be explained by heavier elements, the origins of which have been uncertain. However, one paper (by Stephen Smartt and colleagues) argues that only light elements are needed for the entire event. Additionally, Eleonora Troja and colleagues report X-ray observations and radio emissions that suggest that the 'kilonova' jet was observed off-axis, which could explain why gamma-ray-burst detections are seen as dim. Gravitational waves were discovered with the detection of binary black-hole mergers 1 and they should also be detectable from lower-mass neutron-star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal. This signal is luminous at optical and infrared wavelengths and is called a kilonova 2 , 3 , 4 , 5 . The gravitational-wave source GW170817 arose from a binary neutron-star merger in the nearby Universe with a relatively well confined sky position and distance estimate 6 . Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817 and with a weak, short γ-ray burst 7 , 8 . The transient has physical parameters that broadly match the theoretical predictions of blue kilonovae from neutron-star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01 solar masses, with an opacity of less than 0.5 square centimetres per gram, at a velocity of 0.2 ± 0.1 times light speed. The power source is constrained to have a power-law slope of −1.2 ± 0.3, consistent with radioactive powering from r-process nuclides. (The r-process is a series of neutron capture reactions that synthesise many of the elements heavier than iron.) We identify line features in the spectra that are consistent with light r-process elements (atomic masses of 90–140). As it fades, the transient rapidly becomes red, and a higher-opacity, lanthanide-rich ejecta component may contribute to the emission. This indicates that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.