Reaching for the (invisible) stars

Supernovae-stellar explosions as bright as an entire galaxy-have fascinated us since time immemorial. Yet, there are more hydrogen-poor supernovae than astrophysicists can explain. Now, a new Assistant Professor at the Institute of Science and Technology Austria (ISTA) has played a pivotal role in identifying the missing precursor star population. The results, now published in Science, go back to a conversation the involved professors had many years ago as junior scientists.

Some stars do not simply die down, but explode in a stellar blast that could outshine entire galaxies. These cosmic phenomena, called supernovae, spread light, elements, energy, and radiation in space and send galactic shock waves that could compress gas clouds and generate new stars. In other words, supernovae shape our universe. Among these, hydrogen-poor supernovae from exploding massive stars have long puzzled astrophysicists. The reason: scientists have not been able to put their finger on their precursor stars. It is almost as if these supernovae appeared out of nowhere.

"There are many more hydrogen-poor supernovae than our current models can explain. Either we can't detect the stars that mature on this path, or we must revise all our models," says ISTA Assistant Professor Ylva Götberg. She pioneered this work together with Maria Drout, an Associated Faculty Member of the Dunlap Institute for Astronomy & Astrophysics, University of Toronto, Canada. "Single stars would typically explode as hydrogen-rich supernovae. Being hydrogen-poor indicates that the precursor star must have lost its thick hydrogen-rich envelope. This happens naturally in a third of all massive stars through envelope stripping by a binary companion star," says Götberg. Now, Götberg and Drout combined their areas of expertise in theoretical modeling and observation to hunt down the missing stars. Their quest is successful: they document a first-of-its-kind star population that finally bridges a large knowledge gap and sheds light on the origin of hydrogen-poor supernovae.

Binary stars and envelope stripping

The stars that Götberg and Drout search for go in pairs: interlocked in a binary star system. Some binary systems are well-known to us Earthlings: these include the brightest star in our night sky, Sirius A, and its faint companion star Sirius B. The Sirius binary system is located only 8.6 light-years away from Earth-a stone's throw in cosmic terms. This explains Sirius A's observed brightness in our night sky.

Astrophysicists expect the missing stars to be initially formed from massive binary systems. In a binary system, the stars would orbit around one another until the more massive star's thick, hydrogen-rich envelope expands. Eventually, the expanding envelope experiences a stronger gravitational pull to the companion star than to its own core. This causes a transfer of mass to begin, which eventually leads the entire hydrogen-rich envelope to be stripped off, leaving the hot and compact helium core exposed-more than 10 times hotter than the Sun's surface. This is precisely the type of stars that Götberg and Drout are looking for. "Intermediate mass helium stars stripped through binary interaction are predicted to play important roles in astrophysics. Yet, they were not observed until now," says Götberg. In fact, there is an important mass gap between the known classes of helium stars: the more massive Wolf-Rayet (WR) stars have more than 10 times the Sun's mass, and the low-mass subdwarf stars could have around half the Sun's mass. However, models have predicted the precursors of hydrogen-poor supernovae to lie between 2 and 8 solar masses following stripping.

Not just a needle in the haystack

Before Götberg and Drout's study, only one star was found to fulfill the expected mass and composition criteria and was called "Quasi-WR" (or "Almost Wolf-Rayet"). "Yet, the stars that follow this path have such a long lifetime that many must be scattered all over the observable universe," says Götberg. Did the scientists simply not "see" them? Thus, Götberg and Drout drew on their complementary expertise. With the help of UV photometry and optical spectroscopy, they identified a population of 25 stars that are consistent with the expectations for intermediate-mass helium stars. The stars are located in two well-studied neighboring galaxies, the Large and the Small Magellanic Clouds. "We showed that these stars were bluer than the stellar birthline, the bluest phase in a single star's lifetime. Single stars mature by evolving towards the redder region of the spectrum. A star only shifts in the opposite direction if its outer layers are removed-something that is expected to be common in interacting binary stars and rare among single massive stars," explains Götberg.

The scientists then verified their candidate star population using optical spectroscopy: they showed that the stars had strong spectral signatures of ionized helium. "Strong ionized helium lines tell us two important things: first, they confirm that the stars' outermost layers are dominated by helium and, second, that their surface is very hot. This is what happens to stars left as an exposed, compact, helium-rich core following stripping," says Götberg. Yet, both stars in a binary system contribute to the observed spectra. Thus, this technique allowed the researchers to classify their candidate population depending on which star contributed the most to the spectrum. "This work allowed us to find the missing population of intermediate-mass, stripped helium stars, the predicted progenitors of hydrogen-poor supernovae. These stars have always been there and there are probably many more out there. We must simply come up with ways to find them," says Götberg. "Our work may be one of the first attempts, but there should be other ways possible."

From graduate students at a conference to group leaders

The idea behind this project sparked in a discussion following a talk by Götberg at a conference that she and Drout attended during their graduate studies. Both scientists, then Early Career Researchers reaching for the stars, are now group leaders in their field. Götberg joined ISTA in September following her research at the Carnegie Observatories in Pasadena, California, as a NASA Hubble postdoctoral fellow. At ISTA, Götberg joins the Institute's growing ranks of young group leaders in astrophysics and leads her own group focused on studying the binary interactions of stars.

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Astronomers find distant gas clouds with leftovers of the first stars

Using ESO's Very Large Telescope (VLT), researchers have found for the first time the fingerprints left by the explosion of the first stars in the Universe. They detected three distant gas clouds whose chemical composition matches what we expect from the first stellar explosions. These findings bring us one step closer to understanding the nature of the first stars that formed after the Big Bang.

"For the first time ever, we were able to identify the chemical traces of the explosions of the first stars in very distant gas clouds," says Andrea Saccardi, a PhD student at the Observatoire de Paris -- PSL, who led this study during his master's thesis at the University of Florence.

Researchers think that the first stars that formed in the Universe were very different from the ones we see today. When they appeared 13.5 billion years ago, they contained just hydrogen and helium, the simplest chemical elements in nature. These stars, thought to be tens or hundreds of times more massive than our Sun, quickly died in powerful explosions known as supernovae, enriching the surrounding gas with heavier elements for the first time. Later generations of stars were born out of that enriched gas, and in turn ejected heavier elements as they too died. But the very first stars are now long gone, so how can researchers learn more about them? "Primordial stars can be studied indirectly by detecting the chemical elements they dispersed in their environment after their death," says Stefania Salvadori, Associate Professor at the University of Florence and co-author of the study published today in the Astrophysical Journal.

Using data taken with ESO's VLT in Chile, the team found three very distant gas clouds, seen when the Universe was just 10-15% of its current age, and with a chemical fingerprint matching what we expect from the explosions of the first stars. Depending on the mass of these early stars and the energy of their explosions, these first supernovae released different chemical elements such as carbon, oxygen and magnesium, which are present in the outer layers of stars. But some of these explosions were not energetic enough to expel heavier elements like iron, which is found only in the cores of stars. To search for the telltale sign of these very first stars that exploded as low energy supernovae, the team therefore looked for distant gas clouds poor in iron but rich in the other elements. And they found just that: three faraway clouds in the early Universe with very little iron but plenty of carbon and other elements -- the fingerprint of the explosions of the very first stars.

This peculiar chemical composition has also been observed in many old stars in our own galaxy, which researchers consider to be second-generation stars that formed directly from the 'ashes' of the first ones. This new study has found such ashes in the early Universe, thus adding a missing piece to this puzzle. "Our discovery opens new avenues to indirectly study the nature of the first stars, fully complementing studies of stars in our galaxy," explains Salvadori.

To detect and study these distant gas clouds, the team used light beacons known as quasars -- very bright sources powered by supermassive black holes at the centres of faraway galaxies. As the light from a quasar travels through the Universe, it passes through gas clouds where different chemical elements leave an imprint on the light.

To find these chemical imprints, the team analysed data on several quasars observed with the X-shooter instrument on ESO's VLT. X-shooter splits light into an extremely wide range of wavelengths, or colours, which makes it a unique instrument with which to identify many different chemical elements in these distant clouds.

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Webb reveals never-before-seen details in Cassiopeia A

The explosion of a star is a dramatic event, but the remains the star leaves behind can be even more dramatic. A new mid-infrared image from NASA's James Webb Space Telescope provides one stunning example. It shows the supernova remnant Cassiopeia A (Cas A), created by a stellar explosion seen from Earth 340 years ago. Cas A is the youngest known remnant from an exploding, massive star in our galaxy, which makes it a unique opportunity to learn more about how such supernovae occur.

"Cas A represents our best opportunity to look at the debris field of an exploded star and run a kind of stellar autopsy to understand what type of star was there beforehand and how that star exploded," said Danny Milisavljevic of Purdue University in West Lafayette, Indiana, principal investigator of the Webb program that captured these observations.

"Compared to previous infrared images, we see incredible detail that we haven't been able to access before," added Tea Temim of Princeton University in Princeton, New Jersey, a co-investigator on the program.

Cassiopeia A is a prototypical supernova remnant that has been widely studied by a number of ground-based and space-based observatories, including NASA's Chandra X-ray Observatory. The multi-wavelength observations can be combined to provide scientists with a more comprehensive understanding of the remnant.

Dissecting the Image

The striking colors of the new Cas A image, in which infrared light is translated into visible-light wavelengths, hold a wealth of scientific information the team is just beginning to tease out. On the bubble's exterior, particularly at the top and left, lie curtains of material appearing orange and red due to emission from warm dust. This marks where ejected material from the exploded star is ramming into surrounding circumstellar gas and dust.

Interior to this outer shell lie mottled filaments of bright pink studded with clumps and knots. This represents material from the star itself, which is shining due to a mix of various heavy elements, such as oxygen, argon, and neon, as well as dust emission.

"We're still trying to disentangle all these sources of emission," said Ilse De Looze of Ghent University in Belgium, another co-investigator on the program.

The stellar material can also be seen as fainter wisps near the cavity's interior.

Perhaps most prominently, a loop represented in green extends across the right side of the central cavity. "We've nicknamed it the Green Monster in honor of Fenway Park in Boston. If you look closely, you'll notice that it's pockmarked with what look like mini-bubbles," said Milisavljevic. "The shape and complexity are unexpected and challenging to understand."

Origins of Cosmic Dust -- and Us

Among the science questions that Cas A may help answer is: Where does cosmic dust come from? Observations have found that even very young galaxies in the early universe are suffused with massive quantities of dust. It's difficult to explain the origins of this dust without invoking supernovae, which spew large quantities of heavy elements (the building blocks of dust) across space.

However, existing observations of supernovae have been unable to conclusively explain the amount of dust we see in those early galaxies. By studying Cas A with Webb, astronomers hope to gain a better understanding of its dust content, which can help inform our understanding of where the building blocks of planets and ourselves are created.

"In Cas A, we can spatially resolve regions that have different gas compositions and look at what types of dust were formed in those regions," explained Temim.

Supernovae like the one that formed Cas A are crucial for life as we know it. They spread elements like the calcium we find in our bones and the iron in our blood across interstellar space, seeding new generations of stars and planets.

"By understanding the process of exploding stars, we're reading our own origin story," said Milisavljevic. "I'm going to spend the rest of my career trying to understand what's in this data set."

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A supernova in distant space allows us to understand the origin of the elements in the universe

A supernova is a stellar explosion, which occurs when the lives of some really massive stars come to an end. In this violent epilogue, the star expels the material from its outer layers by means of a shock wave, allowing us to see the various elements it was composed of.

The research team developed a model of the gravitational field of the galaxy that acted as a lens, and that way it was possible to determine that the light from these three images travelled along three different paths, differing in distance by a few days. This accounts for the three colours obtained in the images, because a variation in the colour emitted takes place as the gas in the supernova expands and cools. The higher the temperature is, the bluer the light emitted will be, and as the temperature falls, the light emitted tends towards red. So the blue image is a photograph of the supernova a few hours after the stellar explosion, while the green and red images correspond to 2 and 8 days, respectively, after the explosion.

This information enabled the radius of the star that exploded to be determined; it was a red super-giant with a radius equal to 500 times that of the Sun, and exploded 11.5 billion years ago, long before the Earth was born, specifically at the moment when our Galaxy is thought to have formed. The images of this supernova captured by the Hubble Space Telescope are highly magnified by the gravitational field of a nearby galaxy that acts as a lens and allows us to see much further in distance and in time than all local supernovae in nearby galaxies.

The study of the explosions of these red super-giant stars tallies with the current understanding of how the heavier atomic elements were created inside stars and during supernova explosions: elements forged inside stars are released in these supernova explosions to become the next generation of gas and material from which solar systems and life as we know it are created. Without these explosions, the gas in today's galaxies would only include the hydrogen and helium that formed during the Big Bang and would not support complex life that requires other heavier chemical elements. Moreover, this supernova observed through a gravitational lens demonstrates that an event taking place in the distant Universe can be witnessed several times, so in principle we could focus our instruments in advance to get a detailed view of the eruption of a star turning into a supernova.

From Science Daily

Supernova reveals secrets to astronomers

An international group of astronomers led by Benjamin Thomas of The University of Texas at Austin has used observations from the Hobby-Eberly Telescope (HET) at the university's McDonald Observatory to unlock a puzzling mystery about a stellar explosion discovered several years ago and evolving even now. The results, published in today's issue of The Astrophysical Journal, will help astronomers better understand the process of how massive stars live and die.

When an exploding star is first detected, astronomers around the world begin to follow it with telescopes as the light it gives off changes rapidly over time. They see the light from a supernova get brighter, eventually peak, and then start to dim. By noting the times of these peaks and valleys in the light's brightness, called a "light curve," as well as the characteristic wavelengths of light emitted at different times, they can deduce the physical characteristics of the system.

"I think what's really cool about this kind of science is that we're looking at the emission that's coming from matter that's been cast off from the progenitor system before it exploded as a supernova," Thomas said. "And so this makes a sort of time machine."

In the case of supernova 2014C, the progenitor was a binary star, a system in which two stars were orbiting each other. The more massive star evolved more quickly, expanded, and lost its outer blanket of hydrogen to the companion star. The first star's inner core continued burning lighter chemical elements into heavier ones, until it ran out of fuel. When that happened, the outward pressure from the core that had held up the star's great weight dropped. The star's core collapsed, triggering a gigantic explosion.

This makes it a type of supernova astronomers call a "Type Ib." In particular, Type Ib supernovae are characterized by not showing any hydrogen in their ejected material, at least at first.

Thomas and his team have been following SN 2014C from telescopes at McDonald Observatory since its discovery that year. Many other teams around the world also have studied it with telescopes on the ground and in space, and in different types of light, including radio waves from the ground-based Very Large Array, infrared light, and X-rays from the space-based Chandra Observatory.

But the studies of SN 2014C from all of the various telescopes did not add up into a cohesive picture of how astronomers thought a Type Ib supernova should behave.

For one thing, the optical signature from the Hobby-Eberly Telescope (HET) showed SN 2014C contained hydrogen -- a surprising finding that also was discovered independently by another team using a different telescope.

"For a Type Ib supernova to begin showing hydrogen is completely weird," Thomas said. "There's just a handful of events that have been shown to be similar."

For a second thing, the optical brightness (light curve) of that hydrogen was behaving strangely.

Most of the light curves from SN 2014C -- radio, infrared, and X-rays -- followed the expected pattern: they got brighter, peaked, and started to fall. But the optical light from the hydrogen stayed steady.

"The mystery that we've wrestled with has been 'How do we fit our Texas HET observations of hydrogen and its characteristics into that [Type Ib] picture?'," said UT Austin professor and team member J. Craig Wheeler.

The problem, the team realized, was that previous models of this system assumed that the supernova had exploded and sent out its shockwave in a spherical manner. The data from HET showed that this hypothesis was impossible -- something else must have happened.

"It just would not fit into a spherically symmetric picture," Wheeler said.

The team proposes a model where the hydrogen envelopes of the two stars in the progenitor binary system merged to form a "common-envelope configuration," where both were contained within a single envelope of gas. The pair then expelled that envelope in an expanding, disk-like structure surrounding the two stars. When one of the stars exploded, its fast-moving ejecta collided with the slow-moving disk, and also slid along the disk surface at a "boundary layer" of intermediate velocity.

The team suggests that this boundary layer is the origin of the hydrogen they detected and then studied for seven years with HET.

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Astronomers discover micronovae, a new kind of stellar explosion

A team of astronomers, with the help of the European Southern Observatory's Very Large Telescope (ESO's VLT), have observed a new type of stellar explosion -- a micronova. These outbursts happen on the surface of certain stars, and can each burn through around 3.5 billion Great Pyramids of Giza of stellar material in only a few hours.

"We have discovered and identified for the first time what we are calling a micronova," explains Simone Scaringi, an astronomer at Durham University in the UK who led the study on these explosions published today in Nature. "The phenomenon challenges our understanding of how thermonuclear explosions in stars occur. We thought we knew this, but this discovery proposes a totally new way to achieve them," he adds.

Micronovae are extremely powerful events, but are small on astronomical scales; they are much less energetic than the stellar explosions known as novae, which astronomers have known about for centuries. Both types of explosions occur on white dwarfs, dead stars with a mass about that of our Sun, but as small as Earth.

A white dwarf in a two-star system can steal material, mostly hydrogen, from its companion star if they are close enough together. As this gas falls onto the very hot surface of the white dwarf star, it triggers the hydrogen atoms to fuse into helium explosively. In novae, these thermonuclear explosions occur over the entire stellar surface. "Such detonations make the entire surface of the white dwarf burn and shine brightly for several weeks," explains co-author Nathalie Degenaar, an astronomer at the University of Amsterdam, the Netherlands.

Micronovae are similar explosions that are smaller in scale and faster, lasting just several hours. They occur on some white dwarfs with strong magnetic fields, which funnel material towards the star's magnetic poles. "For the first time, we have now seen that hydrogen fusion can also happen in a localised way. The hydrogen fuel can be contained at the base of the magnetic poles of some white dwarfs, so that fusion only happens at these magnetic poles," says Paul Groot, an astronomer at Radboud University in the Netherlands and co-author of the study.

"This leads to micro-fusion bombs going off, which have about one millionth of the strength of a nova explosion, hence the name micronova," Groot continues. Although 'micro' may imply these events are small, do not be mistaken: just one of these outbursts can burn through about 20,000,000 trillion kg, or about 3.5 billion Great Pyramids of Giza, of material.*

These new micronovae challenge astronomers' understanding of stellar explosions and may be more abundant than previously thought. "It just goes to show how dynamic the Universe is. These events may actually be quite common, but because they are so fast they are difficult to catch in action," Scaringi explains.

The team first came across these mysterious micro-explosions when analysing data from NASA's Transiting Exoplanet Survey Satellite (TESS). "Looking through astronomical data collected by NASA's TESS, we discovered something unusual: a bright flash of optical light lasting for a few hours. Searching further, we found several similar signals," says Degenaar.

The team observed three micronovae with TESS: two were from known white dwarfs, but the third required further observations with the X-shooter instrument on ESO's VLT to confirm its white dwarf status.

"With help from ESO's Very Large Telescope, we found that all these optical flashes were produced by white dwarfs," says Degenaar. "This observation was crucial in interpreting our result and for the discovery of micronovae," Scaringi adds.

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Part of the Universe’s missing matter found

Galaxies can receive and exchange matter with their external environment thanks to the galactic winds created by stellar explosions. Thanks to the MUSE instrument1 from the Very Large Telescopeat the ESO, an international research team, led on the French side by the CNRS and l'Université Claude Bernard Lyon 12, has mapped a galactic wind for the first time. This unique observation, which is detailed in a study published in MNRAS on 16 September 2021, helped to reveal where some of the Universe's missing matter is located and to observe the formation of a nebula around a galaxy.

Galaxies are like islands of stars in the Universe, and possess ordinary or baryonic matter, which consists of elements from the periodic table, as well as dark matter, whose composition remains unknown. One of the major problems in understanding the formation of galaxies is that approximately 80% of the baryons3 that make up the normal matter of galaxies is missing. According to models, they were expelled from galaxies into inter-galactic space by the galactic winds created by stellar explosions.

An international team4, led on the French side by researchers from the CNRS and l'Université Claude Bernard Lyon 1, successfully used the MUSE instrument to generate a detailed map of the galactic wind driving exchanges between a young galaxy in formation and a nebula (a cloud of gas and interstellar dust).

The team chose to observe galaxy Gal1 due to the proximity of a quasar, which served as a "lighthouse" for the scientists by guiding them toward the area of study. They also planned to observe a nebula around this galaxy, although the success of this observation was initially uncertain, as the nebula's luminosity was unknown.

The perfect positioning of the galaxy and the quasar, as well as the discovery of gas exchange due to galactic winds, made it possible to draw up a unique map. This enabled the first observation of a nebula in formation that is simultaneously emitting and absorbing magnesium -- some of the Universe's missing baryons -- with the Gal1 galaxy.

This type of normal matter nebula is known in the near Universe, but their existence for young galaxies in formation had only been supposed.

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Why is this weird, metallic star hurtling out of the Milky Way?

About 2,000 light-years away from Earth, there is a star catapulting toward the edge of the Milky Way. This particular star, known as LP 40?365, is one of a unique breed of fast-moving stars -- remnant pieces of massive white dwarf stars -- that have survived in chunks after a gigantic stellar explosion.

"This star is moving so fast that it's almost certainly leaving the galaxy…[it's] moving almost two million miles an hour," says JJ Hermes, Boston University College of Arts & Sciences assistant professor of astronomy. But why is this flying object speeding out of the Milky Way? Because it's a piece of shrapnel from a past explosion -- a cosmic event known as a supernova -- that's still being propelled forward.

"To have gone through partial detonation and still survive is very cool and unique, and it's only in the last few years that we've started to think this kind of star could exist," says Odelia Putterman, a former BU student who has worked in Hermes' lab.

In a new paper published in The Astrophysical Journal Letters, Hermes and Putterman uncover new observations about this leftover "star shrapnel" that gives insight to other stars with similar catastrophic pasts.

Putterman and Hermes analyzed data from NASA's Hubble Space Telescope and Transiting Exoplanet Survey Satellite (TESS), which surveys the sky and collects light information on stars near and far. By looking at various kinds of light data from both telescopes, the researchers and their collaborators found that LP 40?365 is not only being hurled out of the galaxy, but based on the brightness patterns in the data, is also rotating on its way out.

"The star is basically being slingshotted from the explosion, and we're [observing] its rotation on its way out," says Putterman, who is second author on the paper.

"We dug a little deeper to figure out why that star [was repeatedly] getting brighter and fainter, and the simplest explanation is that we're seeing something at [its] surface rotate in and out of view every nine hours," suggesting its rotation rate, Hermes says. All stars rotate -- even our sun slowly rotates on its axis every 27 days. But for a star fragment that's survived a supernova, nine hours is considered relatively slow.

Supernovas occur when a white dwarf gets too massive to support itself, eventually triggering a cosmic detonation of energy. Finding the rotation rate of a star like LP 40?365 after a supernova can lend clues into the original two-star system it came from. It's common in the universe for stars to come in close pairs, including white dwarfs, which are highly dense stars that form toward the end of a star's life. If one white dwarf gives too much mass to the other, the star being dumped on can self-destruct, resulting in a supernova. Supernovas are commonplace in the galaxy and can happen in many different ways, according to the researchers, but they are usually very hard to see. This makes it hard to know which star did the imploding and which star dumped too much mass onto its star partner.

Based on LP 40?365's relatively slow rotation rate, Hermes and Putterman feel more confident that it is shrapnel from the star that self-destructed after being fed too much mass by its partner, when they were once orbiting each other at high speed. Because the stars were orbiting each other so quickly and closely, the explosion slingshotted both stars, and now we only see LP 40-365.

"This [paper] adds one more layer of knowledge into what role these stars played when the supernova occurred," and what can happen after the explosion, Putterman says. "By understanding what's happening with this particular star, we can start to understand what's happening with many other similar stars that came from a similar situation."

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The Goldilocks Supernova

A worldwide team led by UC Santa Barbara scientists at Las Cumbres Observatory has discovered the first convincing evidence for a new type of stellar explosion -- an electron-capture supernova. While they have been theorized for 40 years, real-world examples have been elusive. They are thought to arise from the explosions of massive super-asymptotic giant branch (SAGB) stars, for which there has also been scant evidence. The discovery, published in Nature Astronomy, also sheds new light on the thousand-year mystery of the supernova from A.D. 1054 that was visible all over the world in the daytime, before eventually becoming the Crab Nebula.

Historically, supernovae have fallen into two main types: thermonuclear and iron-core collapse. A thermonuclear supernova is the explosion of a white dwarf star after it gains matter in a binary star system. These white dwarfs are the dense cores of ash that remain after a low-mass star (one up to about 8 times the mass of the sun) reaches the end of its life. An iron core-collapse supernova occurs when a massive star -- one more than about 10 times the mass of the sun -- runs out of nuclear fuel and its iron core collapses, creating a black hole or neutron star. Between these two main types of supernovae are electron-capture supernovae. These stars stop fusion when their cores are made of oxygen, neon and magnesium; they aren't massive enough to create iron.

While gravity is always trying to crush a star, what keeps most stars from collapsing is either ongoing fusion or, in cores where fusion has stopped, the fact that you can't pack the atoms any tighter. In an electron capture supernova, some of the electrons in the oxygen-neon-magnesium core get smashed into their atomic nuclei in a process called electron capture. This removal of electrons causes the core of the star to buckle under its own weight and collapse, resulting in an electron-capture supernova.

If the star had been slightly heavier, the core elements could have fused to create heavier elements, prolonging its life. So it is a kind of reverse Goldilocks situation: The star isn't light enough to escape its core collapsing, nor is it heavy enough to prolong its life and die later via different means.

That's the theory that was formulated beginning in 1980 by Ken'ichi Nomoto of the University of Tokyo and others. Over the decades, theorists have formulated predictions of what to look for in an electron-capture supernova and their SAGB star progenitors. The stars should have a lot of mass, lose much of it before exploding, and this mass near the dying star should be of an unusual chemical composition. Then the electron-capture supernova should be weak, have little radioactive fallout, and have neutron-rich elements in the core.

The new study is led by Daichi Hiramatsu, a graduate student at UC Santa Barbara and Las Cumbres Observatory (LCO). Hiramatsu is a core member of the Global Supernova Project, a worldwide team of scientists using dozens of telescopes around and above the globe. The team found that the supernova SN 2018zd had many unusual characteristics, some of which were seen for the first time in a supernova.

It helped that the supernova was relatively nearby -- only 31 million light-years away -- in the galaxy NGC 2146. This allowed the team to examine archival images taken by the Hubble Space Telescope prior to the explosion and to detect the likely progenitor star before it exploded. The observations were consistent with another recently identified SAGB star in the Milky Way, but inconsistent with models of red supergiants, the progenitors of normal iron core-collapse supernovae.

The authors looked through all published data on supernovae, and found that while some had a few of the indicators predicted for electron-capture supernovae, only SN 2018zd had all six: an apparent SAGB progenitor, strong pre-supernova mass loss, an unusual stellar chemical composition, a weak explosion, little radioactivity and a neutron-rich core.

"We started by asking 'what's this weirdo?'" Hiramatsu said. "Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario."

The new discoveries also illuminate some mysteries of the most famous supernova of the past. In A.D. 1054 a supernova happened in the Milky Way Galaxy that, according to Chinese and Japanese records, was so bright that it could be seen in the daytime for 23 days, and at night for nearly two years. The resulting remnant, the Crab Nebula, has been studied in great detail.

The Crab Nebula was previously the best candidate for an electron-capture supernova, but its status was uncertain partly because the explosion happened nearly a thousand years ago. The new result increases the confidence that the historic SN 1054 was an electron-capture supernova. It also explains why that supernova was relatively bright compared to the models: Its luminosity was probably artificially enhanced by the supernova ejecta colliding with material cast off by the progenitor star as was seen in SN 2018zd.

Ken Nomoto at the Kavli IPMU of the University of Tokyo expressed excitement that his theory had been confirmed. "I am very pleased that the electron-capture supernova was finally discovered, which my colleagues and I predicted to exist and have a connection to the Crab Nebula 40 years ago," he said. "I very much appreciate the great efforts involved in obtaining these observations. This is a wonderful case of the combination of observations and theory."

Hiramatsu added, "It was such a 'Eureka moment' for all of us that we can contribute to closing the 40-year-old theoretical loop, and for me personally because my career in astronomy started when I looked at the stunning pictures of the Universe in the high school library, one of which was the iconic Crab Nebula taken by the Hubble Space Telescope."

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Front-row view reveals exceptional cosmic explosion

Scientists have gained the best view yet of the brightest explosions in the universe: A specialised observatory in Namibia has recorded the most energetic radiation and longest gamma-ray afterglow of a so-called gamma-ray burst (GRB) to date. The observations with the High Energy Stereoscopic System (H.E.S.S.) challenge the established idea of how gamma-rays are produced in these colossal stellar explosions which are the birth cries of black holes, as the international team reports in the journal Science.

"Gamma-ray bursts are bright X-ray and gamma-ray flashes observed in the sky, emitted by distant extragalactic sources," explains DESY scientist Sylvia Zhu, one of the authors of the paper. "They are the biggest explosions in the universe and associated with the collapse of a rapidly rotating massive star to a black hole. A fraction of the liberated gravitational energy feeds the production of an ultrarelativistic blast wave. Their emission is divided into two distinct phases: an initial chaotic prompt phase lasting tens of seconds, followed by a long-lasting, smoothly fading afterglow phase."

On 29 August 2019 the satellites Fermi and Swift detected a gamma-ray burst in the constellation of Eridanus. The event, catalogued as GRB 190829A according to its date of occurrence, turned out to be one of the nearest gamma-ray bursts observed so far, with a distance of about one billion lightyears. For comparison: The typical gamma-ray burst is about 20 billion lightyears away. "We were really sitting in the front row when this gamma-ray burst happened," explains co-author Andrew Taylor from DESY. The team caught the explosion's afterglow immediately when it became visible to the H.E.S.S. telescopes. "We could observe the afterglow for several days and to unprecedented gamma-ray energies," reports Taylor.

The comparatively short distance to this gamma-ray burst allowed detailed measurements of the afterglow's spectrum, which is the distribution of "colours" or photon energies of the radiation, in the very-high energy range. "We could determine GRB 190829A's spectrum up to an energy of 3.3 tera-electronvolts, that's about a trillion times as energetic as the photons of visible light," explains co-author Edna Ruiz-Velasco from the Max Planck Institute for Nuclear Physics in Heidelberg. "This is what's so exceptional about this gamma-ray burst -- it happened in our cosmic backyard where the very-high-energy photons were not absorbed in collisions with background light on their way to Earth, as it happens over larger distances in the cosmos."

The team could follow the afterglow up to three days after the initial explosion. The result came as a surprise: "Our observations revealed curious similarities between the X-ray and very-high energy gamma-ray emission of the burst's afterglow," reports Zhu. Established theories assume that the two emission components must be produced by separate mechanisms: the X-ray component originates from ultra-fast electrons that are deflected in the strong magnetic fields of the burst's surroundings. This "synchrotron" process is quite similar to how particle accelerators on Earth produce bright X-rays for scientific investigations.

However, according to existing theories it seemed very unlikely that even the most powerful explosions in the universe could accelerate electrons enough to directly produce the observed very-high-energy gamma rays. This is due to a "burn-off limit," which is determined by the balance of acceleration and cooling of particles within an accelerator. Producing very-high energy gamma-rays requires electrons with energies well beyond the burn-off limit. Instead, current theories assume that in a gamma-ray burst, fast electrons collide with synchrotron photons and thereby boost them to gamma-ray energies in a process dubbed synchrotron self-Compton.

But the observations of GRB 190829A's afterglow now show that both components, X-ray and gamma ray, faded in sync. Also, the gamma-ray spectrum clearly matched an extrapolation of the X-ray spectrum. Together, these results are a strong indication that X-rays and very-high-energy gamma rays in this afterglow were produced by the same mechanism. "It is rather unexpected to observe such remarkably similar spectral and temporal characteristics in the X-ray and very-high energy gamma-ray energy bands, if the emission in these two energy ranges had different origins," says co-author Dmitry Khangulyan from Rikkyo University in Tokyo. This poses a challenge for the synchrotron self-Compton origin of the very-high energy gamma-ray emission.

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Spectacular 'honeycomb heart' revealed in iconic stellar explosion

A unique 'heart-shape', with wisps of gas filaments showing an intricate honeycomb-like arrangement, has been discovered at the centre of the iconic supernova remnant, the Crab Nebula. Astronomers have mapped the void in unprecedented detail, creating a realistic three-dimensional reconstruction. The new work is published in Monthly Notices of the Royal Astronomical Society.

The Crab, formally known as Messier 1, exploded as a dramatic supernova in 1054 CE, and was observed over the subsequent months and years by ancient astronomers across the world. The resulting nebula -- the remnant of this enormous explosion -- has been studied by amateur and professional astronomers for centuries. However, despite this rich history of investigation, many questions remain about what type of star was originally there and how the original explosion took place.

Thomas Martin, the researcher at Université Laval who led the study, hopes to answer these questions using a new 3D reconstruction of the nebula. "Astronomers will now be able to move around and inside the Crab Nebula and study its filaments one by one," said Martin.

The team used the powerful SITELLE imaging spectrometer on the Canada-Hawaii-France Telescope (CFHT) in Mauna Kea, Hawaii, to compare the 3D shape of the Crab to two other supernova remnants. Remarkably, they found that all three remnants had ejecta arranged in large-scale rings, suggesting a history of turbulent mixing and radioactive plumes expanding from a collapsed iron core.

Co-author Dan Milisavljevic, an assistant professor at Purdue University and supernova expert, concludes that the fascinating morphology of the Crab seems to go against the most popular explanation of the original explosion.

"The Crab is often understood as being the result of an electron-capture supernova triggered by the collapse of an oxygen-neon-magnesium core, but the observed honeycomb structure may not be consistent with this scenario," Milisavljevic said.

The new reconstruction was made possible by the ground-breaking technology used by SITELLE, which incorporates a Michelson interferometer design allowing scientists to obtain over 300,000 high-resolution spectra of every single point of the nebula.

"SITELLE was designed with objects like the Crab Nebula in mind; but its wide field of view and adaptability make it ideal to study nearby galaxies and even clusters of galaxies at large distances," said co-author Laurent Drissen.

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Rare blast's remains discovered in Milky Way's center

 Astronomers may have found our galaxy's first example of an unusual kind of stellar explosion. This discovery, made with NASA's Chandra X-ray Observatory, adds to the understanding of how some stars shatter and seed the universe with elements critical for life on Earth.

This intriguing object, located near the center of the Milky Way, is a supernova remnant called Sagittarius A East, or Sgr A East for short. Based on Chandra data, astronomers previously classified the object as the remains of a massive star that exploded as a supernova, one of many kinds of exploded stars that scientists have catalogued.

Using longer Chandra observations, a team of astronomers has now instead concluded that the object is left over from a different type of supernova. It is the explosion of a white dwarf, a shrunken stellar ember from a fuel-depleted star like our Sun. When a white dwarf pulls too much material from a companion star or merges with another white dwarf, the white dwarf is destroyed, accompanied by a stunning flash of light.

Astronomers use these "Type Ia supernovae" because most of them mete out almost the same amount of light every time no matter where they are located. This allows scientists to use them to accurately measure distances across space and study the expansion of the universe.

Data from Chandra have revealed that Sgr A East, however, did not come from an ordinary Type Ia. Instead, it appears that it belongs to a special group of supernovae that produce different relative amounts of elements than traditional Type Ias do, and less powerful explosions. This subset is referred to as "Type Iax," a potentially important member of the supernova family.

"While we've found Type Iax supernovae in other galaxies, we haven't identified evidence for one in the Milky Way until now," said Ping Zhou of Nanjing University in China, who led the new study while at the University of Amsterdam. "This discovery is important for getting a handle of the myriad ways white dwarfs explode."

The explosions of white dwarfs is one of the most important sources in the universe of elements like iron, nickel, and chromium. The only place that scientists know these elements can be created is inside the nuclear furnace of stars or when they explode.

"This result shows us the diversity of types and causes of white dwarf explosions, and the different ways that they make these essential elements," said co-author Shing-Chi Leung of Caltech in Pasadena, California. "If we're right about the identity of this supernova's remains, it would be the nearest known example to Earth."

Astronomers are still debating the cause of Type Iax supernova explosions, but the leading theory is that they involve thermonuclear reactions that travel much more slowly through the star than in Type Ia supernovae. This relatively slow walk of the blast leads to weaker explosions and, hence, different amounts of elements produced in the explosion. It is also possible that part of the white dwarf is left behind.

Sgr A East is located very close to Sagittarius A*, the supermassive black hole in the center of our Milky Way galaxy, and likely intersects with the disk of material surrounding the black hole. The team was able to use Chandra observations targeting the supermassive black hole and the region around it for a total of about 35 days to study Sgr A East and find the unusual pattern of elements in the X-ray data. The Chandra results agree with computer models predicting a white dwarf that has undergone slow-moving nuclear reactions, making it a strong candidate for a Type Iax supernova remnant.

"This supernova remnant is in the background of many Chandra images of our galaxy's supermassive black hole taken over the last 20 years," said Zhiyuan Li, also of Nanjing University. "We finally may have worked out what this object is and how it came to be."

In other galaxies, scientists observe that Type Iax supernovae occur at a rate that is about one third that of Type Ia supernovae. In the Milky Way, there have been three confirmed Type Ia supernova remnants and two candidates that are younger than 2,000 years, corresponding to an age when remnants are still relatively bright before fading later. If Sgr A East is younger than 2,000 years and resulted from a Type Iax supernova, this study suggests that our galaxy is in alignment with respect to the relative numbers of Type Iax supernovae seen in other galaxies.

Along with the suggestion that Sgr A East is the remnant from the collapse of a massive star, previous studies have also pointed out that a normal Type Ia supernova had not been ruled out. The latest study conducted with this deep Chandra data argue against both the massive star and the normal Type Ia interpretations.

These results have been published today in The Astrophysical Journal, and a preprint is available online. The other co-authors of the paper are Ken'ichi Nomoto of The University of Tokyo in Japan, Jacco Vink of the University of Amsterdam in The Netherlands, and Yang Chen, also of Nanjing University.

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Stellar explosion in Earth's proximity, eons ago

 When the brightness of the star Betelgeuse dropped dramatically a few months ago, some observers suspected an impending supernova -- a stellar explosion that could also cause damage on Earth. While Betelgeuse has returned to normal, physicists from the Technical University of Munich (TUM) have found evidence of a supernova that exploded near the Earth around 2.5 million years ago.

The life of stars with a mass more than ten times that of our sun ends in a supernova, a colossal stellar explosion. This explosion leads to the formation of iron, manganese and other heavy elements.

In layers of a manganese crust that are around two and a half million years old a research team led by physicists from the Technical University of Munich has now confirmed the existence of both iron-60 and manganese-53.

"The increased concentrations of manganese-53 can be taken as the "smoking gun" -- the ultimate proof that this supernova really did take place," says first author Dr. Gunther Korschinek.

While a very close supernova could inflict massive harm to life on Earth, this one was far enough away. It only caused a boost in cosmic rays over several thousand years. "However, this can lead to increased cloud formation," says co-author Dr. Thomas Faestermann. "Perhaps there is a link to the Pleistocene epoch, the period of the Ice Ages, which began 2.6 million years ago."

Ultra-trace analysis

Typically, manganese occurs on earth as manganese-55. Manganese-53, on the other hand, usually stems from cosmic dust, like that found in the asteroid belt of our solar system. This dust rains down onto the earth continuously; but only rarely do we perceive larger specks of dust that glow as meteorites.

New sediment layers that accumulate year for year on the sea floor preserve the distribution of the elements in manganese crusts and sediment samples. Using accelerator mass spectrometry, the team of scientists has now detected both iron-60 and increased levels of manganese-53 in layers that were deposited about two and a half million years ago.

"This is investigative ultra-trace analysis," says Korschinek. "We are talking about merely a few atoms here. But accelerator mass spectrometry is so sensitive that it even allows us to calculate from our measurements that the star that exploded must have had around 11 to 25 times the size of the sun."

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