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News Feature: How to light a cosmic candle Astronomers are still struggling to identify the companions that help white dwarf stars self-destruct in violent supernovae explosions. Nadia Drake Science Writer
Billions of years ago, an ill-fated star not so different from our Sun began to age and balloon outward. As it grew, the star’s glow darkened to a deep, foreboding red, and its outer layers sloughed off into space. Eventually, the star’s nuclear furnace blinked out. All that remained was a dense, lifeless core about the size of Earth: a white dwarf star.
That was the first time the star died. However, a spectacular resurrection was at hand. Material stolen from a nearby stellar companion ignited the dwarf’s carbon and oxygen layers and triggered a ferocious thermonuclear explosion that flung smoldering material outward at about 10% of the speed of light. Decaying radioactive
In 1572, the Danish astronomer Tycho Brahe observed and studied the explosion of a star that became known as Tycho’s supernova. More than four centuries later, Chandra’s image of the supernova remnant shows an expanding bubble of multimillion-degree debris (green and red) inside a more rapidly moving shell of extremely high energy electrons (filamentary blue). Image courtesy of NASA/CXC/Rutgers/J. Warren and J. Hughes, et al. www.pnas.org/cgi/doi/10.1073/pnas.1413121111
elements transformed the billowing material into a blinding beacon of light. In its second death, the star blazed with the brilliance of three billion suns. Photons expelled by this terminal stellar spasm zoomed across space. For 21 million years, they traveled from their home in the Pinwheel Galaxy through dust and clouds until—quite improbably—they collided with a telescope perched atop a mountain in southern California. It was August 24, 2011, and the Palomar Transient Factory had just seen the nearest, freshest type 1a supernova that had yet been detected (1, 2). In the days and weeks that followed, that twice-dead star—now called supernova 2011fe—was the most studied object in the sky. Ordinary in every way, it was just what astronomers needed to learn more about how white dwarf stars explode. Type 1a supernovae have been observed for millennia, but in recent decades, their predictable brightness has made then invaluable as cosmic distance markers. In the late 1990s, Nobel Prize-winning observations based on these stellar explosions revealed that the Universe is expanding at an accelerating rate, propelled by a poorly understood phenomenon dubbed dark energy (3, 4). However, the processes that produce type 1a supernovae are still fundamentally mysterious. “That’s a scary thing,” says astrophysicist Brad Tucker of Australian National University and the University of California, Berkeley. “These are very powerful tools in cosmology, but we really don’t know what’s going on with them.” he says. Before 2011fe exploded, astronomers did not have much observational evidence that exploding white dwarfs were responsible for the supernovae. Just as the discovery answered one question, it also added tinder to a debate that burns brightly today: What kind of starry companion is donating material to the doomed dwarf? “People are now really embracing the idea there may be more than one way to make a type 1a supernova,” says Peter Nugent, an
PNAS | August 19, 2014 | vol. 111 | no. 33 | 11909–11911
Supernova 2011fe was discovered just hours after it exploded in the Big Dipper. Studies by the Nearby Supernova Factory of its spectrum as it evolved over time have produced a benchmark atlas of data by which to measure all future type Ia supernovas. Image courtesy of B. J. Fulton (Las Cumbres Observatory Global Telescope Network, Goleta, CA).
astrophysicist at Lawrence Berkeley National Quite by chance, PIRATE had looked at the Pinwheel Galaxy just 4 hours after 2011fe Laboratory in California. went off. The image showed no trace of a The Supernova Next Door supernova. The only possibility was that the Every night, the Palomar Transient Factory exploding star was extremely dense and very takes hundreds of images of the sky. Software small—less than 2% of the Sun’s diameter subtracts those images from one another, and (5). It was the strongest evidence yet that type flags anything that changes between shots— 1a supernovae erupt from white dwarfs. what astronomers call an astrophysical transient. These objects include variable stars, The Unusual Suspect gamma ray bursts, and supernovae. Two and Carbon-oxygen white dwarfs are dense, a half years ago, however, Nugent was sifting containing roughly a Sun’s mass of material through the images himself because the sys- squeezed into an Earth-size object. In these tem’s software had crashed the night before. stars, the inward crush of gravity is counteracted by electron degeneracy pressure, a That’s when he discovered 2011fe. The explosion was remarkably close and, quantum mechanical property that conalthough it was just 11 hours old, the bal- strains how tightly packed electrons can be looning debris cloud was already big enough (this is the reason white dwarfs are called to fill the orbit of Jupiter. It was noon in degenerate stars). Degeneracy pressure usuBerkeley, so he asked a colleague to aim ally prevents a catastrophic collapse, leaving a telescope in the Canary Islands at the the stable white dwarf to slowly fade away growing spot of light. That observation over billions of years. However, if the star crosses a crucial revealed the classic spectrum of a normal, young 1a supernova, containing silicon, cal- threshold around 1.4 solar masses, it becomes cium, a little bit of iron—and no hydrogen. massive enough to both overwhelm deMany of the major space observatories and generacy pressure and begin fusing carbon telescope arrays on the ground soon joined nuclei, causing a runaway reaction that in, eager to gather as much data as possible ends in a type 1a supernova. “Once that about the earliest stages of the star’s final process starts, for the most part, we think it performance. However, it was a tiny tele- just continues. It’ll burn through the entire scope in Mallorca, known by the acronym star,” says astronomer Ryan Foley of the PIRATE, that garnered a crucial observation. University of Illinois, Urbana–Champaign. 11910 | www.pnas.org/cgi/doi/10.1073/pnas.1413121111
A solitary white dwarf is in no danger of exceeding this mass threshold, but a dwarf in a binary star system is. When two gravitationally bound stars circle one another, the dwarf can steal material from its companion, growing and growing until it explodes. In a normal galaxy, this kind of kleptomaniacal relationship ends catastrophically every 200 years or so. A decade ago, many astronomers favored a model in which the dwarf’s companion was large and gassy, such as a red giant star. “Everybody was settled with that. That’s what we taught in our introductory astrophysics classes,” Foley says. However, some type 1a supernovae seem to have taken about 10 billion years to grow up and die—much too long to be explained by a shorter-lived red giant binary system. The discrepancy suggests other partners must be involved. “There’s quite strong evidence that many, if not most, type 1a supernovae come from a different system, with two white dwarfs,” says astronomer Alexei Filippenko of the University of California, Berkeley. If they’re close enough, two dwarfs will slowly spiral in toward one another as they emit gravitational waves—ripples in the fabric of space-time that draw energy from the stars. As they come together, either the more massive dwarf steals material from its companion until it explodes and obliterates both of them or the two dwarfs collide and are annihilated. The double-degenerate scenario provoked skepticism within the field for years. Early models of their death spiral could not explain how the dwarfs transferred mass or came together quickly enough to explode. Plus, the explosion physics weren’t quite right: scientists could not make the dwarfs in their models shine brightly enough or explain the many layers of chemical elements emerging over the course of the explosion. However, new observations and better models are now changing astronomers’ minds. “Almost everything has been flipped on its head,” says Nugent. Missing Companions
One of the most telling flaws in the case for large companion stars is that astronomers have not seen much evidence of them around type 1a supernovae. When a dwarf detonates, the explosion should tear some of the hydrogen gas from a nondegenerate companion star and fling it outward. “And yet, we don’t see any evidence for that gas,” Filippenko says. Indeed, one of the hallmarks of a 1a explosion is a lack of hydrogen gas flying outward at high speed. If an explosion is caught early enough, like 2011fe, models suggest that astronomers Drake
Drake
However, the race to accept multiple progenitor systems worries Kerzendorf. “We have to try to find one model that fits all. I’m not saying it’s impossible that there are two progenitor systems,” he says. Kerzendorf adds, “But if you open that door, then there could be three. And then every single supernova could have its own progenitor system. That might not be the truth.”
million light-years away in the Cigar Galaxy, are more constraining—but they do not rule out dimmer nondegenerate stars like red m-dwarfs, which could easily evade detection. “M-dwarfs are the most common kind of star in the galaxy,” says J. Craig Wheeler, an astrophysicist at the University of Texas at Austin. “How often would white dwarfs and m-dwarfs pair up? The answer is a lot. There are billions of each of them,” he says. There is at least one strong, recent piece of evidence for the traditional scenario: a supernova called PTF 11kx, surrounded by complex shells of gas and ejected material that suggest it blew up with the help of a red giant companion (9). Foley has also studied distant supernova remnants and found that about 20% have tell-tale outflows of sodium gas that hint at a relatively large and gassy companion star (10). “The simplest explanation for that is they come from single degenerate systems,” he says. With a growing number of observations supporting each scenario, many scientists now strongly suspect that doomed white dwarfs could be dancing with a variety of companions. “There’s reasonable circumstantial evidence for both channels,” say Saurab Jha, an astrophysicist at Rutgers University. That’s a real surprise, he adds, because it would mean that different combinations of ingredients and varied cooking times could produce remarkably similar type 1a supernovae.
Although scientists do not fully understand how to explode a white dwarf, the dead stars’ role as cosmic milestones is on solid ground. However, a better understanding of how these candles light up will make distance measurements more accurate and help scientists figure out whether dark energy has changed over time. To do that, scientists need to find some really old, distant supernovae and be confident that they understand the mechanics of the explosions. “The concern is that maybe the star systems that are exploding in type 1a supernovae are different,” says Jha, “but we don’t know how they’re going to be different.” Still, scientists are confident that they will crack the mystery of these twice-dead candles. “It’s just a hard slog, the way science sometimes is,” Wheeler admits, “but there are so many people getting so much data, I suspect we’ll figure it out.”
1 Nugent PE, et al. (2011) Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star. Nature 480(7377):344–347. 2 Nugent PE, et al. (2011) Young type 1a supernova PTF 11kly in M101. The Astronomer’s Telegram. Available at http://www. astronomerstelegram.org/?read=3581. Accessed July 25, 2014. 3 Perlmutter S, et al. (1998) Measurements of Ω and Λ from 42 high-redshift supernovae. Astrophysical Journal 517(2):565–586. 4 Riess AG, et al. (1998) Observational evidence from supernovae for an accelerating universe and a cosmological constant. Astron J 116(3):1009–1038. 5 Bloom JS, et al. (2012) A compact degenerate primary-star progenitor of SN 2011fe. Astrophysical Journal Letters 744(2):L17.
6 Ruiz-Lapuente P, et al. (2004) The binary progenitor of Tycho Brahe’s 1572 supernova. Nature 431(7012):1069–1072. 7 Schaefer BE, Pagnotta A (2012) An absence of ex-companion stars in the type Ia supernova remnant SNR 0509-67.5. Nature 481(7380): 164–166. 8 Li W, et al. (2011) Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe. Nature 480(7377):348–350. 9 Dilday B, et al. (2012) PTF 11kx: A type Ia supernova with a symbiotic nova progenitor. Science 337(6097):942–945. 10 Foley R, et al. (2012) Linking Type 1a supernova progenitors and their resulting explosions. ApJ 752:101.
Illuminating Dark Energy
PNAS | August 19, 2014 | vol. 111 | no. 33 | 11911
NEWS FEATURE
should also see what’s called a shock breakout. This is when a large companion star acts as a roadblock for some of the supernova ejecta; instead of flying off into space, the material piles up behind the companion. Eventually, the material will heat up and produce a rapid brightening that appears as an abnormal bump in the supernova’s early light curves. However, no one has found much evidence for shock breakouts. Astronomers also expected to find beatenup former companions within the debris of ancient 1a supernovae. “If we find an actual companion in the remnant, it’s the best proof we’re ever going to get to finding out what the progenitor system is,” says Wolfgang Kerzendorf, an astronomer at the University of Toronto, Canada. Rocketing through space with peculiar speeds and spins and possibly carrying chemical scars from the explosion, these stars should be odd enough to identify. However, with one possible exception (6), scientists have not seen them. Kerzendorf has scrutinized the remnants of Tycho’s supernova, which exploded in 1572, Kepler’s supernova of 1604, and the remains of a supernova that exploded in the year 1006. Of those, Tycho’s is the only remnant with a possible companion star, but the claim is disputed. Conversely, a white dwarf companion would not survive the explosion, so there would be no leftover star to find. That’s what Ashley Pagnotta and Bradley Schaefer, then both at Louisiana State University in Baton Rouge, concluded in 2012 after studying a supernova remnant called SNR 0509-67.5 in the Large Magellanic Cloud (7). However, absence of evidence is not evidence of absence, and there has been no direct confirmation of a white dwarf companion. Even archival Hubble images of 2011fe could only rule out companions dimmer than a Sun-like star (8). Observations of 2014J, a recent supernova some 11.5
NEWS FEATURE
News Feature: How to light a cosmic candle Astronomers are still struggling to identify the companions that help white dwarf stars self-destruct in violent supernovae explosions. Nadia Drake Science Writer
Billions of years ago, an ill-fated star not so different from our Sun began to age and balloon outward. As it grew, the star’s glow darkened to a deep, foreboding red, and its outer layers sloughed off into space. Eventually, the star’s nuclear furnace blinked out. All that remained was a dense, lifeless core about the size of Earth: a white dwarf star.
That was the first time the star died. However, a spectacular resurrection was at hand. Material stolen from a nearby stellar companion ignited the dwarf’s carbon and oxygen layers and triggered a ferocious thermonuclear explosion that flung smoldering material outward at about 10% of the speed of light. Decaying radioactive
In 1572, the Danish astronomer Tycho Brahe observed and studied the explosion of a star that became known as Tycho’s supernova. More than four centuries later, Chandra’s image of the supernova remnant shows an expanding bubble of multimillion-degree debris (green and red) inside a more rapidly moving shell of extremely high energy electrons (filamentary blue). Image courtesy of NASA/CXC/Rutgers/J. Warren and J. Hughes, et al. www.pnas.org/cgi/doi/10.1073/pnas.1413121111
elements transformed the billowing material into a blinding beacon of light. In its second death, the star blazed with the brilliance of three billion suns. Photons expelled by this terminal stellar spasm zoomed across space. For 21 million years, they traveled from their home in the Pinwheel Galaxy through dust and clouds until—quite improbably—they collided with a telescope perched atop a mountain in southern California. It was August 24, 2011, and the Palomar Transient Factory had just seen the nearest, freshest type 1a supernova that had yet been detected (1, 2). In the days and weeks that followed, that twice-dead star—now called supernova 2011fe—was the most studied object in the sky. Ordinary in every way, it was just what astronomers needed to learn more about how white dwarf stars explode. Type 1a supernovae have been observed for millennia, but in recent decades, their predictable brightness has made then invaluable as cosmic distance markers. In the late 1990s, Nobel Prize-winning observations based on these stellar explosions revealed that the Universe is expanding at an accelerating rate, propelled by a poorly understood phenomenon dubbed dark energy (3, 4). However, the processes that produce type 1a supernovae are still fundamentally mysterious. “That’s a scary thing,” says astrophysicist Brad Tucker of Australian National University and the University of California, Berkeley. “These are very powerful tools in cosmology, but we really don’t know what’s going on with them.” he says. Before 2011fe exploded, astronomers did not have much observational evidence that exploding white dwarfs were responsible for the supernovae. Just as the discovery answered one question, it also added tinder to a debate that burns brightly today: What kind of starry companion is donating material to the doomed dwarf? “People are now really embracing the idea there may be more than one way to make a type 1a supernova,” says Peter Nugent, an
PNAS | August 19, 2014 | vol. 111 | no. 33 | 11909–11911
Supernova 2011fe was discovered just hours after it exploded in the Big Dipper. Studies by the Nearby Supernova Factory of its spectrum as it evolved over time have produced a benchmark atlas of data by which to measure all future type Ia supernovas. Image courtesy of B. J. Fulton (Las Cumbres Observatory Global Telescope Network, Goleta, CA).
astrophysicist at Lawrence Berkeley National Quite by chance, PIRATE had looked at the Pinwheel Galaxy just 4 hours after 2011fe Laboratory in California. went off. The image showed no trace of a The Supernova Next Door supernova. The only possibility was that the Every night, the Palomar Transient Factory exploding star was extremely dense and very takes hundreds of images of the sky. Software small—less than 2% of the Sun’s diameter subtracts those images from one another, and (5). It was the strongest evidence yet that type flags anything that changes between shots— 1a supernovae erupt from white dwarfs. what astronomers call an astrophysical transient. These objects include variable stars, The Unusual Suspect gamma ray bursts, and supernovae. Two and Carbon-oxygen white dwarfs are dense, a half years ago, however, Nugent was sifting containing roughly a Sun’s mass of material through the images himself because the sys- squeezed into an Earth-size object. In these tem’s software had crashed the night before. stars, the inward crush of gravity is counteracted by electron degeneracy pressure, a That’s when he discovered 2011fe. The explosion was remarkably close and, quantum mechanical property that conalthough it was just 11 hours old, the bal- strains how tightly packed electrons can be looning debris cloud was already big enough (this is the reason white dwarfs are called to fill the orbit of Jupiter. It was noon in degenerate stars). Degeneracy pressure usuBerkeley, so he asked a colleague to aim ally prevents a catastrophic collapse, leaving a telescope in the Canary Islands at the the stable white dwarf to slowly fade away growing spot of light. That observation over billions of years. However, if the star crosses a crucial revealed the classic spectrum of a normal, young 1a supernova, containing silicon, cal- threshold around 1.4 solar masses, it becomes cium, a little bit of iron—and no hydrogen. massive enough to both overwhelm deMany of the major space observatories and generacy pressure and begin fusing carbon telescope arrays on the ground soon joined nuclei, causing a runaway reaction that in, eager to gather as much data as possible ends in a type 1a supernova. “Once that about the earliest stages of the star’s final process starts, for the most part, we think it performance. However, it was a tiny tele- just continues. It’ll burn through the entire scope in Mallorca, known by the acronym star,” says astronomer Ryan Foley of the PIRATE, that garnered a crucial observation. University of Illinois, Urbana–Champaign. 11910 | www.pnas.org/cgi/doi/10.1073/pnas.1413121111
A solitary white dwarf is in no danger of exceeding this mass threshold, but a dwarf in a binary star system is. When two gravitationally bound stars circle one another, the dwarf can steal material from its companion, growing and growing until it explodes. In a normal galaxy, this kind of kleptomaniacal relationship ends catastrophically every 200 years or so. A decade ago, many astronomers favored a model in which the dwarf’s companion was large and gassy, such as a red giant star. “Everybody was settled with that. That’s what we taught in our introductory astrophysics classes,” Foley says. However, some type 1a supernovae seem to have taken about 10 billion years to grow up and die—much too long to be explained by a shorter-lived red giant binary system. The discrepancy suggests other partners must be involved. “There’s quite strong evidence that many, if not most, type 1a supernovae come from a different system, with two white dwarfs,” says astronomer Alexei Filippenko of the University of California, Berkeley. If they’re close enough, two dwarfs will slowly spiral in toward one another as they emit gravitational waves—ripples in the fabric of space-time that draw energy from the stars. As they come together, either the more massive dwarf steals material from its companion until it explodes and obliterates both of them or the two dwarfs collide and are annihilated. The double-degenerate scenario provoked skepticism within the field for years. Early models of their death spiral could not explain how the dwarfs transferred mass or came together quickly enough to explode. Plus, the explosion physics weren’t quite right: scientists could not make the dwarfs in their models shine brightly enough or explain the many layers of chemical elements emerging over the course of the explosion. However, new observations and better models are now changing astronomers’ minds. “Almost everything has been flipped on its head,” says Nugent. Missing Companions
One of the most telling flaws in the case for large companion stars is that astronomers have not seen much evidence of them around type 1a supernovae. When a dwarf detonates, the explosion should tear some of the hydrogen gas from a nondegenerate companion star and fling it outward. “And yet, we don’t see any evidence for that gas,” Filippenko says. Indeed, one of the hallmarks of a 1a explosion is a lack of hydrogen gas flying outward at high speed. If an explosion is caught early enough, like 2011fe, models suggest that astronomers Drake
Drake
However, the race to accept multiple progenitor systems worries Kerzendorf. “We have to try to find one model that fits all. I’m not saying it’s impossible that there are two progenitor systems,” he says. Kerzendorf adds, “But if you open that door, then there could be three. And then every single supernova could have its own progenitor system. That might not be the truth.”
million light-years away in the Cigar Galaxy, are more constraining—but they do not rule out dimmer nondegenerate stars like red m-dwarfs, which could easily evade detection. “M-dwarfs are the most common kind of star in the galaxy,” says J. Craig Wheeler, an astrophysicist at the University of Texas at Austin. “How often would white dwarfs and m-dwarfs pair up? The answer is a lot. There are billions of each of them,” he says. There is at least one strong, recent piece of evidence for the traditional scenario: a supernova called PTF 11kx, surrounded by complex shells of gas and ejected material that suggest it blew up with the help of a red giant companion (9). Foley has also studied distant supernova remnants and found that about 20% have tell-tale outflows of sodium gas that hint at a relatively large and gassy companion star (10). “The simplest explanation for that is they come from single degenerate systems,” he says. With a growing number of observations supporting each scenario, many scientists now strongly suspect that doomed white dwarfs could be dancing with a variety of companions. “There’s reasonable circumstantial evidence for both channels,” say Saurab Jha, an astrophysicist at Rutgers University. That’s a real surprise, he adds, because it would mean that different combinations of ingredients and varied cooking times could produce remarkably similar type 1a supernovae.
Although scientists do not fully understand how to explode a white dwarf, the dead stars’ role as cosmic milestones is on solid ground. However, a better understanding of how these candles light up will make distance measurements more accurate and help scientists figure out whether dark energy has changed over time. To do that, scientists need to find some really old, distant supernovae and be confident that they understand the mechanics of the explosions. “The concern is that maybe the star systems that are exploding in type 1a supernovae are different,” says Jha, “but we don’t know how they’re going to be different.” Still, scientists are confident that they will crack the mystery of these twice-dead candles. “It’s just a hard slog, the way science sometimes is,” Wheeler admits, “but there are so many people getting so much data, I suspect we’ll figure it out.”
1 Nugent PE, et al. (2011) Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star. Nature 480(7377):344–347. 2 Nugent PE, et al. (2011) Young type 1a supernova PTF 11kly in M101. The Astronomer’s Telegram. Available at http://www. astronomerstelegram.org/?read=3581. Accessed July 25, 2014. 3 Perlmutter S, et al. (1998) Measurements of Ω and Λ from 42 high-redshift supernovae. Astrophysical Journal 517(2):565–586. 4 Riess AG, et al. (1998) Observational evidence from supernovae for an accelerating universe and a cosmological constant. Astron J 116(3):1009–1038. 5 Bloom JS, et al. (2012) A compact degenerate primary-star progenitor of SN 2011fe. Astrophysical Journal Letters 744(2):L17.
6 Ruiz-Lapuente P, et al. (2004) The binary progenitor of Tycho Brahe’s 1572 supernova. Nature 431(7012):1069–1072. 7 Schaefer BE, Pagnotta A (2012) An absence of ex-companion stars in the type Ia supernova remnant SNR 0509-67.5. Nature 481(7380): 164–166. 8 Li W, et al. (2011) Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe. Nature 480(7377):348–350. 9 Dilday B, et al. (2012) PTF 11kx: A type Ia supernova with a symbiotic nova progenitor. Science 337(6097):942–945. 10 Foley R, et al. (2012) Linking Type 1a supernova progenitors and their resulting explosions. ApJ 752:101.
Illuminating Dark Energy
PNAS | August 19, 2014 | vol. 111 | no. 33 | 11911
NEWS FEATURE
should also see what’s called a shock breakout. This is when a large companion star acts as a roadblock for some of the supernova ejecta; instead of flying off into space, the material piles up behind the companion. Eventually, the material will heat up and produce a rapid brightening that appears as an abnormal bump in the supernova’s early light curves. However, no one has found much evidence for shock breakouts. Astronomers also expected to find beatenup former companions within the debris of ancient 1a supernovae. “If we find an actual companion in the remnant, it’s the best proof we’re ever going to get to finding out what the progenitor system is,” says Wolfgang Kerzendorf, an astronomer at the University of Toronto, Canada. Rocketing through space with peculiar speeds and spins and possibly carrying chemical scars from the explosion, these stars should be odd enough to identify. However, with one possible exception (6), scientists have not seen them. Kerzendorf has scrutinized the remnants of Tycho’s supernova, which exploded in 1572, Kepler’s supernova of 1604, and the remains of a supernova that exploded in the year 1006. Of those, Tycho’s is the only remnant with a possible companion star, but the claim is disputed. Conversely, a white dwarf companion would not survive the explosion, so there would be no leftover star to find. That’s what Ashley Pagnotta and Bradley Schaefer, then both at Louisiana State University in Baton Rouge, concluded in 2012 after studying a supernova remnant called SNR 0509-67.5 in the Large Magellanic Cloud (7). However, absence of evidence is not evidence of absence, and there has been no direct confirmation of a white dwarf companion. Even archival Hubble images of 2011fe could only rule out companions dimmer than a Sun-like star (8). Observations of 2014J, a recent supernova some 11.5
