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--------------------- 2646 - SUPERNOVAE - how life is being created?
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- See Review 2636 about the two supernovae explosions being studied. This review is about Betelgeuse that is a supernovae we can view with backyard binoculars.
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- Betelgeuse is a star that is now beginning to slowly brighten. This behavior is exactly what astronomers expected. Betelgeuse is a very different star from our Sun. While our Sun is a main-sequence star in its prime of life, Betelgeuse is a red giant star on the verge of death. But the death of a star is not a simple process.
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- Stars shine so brightly and for so long because of a delicate balance of gravity and nuclear fusion. Gravity would like to collapse a star under its weight. Without nuclear fusion, gravity would crush a star into a white dwarf, neutron star, or black hole.
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- But, the crushing pressure gravity creates allows hydrogen in the star’s core to fuse into helium. The process is known as the proton-proton chain and combines four hydrogen nuclei into one helium nucleus. About 3% of the original mass is converted to energy in the form of gamma rays. This energy heats the core even further, letting it push back against gravity.
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- For stars larger than the Sun, another fusion process known as the Carbon-Nitrogen-Oxygen because the process fuses helium into those three elements. This process is why those three elements are the next most abundant in the universe, except for hydrogen and helium.
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- Over time the CNO cycle increases as hydrogen become more scarce and helium more abundant. Since the CNO cycle releases more energy at a faster rate, this means a star’s temperature increases over time. We see this gradual heating in our own Sun. By the time the CNO cycle dominates in a star, it’s core is so hot that the outer layers of a star swell and expand.
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- This is the stage Betelgeuse is in now. For millions of years, it was a main-sequence star of about 20 solar masses. But it is now fusing helium so furiously that it has bloomed into a “red super giant“. Betelgeuse is running out of fuel, and in the end, gravity will win. It’s only a matter of time.
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- But that time isn’t necessarily soon. Betelgeuse has enough helium to stay in the red supergiant stage for about 100,000 years. Even after it runs out of helium, it will be able to fuse carbon into heavier elements for about a millennium.
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- After that things will change fairly quickly. When it runs out of carbon it will try fusing heavier and heavier elements for about a year. Then its core will collapse, Betelgeuse will become a supernova explosion.
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- Betelgeuse is still deep in the red supergiant phase of its life. Even though it has dimmed significantly of recent, it isn’t on the verge of exploding. The gradual dimming and brightening we see suggest that it won’t be exploding in our lifetimes. It suggests that the core of Betelgeuse is still chugging away at a steady pace.
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- The changing brightness of Betelgeuse is due to a process known as convection. The upper layers of the star are heated by the core, and this generates a flow of hotter and cooler regions. Material in the interior is heated and rises to the surface. It then cools and sinks into the star, and the cycle continues.
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- Convection happens in the outer regions of most stars, including our Sun. On the surface of the Sun, these convection regions are known as granules, and they are typically the size of Texas. That sounds large, but for the Sun that’s smaller than most sunspots. So even though the Sun has bright hot regions and dimmer cool regions, they are so small compared to the Sun’s surface there isn’t an overall change in solar luminosity.
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- The outer layer of Betelgeuse is much less dense than that of the Sun. It is even less dense than Earth’s atmosphere. It’s basically a thin soup of glowing gas. That means the convection regions on Betelgeuse can be huge. A single region can cover a large part of the star. When one of those regions rises to the top, Betelgeuse gets brighter, and when it cools the star dims. Betelgeuse is starting to brighten because hot material is convecting to its surface. This is normal for Betelguese and is likely the way things will be for millennia.
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- There other supernovae happening every day somewhere in our universe. The biggest explosion seen in our universe has recently been found. This record-breaking, gargantuan eruption came from a black hole in a distant galaxy cluster hundreds of millions of light years away.
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- You could fit fifteen Milky Way galaxies in a row into the crater this eruption punched into the cluster's hot gas. This unrivaled outburst was detected in the Ophiuchus galaxy cluster, which is about 390 million light years from Earth. Galaxy clusters are the largest structures in the Universe held together by gravity, containing thousands of individual galaxies, dark matter, and hot gas.
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- In the center of the Ophiuchus cluster, there is a large galaxy that contains a supermassive black hole. Although black holes are famous for pulling material toward them, they often expel prodigious amounts of material and energy. This happens when matter falling toward the black hole is redirected into jets, or beams, that blast outward into space and slam into any surrounding material.
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- Chandra observations reported in 2016 first revealed hints of the giant explosion in the Ophiuchus galaxy cluster. The wall of a cavity, because it borders a region filled with radio emission, is from electrons accelerated to nearly the speed of light. The acceleration likely originated from the supermassive black hole.
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- The amount of energy required to create the cavity in Ophiuchus is five times greater than the previous record holder, and hundreds and thousands of times greater than typical clusters.
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- The densest and coolest gas seen in X-rays is currently located at a different position from the central galaxy. If this gas shifted away from the galaxy it will have deprived the black hole of fuel for its growth, turning off the jets.
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- This gas displacement is likely caused by "sloshing" of the gas around the middle of the cluster. Usually the merger of two galaxy clusters triggers such sloshing, but here it could have been set off by the eruption.
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- One puzzle is that only one giant region of radio emission is seen, as these systems usually contain two on opposite sides of the black hole. It is possible that the gas on the other side of the cluster from the cavity is less dense so the radio emission there faded more quickly.
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- As is often the case in astrophysics we really need multiwavelength observations to truly understand the physical processes at work. Having the combined information from X-ray and radio telescopes has revealed this extraordinary source, but more data will be needed to answer the many remaining questions this object poses.
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----------------------- See Review 2636 about two supernovae explosions being studied. This review also has an index listing 20 more reviews available about supernovae. These explosions are how all the elements that make up your body and the essentials for life are created. Fortunately our Earth was formed after many such explosions preceded us.
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- March 2, 2020 2646
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--------------------- Friday, March 6, 2020 --------------------
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