Tuesday, November 13, 2018

SUN - Why is the Sun so hot?



-  2165  -  Why is the Sun so hot?  The Sun’s surface is about 10,000 degrees Fahrenheit. But, surrounding the Sun is an atmosphere of gas known as the corona. This envelope of super heated gas is called a plasma and it measures more than 3,000,000 degrees.  Astronomers are still trying to figure out how the outer layer of this star is so much hotter than what lies beneath it.
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 ------------------  2165  -  SUN  -  Why is the Sun so hot?
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-  The fact that the Sun is hot should not be news. Astronomers tell us the Sun’s surface is about 10,000 degrees Fahrenheit. But surrounding the Sun is an atmosphere called the corona. This envelope of super heated gas, which is a plasma,  measures more than 3,000,000 degrees.  Astronomers are still trying to figure out how the outer layer of this star is so much hotter than what lies beneath it.
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-  The part that confuses scientists is quite simple: Since the Sun’s heat source is at its core, it should more or less cool as you move farther away from the center. But, this isn’t what they observe. Science cannot explain how the corona is so much hotter than other layers that are below it.
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-  Heat is actually a measurement of how fast atoms are moving. Scientists are mostly looking for ways to accelerate the Sun's atmosphere material in a way that happens only in the sizzling corona.

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-  Despite its heat, the corona is usually hidden from view thanks to the intense brightness of the rest of the Sun. Even complex instruments have trouble studying it without being overwhelmed by light from across the Sun’s surface.
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-   Fortunately the Sun's corona appears in rare but predictable occurrences that have fascinated people for millennia, total solar eclipses. In 1869, astronomers took advantage of just such an eclipse to study the Sun’s suddenly visible outermost layer.
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-  They used a spectrometer to analyze the light in order to fingerprint the corona. They spotted an unfamiliar green line that appeared to be a totally new element, calling it coronium. Seventy years later, scientists realized it was actually the familiar element iron, heated to never-before-seen temperatures of millions of degrees. This is hundreds of times the measured temperature of the Sun’s surface, and was totally baffling to the scientists.
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-  An early theory was that acoustic waves were compressing and expanding the Sun’s material like an accordion causing this rise in temperature. But solar probes haven’t been able to find such waves carrying enough power to explain the observed coronal heat.
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-  For almost 150 years, the corona's enormous heat remains one of the mysteries of science.  Scientists are quite sure their temperature readings of both the surface and the corona are reasonably correct, and more certain about the basic physics that the farther you are from a heat source the cooler the temperature should be.

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-  Part of the problem is that we don’t understand a lot of the small-scale happenings on the Sun. We know it does its job of heating our planet, and we know generally how it does it with nuclear fusion. But, the scale of the materials and forces at work simply don’t exist in a more accessible laboratory here on Earth.
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-  Current theories boil down to some version of the Sun being a very complicated magnet. The Earth spins its own magnetic field. But the Earth, despite oceans and underground magma, is still much more solid than the Sun, which is just a big ball of gas and plasma. So the Earth spins more or less like a solid object.
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-  Not so the Sun. The Sun spins, but because it’s not solid, its poles and equator spin at different rates. The Sun also bubbles material up and down through its layers, like a pot of boiling water. The effect is a tangled mess of magnetic field lines. 
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-  The charged particles that make up the Sun’s outer layers travel these lines. The lines snarl and reconnect, releasing massive amounts of energy exiting as solar flares or leaving twists full of charged particles flying off  into space at ludicrous speed , called a coronal mass ejection.
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-   It is possible that underneath what we see, the Sun is undergoing constant nanoflares , tiny flares spiking to tens of millions of degrees that cumulatively could give rise to the corona’s high temperatures.
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-  A longer-standing explanation have been the existence of acoustic waves. Heat is just particles moving very quickly. The faster particles move, the hotter they are.  Waves through the Sun’s interior might fling the Sun’s outermost layer outward.  These become acoustic waves,  travelling through the Sun like sound waves travel through air.
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-   The Sun is full of many different kinds of waves.  There are what is called Alfvén waves that travel  in plasma and along magnetic lines separate from acoustic waves.
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-   There are many satellites already tracking the Sun.  The Parker Solar Probe, launched this year, 2018,  is just starting its observations. It will continue observing until 2025. Scientists hope that by getting the closest ever view of the Sun, it will answer some of these questions about nanoflares or Alfvén waves or,  some even more complicated combination of both mechanisms.
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-  Stay tuned , there is a lot more to learn.  I am 78 years and just getting started.
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-  November 13, 2018.             An Index of recent Reviews is available.
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