Wednesday, 6 September 2017

What should be the cause of death of sun forming into red giant?

The Sun radiates energy at the rate of 3.9 x 108 W and has been doing so for several billion years. Where does all this energy come from? Chemical burning is ruled out; if the Sun had been made of coal and oxygen-in the right proportions for combustion-it would have lasted for only about 1000 y. Another possibility is that the Sun is slowly shrinking, under the action of its own gravitational forces. By transferring gravitational potential energy to thermal energy, the Sun might maintain its temperature and continue to radiate. Calculation shows, however, that this mechanism also fails; it produces a solar lifetime that is too short by a factor of at least 500 that leaves only thermonuclear fusion. The Sun, as you will see, burns not coal but hydrogen, and in a nuclear furnace, not an atomic or chemical one. The fusion reaction in the Sun is a multistep process in which hydrogen is burned into helium, hydrogen being the "fuel and helium the "ashes."

The p-p cycle starts with the collision of two protons ('HH) to form deuteron (H), with the simultaneous creation of a positron (e) and a neutrino. The positron very quickly encounters a free electron (e) in the Sun and both particles annihilate their mass energy appearing as two gamma-ray photons. A pair which is actually extremely rare. In fact, only once in about 1028 proton-proton collisions is deuteron formed: in the vast majority of cases, the two protons simply rebound elastically from each other. It is the slowness of this "bottleneck" process that regulates the rate of energy production and keeps the Sun from exploding. In spite of this slowness, there are so very many protons in the huge and dense volume of the Sun's core that deuterium is produced in just this way at the rate of 1012 kg/s.

Once a deuteron has been produced, it quickly collides with another proton and forms a He nucleus. Two such 'He muclei may eventually (within 10 y: there is plenty of time) find each other, forming an alpha particle He and two protons,  Overall the p-p cycle amounts to the combination of four protons and two electrons to form an alpha particle, two neutrinos, and six gamma-ray photons. obtaining the quantities in the two sets of parentheses then represent atoms of bydrogen and of helium. That allows us to compute the energy release in the overall reactions.

About 05 MeV of this energy is carried out of the Sun by the two neutrinos is deposited in the core of the Sun as thermal energy. That thermal energy is then gradually transported to the Sun's surface where it is radiated away from the Sun as electromagnetic waves, including visible light.

Hydrogen burning has been going on in the Sun for about 5 x 108 y, and calculations show that there is enough hydrogen left to keep the Sun going for about the same length of time into the future. In 5 billion years, however, the Sun's which by that time will be largely helium, will begin to cool and the Sun will to collapes under its own gravity. This will raise the core temperature and cause the outer envelope to expand, turning the Sun into what is called a red ziant. If the core temperature increases to about 10 K again, energy can be produced through fusion once more this time by burning helium to make carbon. As a star evolves further and becomes still hotter, other elements can be formed by other fusion reactions However, elements more massive than those near the peak of the bindinig energy cannot be produced by further fusion processes.

Elements with mass numbers beyond the peak of that curve are thought to be formed by neutron capture during cataclysmic stellar explosions that we call super
novas. In such an event the outer shell of the star is blown outward into space, where it mixes with and becomes part of the tenuous medium that tils the space between the stars. It is from this medium, continually enriched by debris from stellar explosions, that new stars form, by condensation under the influence of the gravitational force. The abundance on Earth of elements heavier than hydrogen and helium suggests that our solar system has condensed out of interstellar material that contained remnants of such explosions. Thus, all the elements around us including those our own body were manufactured in the interios of stars that no longer exist as one scientist put it "In truth we are the children of the stars

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