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--------------------- 2648 - SUPERNOVA - what is the youngest?
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- Today the remnant of that 1678 explosion has an expanding shockwave that is 10 lightyears in diameter. The shockwave has an average velocity of 20,300,000 miles per hour.
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- The Tycho Brahe supernova was first seen in 1572 and Remember? That was 436 years ago.
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- Of course, there was the Supernova 1987A that exploded just 13 years ago. But, that was in another galaxy, the Large Megallenic Cloud, our neighboring galaxy. There are 24 of these neighbor galaxies.
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- The youngest supernova in our galaxy was just now spotted. It has been hidden from view by a thick cloud of interstellar dust. It’s radio waves first hit Earth 140 years ago, in 1860. It is called G1.9+0.3 Supernova.
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- It exploded towards the center of the Milky Way 26,000 lightyears away. It took that long for the radiation to reach us. The visible light portion of the radiation was blocked by the dust but astronomers were able to see it in radio waves and in X-rays that passed through the dust.
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- Supernova are the death of giant stars. It is the last instant when gravity wins over the thermonuclear pressure. Stars exist within a balancing act of these two forces. It starts with a large cloud of interstellar gas, mostly hydrogen gas. Gravity always tries to compress things, including gas, into a sphere.
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- A sphere is the most volume with the least surface area. It is the geometry of the lowest energy state. So, gravity is trying to pull everything in the cloud to a volume of 4/3*pi*r^3, the volume of a sphere.
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- The pressure of gravity increases density and increases temperature. The temperature continually rises until it reaches 18,000,000 F. At this temperature and pressure hydrogen begins a thermonuclear fusion that generates helium and converts a little bit of mass to a lot of radiation, E=mc^2.
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- The outward pressure of the radiation pushes against gravity until the two forces are in balance. Smaller stars will burn hydrogen for 10,000,000,000 years before they run out and die as White Dwarfs.
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- Larger stars, 10 solar masses, will live for only 10,000,000 years, burn all of their hydrogen, helium, neon, magnesium, silicon, sulfur right up to iron. At the iron core fusion stops the star collapses and the star explodes into a supernova.
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- The Milky Way contains about 250 supernovae. This latest discovery is the youngest on that list. The images of this young supernova going back to 1985 compared to today’s images have increased in size by 15%, in 23 years. This puts its age at 140 years old.
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- The supernova has continually increased in radio brightness over these 23 years. We can only view the star in radio and X-ray spectrums because the dust is blocking any visible light. Astronomers have just recently calculated that dust in the Universe is blocking half of all the visible light that is generated.
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- When a large star has fused all of its hydrogen into helium fusion stops momentarily. The star collapses further under the pressure of gravity. The temperature increase even higher until helium begins its thermonuclear fusion into carbon and oxygen. When helium runs dry the carbon fuses into neon, then magnesium, then silicon, then sulfur, then silicon into iron.
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- The star becomes layered like an onion with iron at its core surrounded by layers of burning silicon, magnesium, neon, oxygen, carbon, helium, and hydrogen at the surface. It took several million years to build up these layers but the iron core is built in one day.
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- The iron core will be 2/3 rds the size of Earth yet have the mass more than the Sun. Iron does not fuse into higher elements. Fusion stops. Thermonuclear pressure stops. In less than one second the entire star collapses from 5,000 miles wide to 12 miles wide.
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- This sudden crash releases a high amount of energy, 100 times more energy than our Sun will produce over its entire lifetime. This blast of energy is the supernova.
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- Neutrinos will carry most of the energy off into space with the explosion. The rest of the energy becomes a shockwave blasting radiation through the layers of the onion. The shells blast away at 10,000,000 miles per hour. The supernova burns brighter than a billion Suns for several weeks.
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- The iron core collapses into a Neutron Star, or a Black Hole. The shockwave continues to expand for millions of years plowing through the interstellar medium. All the elements in this explosion become the interstellar dust that has come from older stars and is blocking our view of this one. The shockwave hit us 140 years ago but we could not see it.
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- The thermonuclear pressure that was supporting the star before the explosion is proportional to the temperature, “T“, and the number density of the particles, “n”, particles/ cubic centimeter. Each particle has a mass, “m”. The entire interstellar cloud of particles has a mass, “M”. The number of particles is “M / m”. To get the density of particles we divide the number by the volume:
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------------------- number density = n = 3*M / 4*pi*m*r^3
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------------------ Thermonuclear pressure = n*k*T
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----------------- Force = pressure * area
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- The area can be assumed to be the two halves of the spherical cloud pressing on the center. Area = pi*r^2.
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----------------- Force = n*k*T *pi*r^2
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----------------- Force = (3*M / 4*pi*m*r^3) * (k*T *pi*r^2)
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--------------- Outward Force = 3*M*k*T /4*m*r
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- To get the balancing inward force of gravity we make the same assumption the 2 halves of he spherical gas are attracting each other towards the center, “M/2” separated by the radius, “r”.
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--------------- Force = G* M/2 *M/2 / r^2
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--------------- Inward Force = G*M^2 / 4 r^2
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These equations are only approximations because these forces need to be integrated over the volume of the spherical gas cloud, which isn’t ever a perfect sphere. It requires Calculus. But, these equations are close enough to allow us to understand the balance in forces and pressures that create the star and eventually the supernova explosion.
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- An increase in temperature will increase the force of thermal pressure outward. An increase in mass raise total pressure and gravitational force. But its increase goes up as mass squared. An increase in density corresponds to a decrease in radius raising both pressure and gravitational force.
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- The gravity force depends on 1/r^2 while pressure depends on 1/r. All of these parameters change in unison until the sphere is in balance.
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- If we set these two force equations equal to each other we will define the precise balancing point for all of these parameters.
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---------------- Force of thermonuclear pressure = Force of Gravity
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------------- Outward Force = 3*M*k*T /4*m*r = Inward Force = G*M^2 / 4 r^2
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-------------- M^2 = 3*k*T*r / G
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- The math gets complex but if we ratio Mass to the mass of the Sun and we reduce the other factors this equation becomes:
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-------------- (M / Msun)^2 = 18 * T^3 / n
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- To illustrate this calculation a typical interstellar cloud has a temperature, T = 30 K, pretty cold. The particle density is 300 / cm^3, pretty sparse. The Mass at balance is 171 solar mass. When the cloud compresses to a number density of 300,000 particles / cm^3 a star is formed with 5.4 solar mass.
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- The cloud is not smooth in density, it is lumpy, so it can fragment into several 5.4 solar masses and form several stars. In fact, these typical numbers suggest that each cluster of stars found will have a minimum of 32 stars in them.
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- See Review 2636 for a list of more on supernova reviews. An interesting topic. Supernovae make star dust, we are made of star dust.
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- March 3, 2020 2648
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--------------------- Friday, March 6, 2020 --------------------
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