- 3434 - SUPERNOVAE - brought life to Earth? Life on Earth appears to have evolved under the influence of supernovae activity. Supernova activity in Earth’s neighborhood may have led to more oxygen in the atmosphere. And oxygen is necessary for complex life. The 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.
------------- 3434 - SUPERNOVAE - brought life to Earth?
- 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.
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- 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.
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- However, 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, you wouldn’t be reading this.
- Life on Earth appears to show 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. Cosmic rays travel to our solar system, and some end their journey by colliding with Earth’s atmosphere. They are responsible for ionizing our 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.
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- 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. 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. Supernova activity over time and the increased levels of organic matter that result from supernovae.
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- How does the increased organic matter lead to more oxygen? 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 life needs oxygen.
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- This wouldn’t happen without nearby supernova. Without enough nearby supernova activity, the climate would be warmer. The winds and ocean currents would be weaker, and would move fewer nutrients around.
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- The strong upwelling ocean currents required to deliver chemical nutrients to the ocean’s bio-productive zone would be absent. The consequence of a warmer climate would be less bio-productivity because ocean currents and atmospheric winds would be weaker.
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- Less bio-productivity would mean less organic material (Carbon 12) 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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- Modern Earth’s ocean currents combine to create “thermohaline circulation“, also called the “ocean conveyor belt“. That belt, along with winds and surface run-off from rivers,. drives Earth’s nutrient cycle.
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- A consequence is that moving organic matter to sediments is indirectly the source of oxygen. 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.
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- 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. There is an extraordinary inter-connection between life on Earth and supernovae, mediated by the effect of cosmic rays on clouds and climate.
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- 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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- If a supernova were too close, it would sterilize Earth completely. But 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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- We’re accustomed to thinking of nearby supernovae as potentially devastating to life on Earth, and they are. But this study shows that, like many things in nature, it’s the dosage that matters. If there were no supernova activity in our neighborhood, life on Earth might look much different than it does now.
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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. It might be caused by a series of supernova explosions no more than 35 lightyears 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.
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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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- We’re not exactly sure what triggered that extinction event. There’s no clear smoking gun like there is for the asteroid impact evidence of the one that killed most of the dinosaurs.
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- Fossils of plants remaining from that tumultuous era show signs of 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. In general, intense UV radiation isn’t too great for living beings, hence an extinction event.
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- The researchers estimated that a single supernova blast within 65 light years could have been enough to suppress our ozone layer for about 100,000 years. The fossil record indicates that life was having a tough go at it for three times that length, however, so the researchers speculate that the supernova wasn’t alone.
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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.
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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.
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- This expansion produces a surrounding nebula rich in ionized helium, carbon, and nitrogen, but almost no hydrogen. 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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- At least some Wolf-Rayet stars do become supernovae. The spectrum of a supernova known as “SN 2019hgp“ was discovered by the Zwicky Transient Facility. The supernova’s spectrum had bright emission light indicating the presence of carbon, oxygen, and neon, but not hydrogen or helium.
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- These particular emission lines weren’t caused by elements of the supernova directly. Instead, they were part of a nebula expanding away from the star at more than 1,500 kilometers / second.
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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.
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- This matches the structure of a Wolf-Rayet star extremely well. Because this supernova was identified by spectra of the surrounding nebula, it isn’t clear whether the explosion was a simple supernova, or whether it was a more complex hybrid process where the upper layer of the star exploded while the core collapsed directly to a blackhole.
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- In 2018, a breakout supernova was discovered by an automated facility, “AT2018cow“, and was the first in a new class of superluminous transient events.
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- 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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- Every once in a while, a stellar cataclysm occurs in our Universe, bringing the life of a star to an end. The most common type of cataclysm is a core-collapse supernova, 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.
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- In 2018, a new class of explosions was seen for the first time, the “Cow” class. Detected automatically by a facility that monitors the sky for unexpected brightening events, its randomly generated name came out at AT2018cow.
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- Another event in the same “Cow” class of objects was recently discovered. The first one detected was not by its visible light signatures, but by its spectacular X-ray brightening. Known as AT2020mrf, it literally bathed the Universe in X-rays for billions of light-years.
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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.
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- 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. 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. Some of those features include:
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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 explanation for the event AT2018cow is a cocooned supernova that undergoes a shock breakout. By synthesizing a wide variety of observations from many different observatories, a consistent picture began to emerge. One candidate explanation was that it came from a tidal disruption event, where stars are torn apart through gravitational interactions with a massive yet compact object.
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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. Instead, perhaps it was a supernova after all, one that was in an unusual environment, shrouded by a dense cocoon-like structure of gas.
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- With that realization, the pieces fell into place. 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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- This new class of objects are now known not exclusively as “Cow” class objects, but rather as FBOTs, Fast Blue Optical Transients.
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- The supernova remnant of “SN 1987a” in six different wavelengths of light. Even though it’s been 35 years since this explosion occurred, and even though it’s right here in our own backyard, the material around the central engine has not cleared enough to expose the stellar remnant. For contrast, Cow-like objects have their cores exposed almost immediately.
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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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- If something has brightened, faintened, newly appeared, newly disappeared, or has otherwise changed somehow, in position or color, is it “flagged” as a candidate for a transient event. Almost all of our automated transient searches, however, are restricted to being performed in visible light.
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- The 48-inch “Samuel Oschin Telescope” at Mt. Palomar is where the “Zwicky Transient Facility” takes its data from. Even though it’s only a 48 inches (1.3 meter) telescope, its wide field of view and rapid observing speed allows it to discover optical changes in the night sky that practically every other observatory cannot find.
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- That’s part of what makes this newest event, “AT2020mrf,” so spectacular. It was first found in July of 2020 not by any of the transient facilities explicitly built and designed to find these optical events, but rather by a completely different type of observatory: an X-ray telescope known as the Spektrum-Roentgen-Gamma (SRG) telescope.
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- The Spektrum-Roentgen-Gamma telescope completed its first full survey of the sky in June 2020. The whole point of surveying the sky over and over again is, once again, 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 possessed other features that made it a remarkably interesting object including:
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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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- Perhaps most interestingly, the fact that 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, AT2020mrf, 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.
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- Some 328 days after the explosion began, NASA’s Chandra X-ray telescope pointed its eyes at this object 2 billion light-years away. 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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- We only have a series of measurements of its optical brightness at low resolution and low sensitivity, as the drawback of large-area transient surveys is that it trades sensitivity and resolution for speed.
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- We have no X-ray data from the initial brightening, as we only happened to observe this region some 35 to 37 days after the initial brightness peaked, and we don’t have data in between the SRG observation and the Chandra X-ray observations: a gap of nearly 300 days.
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- We know that the X-ray emissions have dropped off but we don’t know how they decayed. We know that there was both hydrogen and helium in the AT2018cow event but we don’t know whether hydrogen and helium were present or absent in this one, as it’s already too late to make those critical follow-up observations.
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- We don’t know whether the substantial, record-breaking X-ray emissions that were first seen by SRG, again, more than a month after the optical brightness peaked, actually represents the true peak of emissions or was truly an even brighter event than we were able to observe.
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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.
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- 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. Nobody knows why. They’re only found in star-forming regions around dwarf galaxies and we don’t understand why that’s the case.
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- 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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- Whatever the resolution to this puzzle is, the only way we’ll uncover it is by discovering and more thoroughly examining even more of these events.
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- With more advanced all-sky X-ray surveys on the way, our best bet, as always, 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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- Still more to learn. The more you learn the more you know you don’t understand.
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January 31, 2022 SUPERNOVAE - brought life to Earth? 3434
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