- 3482 - SUPERNOVAE - learning from explosions? It’s almost impossible to comprehend a supernova explosion’s violent, destructive power. An exploding supernova can outshine its host galaxy for a few weeks or even months. That seems almost impossible when considering that a galaxy can contain hundreds of billions of stars all shining bright.
--------------------- 3482 - SUPERNOVAE - learning from explosions?
- Any planet too close to a supernova would be completely sterilized by all the energy released, its atmosphere would be stripped away, and it may even be shredded into pieces. But like many things in nature, it all comes down to dose. A certain amount of supernova activity might be necessary for life to exist.
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- “Nucleosynthesis” is how supernova explosions forge heavy chemical elements necessary for life. Supernovae explosions create and spread elements like iron out into space to be taken up during the formation of stars and planets. Without them, we wouldn’t be here. Life on Earth appears to have evolved under the influence of supernovae activity.
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- Supernova activity in Earth’s neighborhood may have led to more oxygen in the atmosphere. And oxygen is necessary for complex life. Oxygen is at the end of a long chain of cause and effect, and it all begins with the Galactic Cosmic Rays released by supernovae. Evidence shows a connection between climate, clouds, and cosmic rays from supernovae.
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- When heavy stars explode, they produce cosmic rays made of elementary particles with enormous energies. They are misnomer as rays, they were later to be discovered as particles instead. Cosmic rays travel to our solar system, and some end their journey by colliding with Earth’s atmosphere. They are responsible for ionizing the atmosphere.
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- The ionizing energy from those cosmic rays creates aerosols in Earth’s upper atmosphere. That increases cloud formation. Clouds block solar radiation from reaching Earth’s surface, cooling the climate. A cooler climate has greater temperature differences between polar regions and mid-latitudes. Those differences create stronger winds and ocean currents, which in turn drive stronger nutrient cycles.
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- There is a correlation between supernova rates and trace elements, or nutrients, in the ocean. The nutrients are found in pyrite and are a proxy for nutrients in the ocean.
Stronger nutrient cycles mean that more chemical elements necessary for life are delivered to the upper 200 meters of the ocean, near continental shelves, where bio-productivity is highest.
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- When there’s higher bio-productivity, more organisms live and die, and when they die, they fall to the ocean floor as organic matter, to be encased in sediments. On geological time scales, supernovae activity can fluctuate wildly, by several hundred percent. So the effect on climate can be pronounced on long time scales.
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- The organic matter in ocean sediments in the form of Carbon 12. Life prefers the lighter C12 isotope over C13, and the ratio of C12 to C13 in the sediments reveals the presence of life over geological timescales.
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- All of this activity has consequences for Earth’s oxygen. When organic matter moves into sediments, it becomes an indirect source of oxygen. If all of that organic matter were exposed to the atmosphere, then it would react with atmospheric oxygen as it decomposed and pull the oxygen out of the atmosphere. Instead, since the organic matter is buried, the oxygen remains in the atmosphere. And complex life needs oxygen.
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- Without enough nearby supernova activity, the climate would be warmer. The winds and ocean currents would be weaker, and would move fewer nutrients around. The strong upwelling ocean currents required to deliver chemical nutrients to the ocean’s bio- productive zone would be absent.
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- Less “bioproductivity” would mean less organic material (C12) in the ocean sediments. The available kinetic energy in the ocean-atmosphere system determines the mixing and transport of nutrients in the oceans and atmosphere.
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- Earth's ocean currents combine to create “thermohaline circulation“, “the ocean conveyor belt“. That belt, along with winds, drives Earth's nutrient cycle A fascinating consequence is that moving organic matter to sediments is indirectly the source of oxygen.
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- Photosynthesis produces oxygen and sugar from light, water and CO2. However, if organic material is not moved into sediments, oxygen and organic matter become CO2 and water. The burial of organic material prevents this reverse reaction. Therefore, supernovae indirectly control oxygen production, and oxygen is the foundation of all complex life.
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- Oxygenic photosynthesis and organic matter burial is the primary source of oxygen, and oxygen underpins the evolution of complex life. This new evidence points to an extraordinary interconnection between life on Earth and supernovae, mediated by the effect of cosmic rays on clouds and climate.
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- Supernova activity and life on Earth come down to dosage. According to some scientific evidence, some supernovae have been close enough to Earth to contribute to partial extinction. A supernova explosion may have triggered the “Ordovician Extinction“ , the second-largest extinction in Earth’s history by number of species killed off.
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- According to this research some supernovae activity helped drive life on Earth by stimulating the nutrient cycle and increasing atmospheric oxygen.
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- Did a supernova cause the Devonian mass extinction event? 359 million years ago the Earth suffered one of its worst extinction events. There were a series of supernova explosions no more than 35 light years away. Our nearest star is 4.5 light years away.
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- Every once in a while something disastrous happens to life on Earth. The biggest episodes we call extinction events. The latest big one happened about 65 million years ago, and was a very rough time for dinosaurs but turned out to work well for the mammals.
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- That extinction event was just the latest in a long series of interruptions in the multitude of life on the planet. One of the earliest extinction events happened at the boundary of the Devonian and Carboniferous periods about 359 million years ago.
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- The key piece of evidence is the fact that fossils of plants remaining from that tumultuous era show signs of nasty sunburns: excess UV exposure. The ozone layer of the Earth does a fantastic job of blocking almost all the UV radiation from the sun, so the fact that these critters were getting an extra dose means that our ozone layer had to be depleted. There are a lot of potential geological processes that can scrub away our ozone layer, and there’s also one celestial one.
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- The intense radiation from a close enough supernova blast can strip away our ozone, leaving the surface of the Earth exposed to the UV onslaught from the sun. It is estimated that a single supernova blast within 65 light years could have been enough to suppress our ozone layer for about 100,000 years.
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- The next step is to find evidence in those fossil layers of an excess of certain radioactive elements like plutonium-244. This element isn’t naturally produced on the Earth, and so the only way for it to exist in that layer of sediment is for it to have been put there as the shock-wave of the supernova washed over our planet.
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- The nearest supernova candidate to the Earth is the star Betelgeuse, which is located a safe 600 light years away. We often think of supernova explosions as inevitable for large stars. Big star runs out of fuel, gravity collapses its core and BOOM!
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- Astronomers have long thought at least one type of large star didn’t end with a supernova. Known as “Wolf-Rayet stars“, they were thought to end with a quiet collapse of their core into a blackhole. But a new discovery finds they might become supernovae after all.
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- Wolf-Rayet stars are among the most massive stars known. They are at the end of their short lives, but rather than simply running out of fuel and exploding, they push out their outer layers with an extremely powerful stellar wind. This produces a surrounding nebula rich in ionized helium, carbon, and nitrogen, but almost no hydrogen.
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- The surface temperature of the remaining star can be over 200,000 Kelvin, making them the most luminous stars known. But because most of that light is in the ultraviolet range, they are not particularly bright to the naked eye.
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- Even with the outer layers of a Wolf-Rayet star cast off, the central star is still much more massive than the Sun. So you’d figure it’s only a matter of time before it becomes a supernova. No matter how far up the periodic table fusion occurs, it will eventually run out of fuel, leading to a core-collapse supernova.
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- This latest study is of the spectrum of a supernova known as “SN 2019hgp“ discovered by the Zwicky Transient Facility (ZTF). The supernova’s spectrum had bright emission light indicating the presence of carbon, oxygen, and neon, but not hydrogen or helium. They were part of a nebula expanding away from the star at more than 1,500 km/s.
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- Before the supernova occurred, the progenitor star was surrounded by a nebula rich in carbon, nitrogen, and neon, while lacking the lighter elements of hydrogen and helium. The expansion of the nebula must have been driven by strong stellar winds. This matches the structure of a Wolf-Rayet star extremely well.
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- In 2018, a breakout supernova was discovered, “AT2018cow“, and was the first in a new class of superluminous transient events. Since, only a few others have been seen. But AT2020mrf is unique, hundreds of times more luminous than the others. A central engine, like a magnetar or an actively accreting blackhole, is required to power this explosion, which shows unique X-ray features.
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- A core-collapse supernova is where a massive star’s interior implodes, leading to a runaway fusion reaction and a tremendous explosion, where the energy emitted by the star can briefly shine billions of times brighter than a typical star. And yet, it’s the rarer types of stellar cataclysms, superluminous supernovae, hypernovae, tidal disruptions events, and even more exotic explosions, that can shine brighter than anything else we have observed.
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- As they go through their life cycles, stars transform mass into energy through the process of nuclear fusion. By smashing light atomic nuclei together under tremendous pressures and temperatures, they can trigger the formation of heavier atomic nuclei. If you were to put the total masses of the pre-fusion nuclei and the post-fusion nuclei on a scale, you’d find that the ones produced by fusion were slightly less massive than the ones that went into the reaction.
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- Where did that mass go? It gets transformed into energy through Einstein’s most famous equation: E = mc^2.
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- When AT2018cow was first seen, it appeared simply as a rapidly brightening, high-temperature event: like a supernova, but with some unusual features to it.
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-------------------- The copious detection of iron
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-------------------- An extremely luminous brightening in ultraviolet wavelengths
approximately ten times the intrinsic brightness of a normal supernova
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-------------------- Brightness across all wavelengths of light, from X-ray down to the radio
evidence that it was surrounded by very dense material, with a tremendously fast shockwave moving through it
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- The long-term nature of the X-rays suggested that a remnant was left behind to power it, eliminating that as a potential explanation. Perhaps it was a supernova that was in an unusual environment, shrouded by a dense cocoon-like structure of gas. If there were a cocoon of gas surrounding a star that was reaching the end of its life, then:
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------------------------ An initial supernova would shock the surrounding cocoon
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------------------------ The material would heat to exceedingly high temperatures
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------------------------ The injected energy would cause a “breakout” event, creating the extreme brightness, the rapid increase in luminosity, and the ultra-fast shock wave
the remnant of the supernova, like a neutron star, would continue to inject energy for long periods of time after the initial explosion.
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- What makes an explosion a “Fast Blue Optical Transient“? There must be a rapid increase in brightness; that’s the “fast” part. You have to have lots of energy in the ultraviolet portion of the spectrum; that’s the “blue” part. It has to have a large brightness increase in the visible light portion of the spectrum; that’s the “optical” part. And it needs to have a time variation in its overall energy output, where it rises, increases to a maximum, and then decreases and fades away; that’s the “transient” part.
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- AT2020mrf was first found in July, 2020, by an X-ray telescope known as the Spektrum-Roentgen-Gamma (SRG) telescope. This X-ray telescope is unique that it is the only one that plans on imaging the entire sky numerous times over.
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- The whole point of surveying the sky over and over again is to look for changes, as they signify an astronomical event of interest. In July, 2020, right at the start of that second sweep, something fascinating emerged; an entirely new source of X-ray light where none had been previously just six months prior had not only emerged, but was incredibly bright.
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- The original “Cow” event, AT2018cow had a large and significant X-ray brightness for a supernova. That of AT2020mrf was 20 times brighter in that X-ray light. In addition, both of these events had a substantial but erratic variability in their X-ray brightness, varying rapidly on timescales of less than a day.
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- 35 days before the SRG telescope found the remarkable X-ray brightening, an optical brightening had occurred, just as it had for other FBOT events, including the Cow. It had:
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------------------- A very high temperature of around 20,000 K
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------------------- Significant emission features that indicate a very high speed, of around 10% the speed of light (much faster than a normal supernova’s of 2-3% the speed of light)
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-------------------- A bright set of radio emissions
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-------------------- It belongs to a very small, low-mass, dwarf galaxy: one with a mass of only 100 million stars, or less than 0.1% the mass of our Milky Way.
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- This event is now the fifth event to meet all the criteria for an FBOT, and somehow all five of them have occurred in dwarf galaxies that are forming new stars. Some 328 days after the explosion began, NASA’s Chandra X-ray telescope pointed its eyes at this object 2 billion light-years away.
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- Remarkably, within its first six hours of observation, Chandra saw 29 individual X-ray photons coming from this one object: a remarkably large number. Over a second six-hour observation window, it discovered another 10 X-ray photons. Those two observations, taken nearly a year after the initial explosion occurred, indicate a number of remarkable facts:
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------------------- The X-ray flux coming from this object must be absolutely enormous; some 200 times as luminous in X-ray light as AT2018cow was at a comparable time in its evolution.
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------------------ The X-rays make it, by far, the most luminous Cow-like supernova ever seen in the X-ray.
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------------------ It demonstrates the diversity of Fast Blue Optical Transients, while still supporting the cocooned-supernova breakout model of FBOTs.
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------------------ It shows that even a full year after the supposed supernova first occurred, the rapid X-ray variability on a timescale of ~1 day or less still remains.
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- The only way the X-ray flux could remain this large this long after a supernova explosion is if it’s powered by a still-active central engine either an accreting blackhole or an extremely rapidly spinning, highly magnetized neutron star: a millisecond magnetar.
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- In a normal supernova there’s plenty of surrounding material preventing the core from becoming exposed, even years or decades after the explosion first occurs. However, with a Cow-like supernova, the copious material surrounding the stellar core is broken apart, exposing the core in short order.
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- Normally, when stars are on the path to going supernova, they expel large amounts of material and then, when the core implodes, the injected energy has to propagate through that material, shocking it, rebounding, etc., delaying the arrival of the initial light by hours. But with these FBOTs, or Cow-like events, the central cores of those ripped-apart stars are rapidly exposed with the surrounding debris cleared away. We don’t know why.
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- They’re only found in star-forming regions around dwarf galaxies and we don’t understand why that’s the case. And even though AT2020mrf looks very similar to the original Cow, AT2018cow, in optical wavelengths, it’s scores to hundreds of times intrinsically brighter in the X-rays.
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- With more advanced all-sky X-ray surveys on the way, our best bet, is by conducting a more comprehensive suite of scientific investigations. That’s the only way we can truly, reliably learn precisely what’s out there in the Universe.
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February 27, 2022 SUPERNOVAE - learning from explosions? 3482
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