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  • richardmitnick 10:54 am on March 22, 2017 Permalink | Reply
    Tags: , , , , , , Supernovae   

    From Ethan Siegel: “What Will Happen When Betelgeuse Explodes?” 

    Ethan Siegel
    Mar 22, 2017

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    The constellation of Orion, along with the great molecular cloud complex and including its brightest stars. Betelgeuse, the nearby, bright red supergiant (and supernova candidate), is at the lower left. Rogelio Bernal Andreo

    Every star will someday run out of fuel in its core, bringing an end to its run as natural source of nuclear fusion in the Universe. While stars like our Sun will fuse hydrogen into helium and then — swelling into a red giant — helium into carbon, there are other, more massive stars which can achieve hot enough temperatures to further fuse carbon into even heavier elements. Under those intense conditions, the star will swell into a red supergiant, destined for an eventual supernova after around 100,000 years or so. And the brightest red supergiant in our entire night sky? That’s Betelgeuse, which could go supernova at any time.

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    The color-magnitude diagram of notable stars. The brightest red supergiant, Betelgeuse, is shown at the upper right. European Southern Observatory.

    Honestly, at its distance of 640 light years from us, it could have gone supernova at any time from the 14th century onwards, and we still wouldn’t know. Betelgeuse is one of the ten brightest stars in the sky in visible light, but only 13% of its energy output is detectable to human eyes. If we could see the entire electromagnetic spectrum — including into the infrared — Betelgeuse would, from our perspective, outshine every other star in the Universe except our Sun.

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    Three of the major stars in Orion — Betelgeuse, Meissa and Bellatrix — as revealed in the infrared. In IR light, Betelgeuse (lower left) is the brightest star in the night sky. NASA / WISE.

    It was the first star ever to be resolved as more than a point source. At 900 times the size of our Sun, it would engulf Mercury, Venus, Earth, Mars and even the asteroid belt if it were to replace our parent star. It’s a pulsating star, so its diameter changes with time.

    In addition, it’s constantly losing mass, as the intense fusion reactions begin to expel the outermost, tenuously-held layers. Direct radio observations can actually detect this blown-off matter, and have found that it extends to beyond the equivalent of Neptune’s orbit.

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    The nebula of expelled matter created around Betelgeuse, which, for scale, is shown in the interior red circle. This structure, resembling flames emanating from the star, forms because the behemoth is shedding its material into space. ESO/P. Kervella

    But when we study the night sky, we’re studying the past. We know that Betelgeuse, with an uncertain mass between about 12 and 20 times that of our Sun, was never destined to live very long: maybe around 10 million years only. The more massive a star is, the faster it burns through its fuel, and Betelgeuse is burning so very, very brightly: at around 100,000 times the luminosity of our Sun. It’s currently in the final stages of its life — as a red supergiant — meaning that when the innermost core begins fusing silicon and sulphur into iron, nickel and cobalt, the star itself will only have minutes left.

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    The anatomy of a very massive star throughout its life, culminating in a Type II Supernova. Nicole Rager Fuller for the NSF.

    At those final moments, the core will be incredibly hot, yet iron, nickel and cobalt will be unable to fuse into anything heavier. It’s energetically unfavorable to do so, and so no new radiation will be produced in the innermost regions. Yet gravitation is still at play, trying to pull the star’s core in on itself. Without nuclear fusion to hold it up, the core has no other options, and begins to implode. The contraction causes it to heat up, become denser, and achieve pressures like it’s never seen before. And once a critical junction has passed, it happens: the atomic nuclei in the star’s core begin a runaway fusion reaction all at once.

    This is what creates a Type II supernova: the core-collapse of an ultra-massive star. After a brief, initial flash, Betelgeuse will brighten tremendously over a period of weeks, rising to a maximum brightness that, intrinsically, will be billions of times as bright as the Sun. It will remain at maximum brightness for months, as radioactive cobalt and expanding gases cause a continuous bright emission of light.

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    At peak brightness, a supernova can shine nearly as brightly as the rest of the stars in a galaxy combined. This 1994 image shows a typical example of a core-collapse supernova relative to its host galaxy. NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team

    Supernovae have occurred in our Milky Way in the past: in 1604, 1572, 1054 and 1006, among others, with a number of them being so bright that they were visible during the day. But none of them were as close at Betelgeuse.

    At only 600-or-so light years distant, Betelgeuse will be far closer than any supernova ever recorded by humanity. It’s fortunately still far away enough that it poses no danger to us. Our planet’s magnetic field will easily deflect any energetic particles that happen to come our way, and it’s distant enough that the high-energy radiation reaching us will be so low-density that it will have less of an impact on you than the banana you had at breakfast. But oh, will it ever appear bright.

    Not only will Betelgeuse be visible during the day, but it will rival the Moon for the second-brightest object in the sky. Some models “only” have Betelgeuse getting as bright as a thick crescent moon, while others will see it rival the entire full moon. It will conceivably be the brightest object in the night sky for more than a year, until it finally fades away to a dimmer state.

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    The ultra-massive star Wolf-Rayet 124, shown with its surrounding nebula, is one of thousands of Milky Way stars that could be our galaxy’s next supernova. Betelgeuse is merely the closest known potential candidate. Hubble Legacy Archive / A. Moffat / Judy Schmidy

    Unfortunately, the key question of “when” is not one we have an answer to; thousands of other stars in the Milky Way may go supernova before Betelgeuse does. Until we develop an ultra-powerful neutrino telescope to measure the energy spectrum of neutrinos being generated by (and hence, which elements are being fused inside) a star like Betelgeuse, hundreds of light years away, we won’t know how close it is to going supernova. It could have exploded already, with the light from the cataclysm already on its way towards us… or it could remain no different than it appears today for another hundred thousand years.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 3:07 pm on March 15, 2017 Permalink | Reply
    Tags: ASCR Discovery, Coding a Starkiller, DOE, , Supernovae   

    From OLCF via ASCR and DOE: “Coding a Starkiller” 

    i1

    Oak Ridge National Laboratory

    OLCF

    ASCR

    March 2017

    The Titan supercomputer and a tool called Starkiller help Stony Brook University-led team simulate key moments in exploding stars.

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    A volume rendering of the density after 0.6 and 0.9 solar mass white dwarfs merge. The image is derived from a calculation performed on the Oak Ridge Leadership Computing facility’s Titan supercomputer. The model used Castro, an adaptive mesh astrophysical radiation hydrodynamics simulation code. Image courtesy of Stony Brook University / Max Katz et al.

    The spectacular Supernova 1987A, whose light reached Earth on Feb. 23 of the year it’s named for, captured the public’s fancy. It’s located at the edge of the Milky Way, in a dwarf galaxy called the Large Magellanic Cloud. It had been four centuries since earthlings had witnessed light from a star exploding in our galaxy.

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    NASA

    A supernova’s awesome light show heralds a giant star’s death, and the next supernova’s post-mortem will generate reams of data, compared to the paltry dozen or so neutrinos and X-rays harvested from the 1987 event.

    Astrophysicists Michael Zingale and Bronson Messer aren’t waiting. They’re aggressively anticipating the next supernova by leading teams in high-performance computer simulations of explosive stellar events, including different supernova types and their accompanying X-ray bursts. Zingale, of Stony Brook University, and Messer, of the Department of Energy’s Oak Ridge National Laboratory (ORNL), are in the midst of an award from the DOE Office of Science’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. It provides an allocation of 45 million processor hours of computer time on Titan, a Cray XK7 that’s one of the world’s most powerful supercomputers, at the Oak Ridge Leadership Computing Facility, or OLCF – a DOE Office of Science user facility.

    The simulations run on workhorse codes developed by the INCITE collaborators and at the DOE’s Lawrence Berkeley National Laboratory – codes that “are often modified toward specific problems,” Zingale says. “And the common problem we share with ORNL is that we have to put more and more of our algorithms on the Titan graphics processor units (GPUs),” specialized computer chips that accelerate calculations. While the phenomena they’re modeling “are really far away and on scales that are hard to imagine,” the codes have other applications closer to home: “terrestrial phenomena, like terrestrial combustion.” The team’s codes – Maestro, Castro, Chimera and FLASH – are available to other modelers free through online code repository Github.

    With a previous INCITE award, the researchers realized the possibility of attacking the GPU problem together. They envisioned codes comprised of multiphysics modules that compute common pieces of most kinds of explosive activities, Messer says. They dubbed the growing collection of GPU-enabled modules Starkiller.

    “Starkiller ties this INCITE project together,” he says. “We realized we didn’t want to reinvent the wheel with each new simulation.” For example, a module that tracks nuclear burning helps the researchers create larger networks for nucleosynthesis, a supernova process in which elements form in the turbulent flow on the stellar surface.

    “In the past, we were able to do only a little more than a dozen different elements, and now we’re routinely doing 150,” Messer says. “We can make the GPU run so much faster. That’s part of Titan’s advantage to us.”

    Supernova 1987A, a type II supernova, arose from the gravitational collapse of a stellar core, the consistent fate of massive stars. Type Ia supernovae follow from intense thermonuclear activities that eventually drive the explosion of a white dwarf – a star that has used up all its hydrogen. Zingale’s group is focused on type Ia, Messer’s on type II. A type II leaves a remnant star; a type Ia does not.

    Stars like the sun burn hydrogen into helium and, over enormous stretches of time, burn the helium into carbon. Once our sun starts burning carbon, it will gradually peter out, Messer says, because it’s not massive enough to turn the carbon into something heavier.

    “A star begins life as a big ball of hydrogen, and its whole life is this fight between gravity trying to suck it into the middle and thermonuclear reactions keeping it supported against its own gravity,” he adds. “Once it gets to the point where it’s burning some carbon, the sun will just give up. It will blow a big smoke ring into space and become a planetary nebula, and at the center it will become a white dwarf.”

    Zingale is modeling two distinct thermonuclear modes. One is for a white dwarf in a binary system – two stars orbiting one another – that consumes additional material from its partner. As the white dwarf grows in mass, it gets hotter and denser in the center, creating conditions that drive thermonuclear reactions.

    “This star is made mostly of carbon and oxygen,” Zingale says. “When you get up to a few hundred million K, you have densities of a few billion grams per cubic centimeter. Carbon nuclei get fused and make things like neon and sodium and magnesium, and the star gets energy out in that process. We are modeling the star’s convection, the creation of a rippling burning front that converts the carbon and oxygen into heavier elements such as iron and nickel. This creates such an enormous amount of energy that it overcomes the force of gravity that’s holding the star together, and the whole thing blows apart.”

    The other mode is being modeled with former Stony Brook graduate student and INCITE co-principal investigator Max Katz, who want to understand whether merging stars can create a burning point that leads to a supernova, as some observations suggest. His simulations feature two white dwarfs so close that they emit gravitational radiation, robbing energy from the system and causing the stars to spiral inward. Eventually, they get so close that the more massive one rips the lesser apart via tidal energy.

    Zingale’s group also continues to model the convective burning on stars, known as X-ray bursts, providing a springboard to more in-depth studies. He says they’re the first to simulate them in three dimensions. That work and additional supernova studies were supported by the DOE Office of Science and performed at OLCF and the National Energy Research Scientific Computing Center, a DOE Office of Science user facility at Lawrence Berkeley National Laboratory.

    See the full article here .

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    ORNL is managed by UT-Battelle for the Department of Energy’s Office of Science. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.

    i2

    The Oak Ridge Leadership Computing Facility (OLCF) was established at Oak Ridge National Laboratory in 2004 with the mission of accelerating scientific discovery and engineering progress by providing outstanding computing and data management resources to high-priority research and development projects.

    ORNL’s supercomputing program has grown from humble beginnings to deliver some of the most powerful systems in the world. On the way, it has helped researchers deliver practical breakthroughs and new scientific knowledge in climate, materials, nuclear science, and a wide range of other disciplines.

    The OLCF delivered on that original promise in 2008, when its Cray XT “Jaguar” system ran the first scientific applications to exceed 1,000 trillion calculations a second (1 petaflop). Since then, the OLCF has continued to expand the limits of computing power, unveiling Titan in 2013, which is capable of 27 petaflops.


    ORNL Cray XK7 Titan Supercomputer

    Titan is one of the first hybrid architecture systems—a combination of graphics processing units (GPUs), and the more conventional central processing units (CPUs) that have served as number crunchers in computers for decades. The parallel structure of GPUs makes them uniquely suited to process an enormous number of simple computations quickly, while CPUs are capable of tackling more sophisticated computational algorithms. The complimentary combination of CPUs and GPUs allow Titan to reach its peak performance.

    The OLCF gives the world’s most advanced computational researchers an opportunity to tackle problems that would be unthinkable on other systems. The facility welcomes investigators from universities, government agencies, and industry who are prepared to perform breakthrough research in climate, materials, alternative energy sources and energy storage, chemistry, nuclear physics, astrophysics, quantum mechanics, and the gamut of scientific inquiry. Because it is a unique resource, the OLCF focuses on the most ambitious research projects—projects that provide important new knowledge or enable important new technologies.

     
  • richardmitnick 2:23 pm on March 15, 2017 Permalink | Reply
    Tags: , , , , , Supernovae   

    From Astronomy: “There’s a supernova occurring right now in NGC 5643” 

    Astronomy magazine

    astronomy.com

    March 15, 2017
    Alison Klesman

    Meet “Bob,” the second Type Ia supernova in the galaxy since 2013

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    Racheal Beaton / Carnegie Institution for Science

    When most people hear the word supernova, they envision a massive star reaching the end of its life and exploding outwards to leave a ghostly remnant in its place. This is called a Type II supernova — the spectacular Supernova 1987A, which recently celebrated its 30th anniversary, was a Type II.

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    Supernova 1987A NASA

    Alternatively, a Type Ia supernova occurs when a white dwarf, the remnant of a Sun-like star, grows too massive after stripping a binary companion star of its outer layers. When the white dwarf reaches a critical mass, a runaway fusion reaction occurs in its core and the star explodes in a Type Ia supernova. Such a supernova has just been spotted occurring in a galaxy about 55 million light-years away.

    Announced by Rachael Beaton at the the Observatories of the Carnegie Institution for Science in Pasadena, CA, and known as 2017cbv (though Beaton has nicknamed it Bob), the explosion was spotted in NGC 5643, a spiral galaxy in the constellation Lupus. The area of the sky it inhabits is also part of the area covered by the Carnegie-Irvine Galaxy Survey, a project aimed at gathering optical and near-infrared images of bright Southern Hemisphere galaxies. NGC 5643 was also the home galaxy of SN 2013aa, which occurred in early 2013.

    Type Ia supernovae play an extremely important role as rungs on the astronomical distance ladder that allows astronomers to measure the distance to faraway galaxies. They’ve also played a critical role in measuring the accelerating expansion of the universe. Because they occur in white dwarfs of exactly the same mass every time (that critical mass mentioned earlier: about 1.4 times the mass of the Sun), Type Ia supernovae are always the same brightness, which means astronomers can use them as standard candles. Knowing how bright the explosion is in terms of absolute luminosity allows astronomers to then work backwards to calculate the distance to the object based on how bright it appears.

    But the word “exactly” is perhaps a bit misleading. Not every star system in which a Type Ia supernova occurs can be exactly the same. Moreover, events in the real world do not always reflect the precise nature of theoretical calculations — as in, some white dwarfs might explode at a mass slightly under 1.4 solar masses, while others might grow a little heavier than this limit before exploding. The fact that 2017cbv is the second recorded Type Ia supernova to occur in NGC 5643 is thus extremely valuable. By comparing the distance to the galaxy as calculated from each supernova, astronomers can better characterize the real-world variance in supernova Type Ia magnitudes that occur, which in turn will improve the accuracy of using these events to measure distance.

    See the full article here .

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  • richardmitnick 11:37 am on March 14, 2017 Permalink | Reply
    Tags: , , , , Supernovae,   

    From Weizmann: “Explosive Material: The Making of a Supernova” 

    Weizmann Institute of Science logo

    Weizmann Institute of Science

    Pre-supernova stars may show signs of instability for months before the big explosion

    14.03.2017

    In the most common type of supernova, the iron core of a massive star suddenly collapses in on itself and the outer layers are thrown out into space in a spectacular explosion. New research led by Weizmann Institute of Science researchers shows that the stars that become so-called core-collapse supernovae might already exhibit instability for several months before the big event, spewing material into space and creating a dense gas shell around themselves. They think that many massive stars, including the red super-giants that are the most common progenitors of these supernovae, may begin the process this way.

    This insight into the conditions leading up to core collapse arose from a unique collaboration called the Palomar Transient Factory, a fully automated sky survey using the telescopes of the Palomar observatory in southern California.


    Palomar Transient Factory, located in San Diego County, California

    Astrophysicists halfway around the globe, in Israel, are on call for the telescope, which scans the California night sky for the sudden appearance of new astronomical “transients” that were not visible before – which can indicate new supernovae. In October, 2013, Dr. Ofer Yaron, in the Weizmann Institute’s Particle Physics and Astrophysics Department, got the message that a potential supernova had been sighted, and he immediately alerted Dr. Dan Perley who was observing that night with the Keck telescope in Hawaii, and NASA’s Swift Satellite.


    Keck Observatory, Mauna Kea, Hawaii, USA


    NASA/SWIFT Telescope

    At Keck, the researchers soon began to record the spectra of the event. Because they had started observing only three hours into the blast, the picture the team managed to assemble was the most detailed ever of the core collapse process. “We had x-rays, ultraviolet, four spectroscopic measurements from between six and ten hours post-explosion to work with,” says Yaron.

    In the most common type of supernova, the iron core of a massive star suddenly collapses in on itself and the outer layers are thrown out into space in a spectacular explosion. New research led by Weizmann Institute of Science researchers shows that the stars that become so-called core-collapse supernovae might already exhibit instability for several months before the big event, spewing material into space and creating a dense gas shell around themselves. They think that many massive stars, including the red super-giants that are the most common progenitors of these supernovae, may begin the process this way.

    This insight into the conditions leading up to core collapse arose from a unique collaboration called the Palomar Transient Factory, a fully automated sky survey using the telescopes of the Palomar observatory in southern California. Astrophysicists halfway around the globe, in Israel, are on call for the telescope, which scans the California night sky for the sudden appearance of new astronomical “transients” that were not visible before – which can indicate new supernovae. In October, 2013, Dr. Ofer Yaron, in the Weizmann Institute’s Particle Physics and Astrophysics Department, got the message that a potential supernova had been sighted, and he immediately alerted Dr. Dan Perley who was observing that night with the Keck telescope in Hawaii, and NASA’s Swift Satellite. At Keck, the researchers soon began to record the spectra of the event. Because they had started observing only three hours into the blast, the picture the team managed to assemble was the most detailed ever of the core collapse process. “We had x-rays, ultraviolet, four spectroscopic measurements from between six and ten hours post-explosion to work with,” says Yaron.

    In a study recently published in Nature Physics, Yaron, Weizmann Institute researchers Profs. Avishay Gal-Yam and Eran Ofek, and their teams, together with researchers from the California Institute of Technology and other institutes in the United States, Denmark, Sweden, Ireland, Israel and the UK, analyzed the unique dataset they had collected from the very first days of the supernova.

    The time window was crucial: It enabled the team to detect material that had surrounded the star pre- explosion, as it heated up and became ionized and was eventually overtaken by the expanding cloud of stellar matter. Comparing the observed early spectra and light-curve data with existing models, accompanied by later radio observations, led the researchers to conclude that the explosion was preceded by a period of instability lasting for around a year. This instability caused material to be expelled from the surface layers of the star, forming the circumstellar shell of gas that was observed in the data. Because this was found to be a relatively standard type II supernova, the researchers believe that the instability they revealed may be a regular warm up act to the immanent explosion.

    “We still don’t really understand the process by which a star explodes as a supernova,” says Yaron, “These findings are raising new questions, for example, about the final trigger that tips the star from merely unstable to explosive. With our globe-spanning collaboration that enables us to alert various telescopes to train their sights on the event, we are getting closer and closer to understanding what happens in that instant, how massive stars end their life and what leads up to the final explosion.”

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    Prof. Avishay Gal-Yam’s research is supported by the Benoziyo Endowment Fund for the Advancement of Science; the Yeda-Sela Center for Basic Research; the Deloro Institute for Advanced Research in Space and Optics; and Paul and Tina Gardner. Prof. Gal-Yam is the recipient of the Helen and Martin Kimmel Award for Innovative Investigation.

    Dr. Eran Ofek’s research is supported by the Helen Kimmel Center for Planetary Science; Paul and Tina Gardner, Austin, TX; Ilan Gluzman, Secaucus, NJ; and the estate of Raymond Lapon.

    See the full article here .

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    Weizmann Institute Campus

    The Weizmann Institute of Science is one of the world’s leading multidisciplinary research institutions. Hundreds of scientists, laboratory technicians and research students working on its lushly landscaped campus embark daily on fascinating journeys into the unknown, seeking to improve our understanding of nature and our place within it.

    Guiding these scientists is the spirit of inquiry so characteristic of the human race. It is this spirit that propelled humans upward along the evolutionary ladder, helping them reach their utmost heights. It prompted humankind to pursue agriculture, learn to build lodgings, invent writing, harness electricity to power emerging technologies, observe distant galaxies, design drugs to combat various diseases, develop new materials and decipher the genetic code embedded in all the plants and animals on Earth.

    The quest to maintain this increasing momentum compels Weizmann Institute scientists to seek out places that have not yet been reached by the human mind. What awaits us in these places? No one has the answer to this question. But one thing is certain – the journey fired by curiosity will lead onward to a better future.

     
  • richardmitnick 1:57 pm on March 10, 2017 Permalink | Reply
    Tags: , , , , , Supernovae   

    From CfA: “Superluminous Supernovae” 

    Harvard Smithsonian Center for Astrophysics


    Center For Astrophysics

    March 10, 2017

    1
    Wikipedia

    Supernovae, the explosive deaths of massive stars, are among the most momentous events in the cosmos because they disburse into space all of the chemical elements that were produced inside their progenitor stars, including the elements essential for making planets and life. Their bright emission also enables them to be used as probes of the very distant universe. Not least, supernovae are astrophysical laboratories for the study of very energetic phenomena. One class of supernovae consists of single stars whose mass is at least eight solar masses as they finish their lives.

    A typical supernova shines about as brightly as ten billion Suns at its peak. In the last decade, a new type of supernova was discovered that is ten to one hundred times more luminous than a normal massive stellar collapse supernova, and today over a dozen of these superluminous supernovae (SLSN) have been seen. Astronomers are in agreement that these objects come from the collapse of massive stars, but their tremendous luminosities cannot be explained by the usual physical mechanisms invoked. Instead, the debate has centered on whether the excess emission results from an external source, for example the interaction of material ejected from the explosion with a circumstellar shell, or instead by some kind of powerful internal engine such as a highly magnetized, spinning neutron star.

    The SLSN “Gaia6apd” was discovered by the European Gaia satellite, and at a distance of about one and one-half billion light-years it is the second-closest SLSN discovered to date.


    ESA/GAIA

    It is also special in another way: it is extraordinarily bright in the ultraviolet, nearly four times brighter than the next nearest known SLSN despite the fact that in the optical both have comparable luminosities. CfA astronomers Matthew Nicholl, Edo Berger, Peter Blanchard, Dan Milisavljevic, and Peter Challis and their colleagues used facilities at the CfA’s MMT and Fred Lawrence Whipple Observatory to track the changing emission of this source from immediately after its discovery and continuing for one hundred and fifty days.


    CfA MMT Telescope at the summit of Mount Hopkins near Tucson, Arizona, USA


    CfA Whipple Observatory, near Amado, Arizona on the slopes of Mount Hopkins

    The long time coverage revealed that the UV emission eventually faded to a level typical for normal supernovae, providing some clues to the mechanisms responsible. The scientists review all the known data and conclude that the most likely source is an internal central engine like a rapidly spinning neutron star. They also emphasize the key role that UV wavelengths played in diagnosing the mechanisms and urge that future studies of SLSN include UV coverage.

    Reference(s):

    “An Ultraviolet Excess in the Superluminous Supernova Gaia16apd Reveals a Powerful Central Engine,” M. Nicholl, E. Berger, R. Margutti, P. K. Blanchard, D. Milisavljevic, P. Challis, B. D. Metzger, and R. Chornock, ApJLett 835, 8, 2017.

    See the full article here .

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    The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

     
  • richardmitnick 6:20 pm on November 28, 2016 Permalink | Reply
    Tags: , , , , Matching Supernovae to Galaxies, Supernovae   

    From AAS NOVA: “Matching Supernovae to Galaxies” 

    AASNOVA

    American Astronomical Society

    28 November 2016
    Susanna Kohler

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    Not every supernova’s host galaxy is as easy to identify as that of SN 1994D, seen in the outskirts of galaxy NGC 4526 in this Hubble image. Automated matching of supernovae to their host galaxies will likely be necessary for large upcoming surveys. [NASA/ESA]

    One of the major challenges for modern supernova surveys is identifying the galaxy that hosted each explosion. Is there an accurate and efficient way to do this that avoids investing significant human resources?

    Why Identify Hosts?

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    One problem in host galaxy identification. Here, the supernova lies between two galaxies — but though the centroid of the galaxy on the right is closer in angular separation, this may be a distant background galaxy that is not actually near the supernova. [Gupta et al. 2016]

    Supernovae are a critical tool for making cosmological predictions that help us to understand our universe. But supernova cosmology relies on accurately identifying the properties of the supernovae — including their redshifts. Since spectroscopic followup of supernova detections often isn’t possible, we rely on observations of the supernova host galaxies to obtain redshifts.

    But how do we identify which galaxy hosted a supernova? This seems like a simple problem, but there are many complicating factors — a seemingly nearby galaxy could be a distant background galaxy, for instance, or a supernova’s host could be too faint to spot.

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    The authors’ algorithm takes into account “confusion”, a measure of how likely the supernova is to be mismatched. In these illustrations of low (left) and high (right) confusion, the supernova is represented by a blue star, and the green circles represent possible host galaxies. [Gupta et al. 2016]

    Turning to Automation

    Before the era of large supernovae surveys, searching for host galaxies was done primarily by visual inspection. But current projects like the Dark Energy Survey’s Supernova Program is finding supernovae by the thousands, and the upcoming Large Synoptic Survey Telescope will likely discover hundreds of thousands. Visual inspection will not be possible in the face of this volume of data — so an accurate and efficient automated method is clearly needed!

    Dark Energy Icon
    Dark Energy Camera. Built at FNAL
    Dark Energy Camera [DECam]. Built at FNAL

    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile
    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo

    LSST
    LSST/Camera, built at SLAC
    LSST/Camera, built at SLAC

    LSST Interior
    LSST telescope, currently under construction at Cerro Pachón Chile
    LSST telescope, currently under construction at Cerro Pachón Chile

    To this end, a team of scientists led by Ravi Gupta (Argonne National Laboratory) has recently developed a new automated algorithm for matching supernovae to their host galaxies. Their work builds on currently existing algorithms and makes use of information about the nearby galaxies, accounts for the uncertainty of the match, and even includes a machine learning component to improve the matching accuracy.

    Gupta and collaborators test their matching algorithm on catalogs of galaxies and simulated supernova events to quantify how well the algorithm is able to accurately recover the true hosts.

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    The matching algorithm’s accuracy (“purity”) as a function of the true supernova-host separation, the supernova redshift, the true host’s brightness, and the true host’s size. [Gupta et al. 2016]

    Successful Matching

    The authors find that when the basic algorithm is run on catalog data, it matches supernovae to their hosts with 91% accuracy. Including the machine learning component, which is run after the initial matching algorithm, improves the accuracy of the matching to 97%.

    The encouraging results of this work — which was intended as a proof of concept — suggest that methods similar to this could prove very practical for tackling future survey data. And the method explored here has use beyond matching just supernovae to their host galaxies: it could also be applied to other extragalactic transients, such as gamma-ray bursts, tidal disruption events, or electromagnetic counterparts to gravitational-wave detections.

    Citation

    Ravi R. Gupta et al 2016 AJ 152 154. doi:10.3847/0004-6256/152/6/154

    See the full article here .

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  • richardmitnick 9:29 am on November 8, 2016 Permalink | Reply
    Tags: , , , Scraps of brightest exploding stars stretch over time, Supernovae   

    From COSMOS: “Scraps of brightest exploding stars stretch over time” 

    Cosmos Magazine bloc

    COSMOS

    08 November 2016
    Belinda Smith

    The inner layer of a superluminous supernovae has elongated in a matter of weeks, new observations show.

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    RCW 103, the remains of a supernova explosion located about 9,000 light-years from Earth. It’s nothing compared to superluminous supernovae, though – and a new study suggests the big ones have a couple of ejecta layers. X-ray: NASA / CXC / University of Amsterdam / N.Rea et al; Optical: DSS

    Some of the biggest and brightest exploding stars don’t keep a spherical shape, new observations show, but may periodically stretch into a hot dog bun shape.

    Cosimo Inserra from Queen’s University Belfast in the UK and colleagues measured polarised light, which gives information about asymmetries of the source, emanating from the superluminous supernova 2015bn. They found it changed shape over the course of a couple of months, pulling from a ball into an ellipsoid after peak brightness.

    The work, published in The Astrophysical Journal, provides another insight into the lifecycle of these strange cosmic objects.

    Supernovae are produced when a star in its death throes and collapses on itself, blasting a shell of material away from a black hole or a dense, spinning object with an immense magnetic field called a magnetar left in the centre.

    Superluminous supernovae, as their name suggests, are particularly bright – but they’re mysterious.

    While they explode with billions of times the energy of the sun – and last longer than a typical supernova, stretching months instead of weeks – astronomers have only known of their existence for the past six years or so.

    One of the closest superluminous supernovae – SN 2015bn – is fading in visible light, but undulating in the ultraviolet part of the spectrum. This, astronomers think, is the result of a magnetar reheating material around it, which results in a burst of ejecta every 30 to 50 days.

    But while it was ramping up to peak brightness, Cosimo and his colleagues trained a spectrograph on Chile’s Very Large Telescope on SN 2015bn to detect polarised light.

    ESO/VLT at Cerro Paranal, Chile
    ESO/VLT at Cerro Paranal, Chile

    Where unpolarised light waves move in, say, horizontal and vertical planes, polarised light moves in a single plane. Measuring polarised light – called polarimetry – and analysing it with come nifty calculations can give astronomers the rough shape of an object, such as the layers of supernova ejecta.

    The best fitting model comprised two layers of ejecta. Some 24 days before peak brightness, SN 2015bn’s outside ejecta layer was the same shape as the inner – like a soccer ball inside a basketball.

    But 28 days after the brightness started waning, more polarised light intimated that the inner ejecta had morphed into an ellipsoid while the outer later stayed roughly spherical – like a small rounded Australian football in a basketball.

    So what does this mean?

    The axisymmetric shape, the researchers write, is in line with a core-collapse explosion. A central inner engine of a magnetar or black hole pumps energy into the layers, causing the asymmetry over time.

    As to whether the shape is typical for a superluminous supernova or not is unknown. More observations and detailed modelling of other superluminous supernovae – and time – will tell.

    See the full article here .

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  • richardmitnick 9:48 am on November 6, 2016 Permalink | Reply
    Tags: , , , , , , Supernovae   

    From CfA: “Pulsar Wind Nebulae” 

    Harvard Smithsonian Center for Astrophysics


    Center For Astrophysics

    November 4, 2016

    Neutron stars are the detritus of supernova explosions, with masses between one and several suns and diameters only tens of kilometers across. A pulsar is a spinning neutron star with a strong magnetic field; charged particles in the field radiate in a lighthouse-like beam that can sweep past the Earth with extreme regularity every few seconds or less. A pulsar also has a wind, and charged particles, sometimes accelerated to near the speed of light, form a nebula around the pulsar: a pulsar wind nebula. The particles’ high energies make them strong X-ray emitters, and the nebulae can be seen and studied with X-ray observatories. The most famous example of a pulsar wind nebula is the beautiful and dramatic Crab Nebula.

    Supernova remnant Crab nebula. NASA/ESA Hubble
    Supernova remnant Crab nebula. NASA/ESA Hubble

    When a pulsar moves through the interstellar medium, the nebula can develop a bow-shaped shock. Most of the wind particles are confined to a direction opposite to that of the pulsar’s motion and form a tail of nebulosity. Recent X-ray and radio observations of fast-moving pulsars confirm the existence of the bright, extended tails as well as compact nebulosity near the pulsars. The length of an X-ray tail can significantly exceed the size of the compact nebula, extending several light-years or more behind the pulsar.

    CfA astronomer Patrick Slane was a member of a team that used the Chandra X-ray Observatory to study the nebula around the pulsar PSR B0355+54, located about 3400 light-years away.

    NASA/Chandra Telescope
    NASA/Chandra Telescope

    The pulsar’s observed movement over the sky (its proper motion) is measured to be about sixty kilometer per second. Earlier observations by Chandra had determined that the pulsar’s nebula had a long tail, extending over at least seven light-years (it might be somewhat longer, but the field of the detector was limited to this size); it also has a bright compact core. The scientists used deep Chandra observations to examine the nebula’s faint emission structures, and found that the shape of the nebula, when compared to the direction of the pulsar’s motion through the medium, suggests that the spin axis of the pulsar is pointed nearly directly towards us. They also estimate many of the basic parameters of the nebula including the strength of its magnetic field, which is lower than expected (or else turbulence is re-accelerating the particles and modifying the field). Other conclusions include properties of the compact core and details of the physical mechanisms powering the X-ray and radio radiation.
    Reference(s):

    Deep Chandra Observations of the Pulsar Wind Nebula Created by PSR B0355+54</emKlingler, Noel; Rangelov, Blagoy; Kargaltsev, Oleg; Pavlov, George G.; Romani, Roger W.; Posselt, Bettina; Slane, Patrick; Temim, Tea; Ng, C.-Y.; Bucciantini, Niccolò; Bykov, Andrei; Swartz, Douglas A.; Buehler, Rolf, ApJ 2016 (in press).

    See the full article here .

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    The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

     
  • richardmitnick 7:51 pm on August 17, 2016 Permalink | Reply
    Tags: , , , Supernovae   

    From Chandra: “G11.2-0.3: Supernova Ejected from the Pages of History” 

    NASA Chandra Banner
    NASA Chandra Telescope

    NASA Chandra

    August 17, 2016

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    Composite

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    X=ray

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    Optical
    Credit X-ray: NASA/CXC/NCSU/K.Borkowski et al; Optical: DSS
    References Borkowski, K. et al, 2016, ApJ, 819, 160; arXiv:1602.03531

    New Chandra data of the supernova remnant G11.2-0.3 raise new questions about the timing of its origin.

    Previously, G11.2-0.3 was associated with an event recorded by Chinese observers in 386 CE.

    Chandra observations show that dense gas clouds lie along the line of sight between Earth and G11.2-0.3.

    This new information means that the supernova explosion would have been too faint to be seen with the naked eye from Earth.

    A new look at the debris from an exploded star in our galaxy has astronomers re-examining when the supernova actually happened. Recent observations of the supernova remnant called G11.2-0.3 with NASA’s Chandra X-ray Observatory have stripped away its connection to an event recorded by the Chinese in 386 CE.

    Historical supernovas and their remnants can be tied to both current astronomical observations as well as historical records of the event. Since it can be difficult to determine from present observations of their remnant exactly when a supernova occurred, historical supernovas provide important information on stellar timelines. Stellar debris can tell us a great deal about the nature of the exploded star, but the interpretation is much more straightforward given a known age.

    New Chandra data on G11.2-0.3 show that dense clouds of gas lie along the line of sight from the supernova remnant to Earth. Infrared observations with the Palomar 5-meter Hale Telescope had previously indicated that parts of the remnant were heavily obscured by dust.

    Caltech Palomar 200 inch Hale Telescope
    Caltech Palomar 200 inch Hale Telescope interior
    Caltech Palomar 200 inch Hale Telescope

    This means that the supernova responsible for this object would simply have appeared too faint to be seen with the naked eye in 386 CE. This leaves the nature of the observed 386 CE event a mystery.

    A new image of G11.2-0.3 is being released in conjunction with this week’s workshop titled “Chandra Science for the Next Decade” being held in Cambridge, Massachusetts. While the workshop will focus on the innovative and exciting science Chandra can do in the next ten years, G11.2-0.3 is an example of how this “Great Observatory” helps us better understand the complex history of the Universe and the objects within it.

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    Historic Supernova Candidates. Credit NASA/CXC/SAO

    Taking advantage of Chandra’s successful operations since its launch into space in 1999, astronomers were able to compare observations of G11.2-0.3 from 2000 to those taken in 2003 and more recently in 2013. This long baseline allowed scientists to measure how fast the remnant is expanding. Using this data to extrapolate backwards, they determined that the star that created G11.2-0.3 exploded between 1,400 and 2,400 years ago as seen from Earth.

    Previous data from other observatories had shown this remnant is the product of a “core-collapse” supernova, one that is created from the collapse and explosion of a massive star. The revised timeframe for the explosion based on the recent Chandra data suggests that G11.2-0.3 is one of the youngest such supernovas in the Milky Way. The youngest, Cassiopeia A, also has an age determined from the expansion of its remnant, and like G11.2-0.3 was not seen at its estimated explosion date of 1680 CE due to dust obscuration. So far, the Crab nebula, the remnant of a supernova seen in 1054 CE, remains the only firmly identified historical remnant of a massive star explosion in our galaxy.

    Supernova remnant Crab nebula. NASA/ESA Hubble
    Supernova remnant Crab nebula. NASA/ESA Hubble

    This latest image of G11.2-0.3 shows low-energy X-rays in red, the medium range in green, and the high-energy X-rays detected by Chandra in blue. The X-ray data have been overlaid on an optical field from the Digitized Sky Survey, showing stars in the foreground.

    Although the Chandra image appears to show the remnant has a very circular, symmetrical shape, the details of the data indicate that the gas that the remnant is expanding into is uneven. Because of this, researchers propose that the exploded star had lost almost all of its outer regions, either in an asymmetric wind of gas blowing away from the star, or in an interaction with a companion star. They think the smaller star left behind would then have blown gas outwards at an even faster rate, sweeping up gas that was previously lost in the wind, forming the dense shell. The star would then have exploded, producing the G11.2-0.3 supernova remnant seen today.

    The supernova explosion also produced a pulsar – a rapidly rotating neutron star – and a pulsar wind nebula, shown by the blue X-ray emission in the center of the remnant. The combination of the pulsar’s rapid rotation and strong magnetic field generates an intense electromagnetic field that creates jets of matter and anti-matter moving away from the north and south poles of the pulsar, and an intense wind flowing out along its equator.

    A paper describing this result appeared in the March 9th, 2016 issue of The Astrophysical Journal and is available online. The authors are Kazimierz Borkowski and Stephen Reynolds, both of North Carolina State University, as well as Mallory Roberts from New York University.

    See the full article here .

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    NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

     
  • richardmitnick 10:39 am on June 30, 2016 Permalink | Reply
    Tags: , , , , , Supernovae   

    From Ethan Siegel: “Origin of LIGO’s merging black holes finally discovered!” 

    Ethan Siegel

    6.30.16

    The massive black holes that formed LIGO’s first event were a surprise, and then a mystery. Here’s the long-awaited solution!

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    A double black hole. Image credit: NASA, ESA and G. Bacon (STScI).

    “Black holes can bang against space-time as mallets on a drum and have a very characteristic song.” -Janna Levin

    In order to produce the gravitational wave signals that LIGO has seen so far, two extremely massive stars in a close, binary orbit must have both gone supernova an extremely long time ago. Over billions of years, those black holes spiraled into one another, as their orbits slowly decayed over the aeons, emitting small amounts of gravitational radiation at each step along the way. Finally, in the final fractions of a second, those ripples in spacetime were enough to vibrate our detectors here on Earth by less than a thousandth the width of a proton. That’s what it took to deliver our first directly detected gravitational wave signal, a century after Einstein’s relativity first predicted them.

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    The inspiral and merger of the first pair of black holes ever directly observed. Image credit: B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration).

    LSC LIGO Scientific Collaboration

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    VIRGO Collaboration bloc

    Before these gravitational waves were seen, all we had were theoretical models of what stellar-mass black holes might be. Contrary to the supermassive ones at the centers of galaxies, where we could measure the stars in orbit around them, the high-energy radiation emitted from the infalling matter, or the energy of the jets leaving them, all we had for these objects — the most common black holes in the Universe — was a story. We knew that stars that were massive enough would not only fuse hydrogen into helium during their main lifetime, and then turn into a red giant, fusing helium into carbon, but would go beyond that, heating up internally to achieve fusion reactions that less than 1% of stars will ever attain. Carbon fusion will begin, then oxygen, then silicon and sulphur and finally the core will be filled with iron, nickel and cobalt: elements too stable to fuse into heavier ones under normal conditions.

    Stars need to be many times the mass of the Sun — at least 8-to-10 but perhaps even more — to reach this stage. At this point, the inner core of the star, since there’s no more fusion occurring, runs out of its prime source of radiation, which was the only thing holding the nuclei inside up against gravitational collapse. So the core of the star collapses, catastrophically, and implodes, giving rise to a Type II Supernova.

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    Type IIb supernova

    The thing is, a star needs to initially be very massive to make a black hole. The overwhelming majority of the star that gives rise to a supernova gets blown off by the explosion; it’s only the innermost core that collapses. Most of the stars that do collapse give rise to neutron stars, just two or three times the mass of the Sun. And the stars that give rise to black holes — the ones 20, 40 or more times the mass of our Sun — were expected to lead to black holes maybe between 5 and 10 solar masses. Maybe the most massive ones would be even 15 or 20 times the mass of our Sun.

    But there’s a limit; high-mass stars tend to do something called quench star formation. The idea is that as a young star gets more and more massive, it burns brighter and hotter, and it not only prevents more matter from falling onto that star and growing it, it ionizes all the matter around it, and blows it off from the entire vicinity. In other words, it prevents all the other stars around it from growing larger; that’s what quench means.

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    The star forming region Sh 2–106, or S106 for short. A newly formed, ultra-massive star at the center, shrouded in dust, is responsible for carving the shape of this nebula. Image credit: NASA and ESA.

    So for two stars to have lived, died in supernovae, and created both a 36 and a 29 solar mass black hole means that something had to happen to avoid this scenario. What actually happens, we think, is more peculiar than you might have imagined altogether. The stars that gave rise to the black holes couldn’t have been formed too late (or with too many heavy elements in them) according to numerical models, which indicate that most likely they had only about 10% of the heavy elements (carbon, oxygen and iron, for example) found in our Sun.

    A new paper by Krzysztof Belczynski, Daniel E. Holz, Tomasz Bulik & Richard O’Shaughnessy, as well as a letter by J.J. Eldridge, suggest, based on simulations, that black hole binaries such as this arose in great numbers very early on in the Universe. Rather than from a Type II supernova, there are likely a whole class of binary black holes of ~30 solar masses (or slightly more) that arose from:

    massive binary star systems,
    between 40 and 100 solar masses to start,
    from when the Universe was only about ~2–3 billion years old,
    and that likely formed either in dwarf galaxies or on the outskirts of what would become a spiral galaxy: where there are fewer heavy elements.

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    Artist’s impression of two merging black holes, with accretion disks. The density and energy of the matter here is woefully insufficient to create gamma ray or X-ray bursts. Image credit: NASA / Dana Berry (Skyworks Digital).

    Over time, the radii of these stars increase as they heat up, making it easier to strip off their outer layers. The first one will go supernova as normal, but the second one will suffer a different fate. What happens in a binary system, rather than getting hotter and hotter and larger and larger, is that the outer layers get thrown off, via gravitational interaction, into the interstellar medium around them. The first black hole to form will also devour some of that material, but black holes are not very good eaters; they spit out most of what falls in. If both stars are massive and close enough, the second can lose its outer envelope. The core inside, then, simply contracts down and collapses without very much fanfare at all. In this way, we can get black holes without the standard, corresponding supernova explosions we know and recognize.

    In addition, the “common envelope” phase shrinks their mutual orbit, bringing them closer and closer to merger status. Despite many years of research, the quantitative answer of how much these orbits shrink by is still an open question with very large uncertainties. Nevertheless, the simulations of Belczynski’s team indicates that these black hole binaries very likely formed more than 10 billion years ago, and their inspirals and merger occurred just 1.3 billion years ago, with the light reaching us today.

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    Hubble space telescope of the merging star clusters at the heart of the Tarantula Nebula, the largest star-forming region known in the local group. Image credit: NASA, ESA, and E. Sabbi (ESA/STScI); Acknowledgment: R. O’Connell (University of Virginia) and the Wide Field Camera 3 Science Oversight Committee.

    There is another possibility they entertain, however: a much younger, massive cluster of stars — with higher mass binaries inside — could have created these black holes much more recently. Perhaps clusters like the one inside the massive Tarantula Nebula in our own local group give rise to black hole binaries, and, with stars up to 260 times the mass of our Sun in there, perhaps ~30–40 times the mass of our Sun isn’t even as large as these black holes get. Regardless of their origin, which we should be able to figure out as more statistics and detections come in, the next generation of gravitational wave observatories should be able to detect perhaps as many as 1,000 of these binary black hole mergers per year. We’re entering, for the first time, the era of direct black hole astronomy, courtesy of gravitational waves. What it means for astrophysics is more than most of us ever anticipated.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
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