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Mstqrha (): Its ultimate; the halt of its movement at the Day hereafter, where it stops and collapses on itself (according to physics, it collapses into a white dwarf star).

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This refers to the fact that the setting of the sun and the Sunrise changes daily, and varies from place to another on Earth.

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The throne is highly above all creatures and the whole universe (Dark Matter: Seven Firmaments, and seven Ardhoan, and Matter: Galaxies, stars, planets, elements, and radiation). Likewise, the Sun continues to move Under the throne.

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(http://www.mutah.edu.jo/eijaz/sevenardhoan.htm)

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[12] Allah is He Who created seven Firmaments and of the Ardh a similar number. Through the midst of them (all) descends His Command: that ye may know that Allah has power over all things, and that Allah comprehends all things in (His) Knowledge.

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[3] Verily your Lord is Allah, Who created the Samawat (seven firmaments) and the Ardh (seven Ardhean) in six Days, then He established Himself on the Throne (of authority), regulating and governing all things. No intercessor (can plead with Him) except after His leave (hath been obtained). This is Allah your Lord; Him therefore serve ye: will yet not receive admonition?

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In Paradise there are hundred levels prepared by God for the mujaahideen. Between each two adjacent levels like what is between Earth and Sama. It is tremendously varying levels of honor. The throne is above the highest level.

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Motion of Our Sun: ([1])

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The ecliptic and celestial equator intersect at two points: the vernal (spring) equinox and autumnal (fall) equinox (Fig. 1). The Sun crosses the celestial equator moving northward at the vernal equinox around March 21 and crosses the celestial equator moving southward at the autumnal equinox around September 22. When the Sun is on the celestial equator at the equinoxes, everybody on the Earth experiences 12 hours of daylight and 12 hours of night for those two days (hence, the name ``equinox'' for ``equal night''). The day of the vernal equinox marks the beginning of the three-month season of spring on our calendar and the day of the autumn equinox marks the beginning of the season of autumn (fall) on our calendar. On those two days of the year, the Sun will rise in the exact east direction, follow an arc right along the celestial equator and set in the exact west direction.

Fig. 1

When the Sun is above the celestial equator during the seasons of spring and summer, you will have more than 12 hours of daylight. The Sun will rise in the northeast, follow a long, high arc north of the celestial equator, and set in the northwest. Where exactly it rises or sets and how long the Sun is above the horizon depends on the day of the year and the latitude of the observer. When the Sun is below the celestial equator during the seasons of autumn and winter, you will have less than 12 hours of daylight. The Sun will rise in the southeast, follow a short, low arc south of the celestial equator, and set in the southwest. The exact path it follows depends on the date and the observer's latitude.

Make sure you understand this. No matter where you are on the Earth, you will see 1/2 of the celestial equator's arc. Since the sky appears to rotate around you in 24 hours, anything on the celestial equator takes 12 hours to go from exact east to exact west. Every celestial object's diurnal (daily) motion is parallel to the celestial equator. So for northern observers, anything south of the celestial equator takes less than 12 hours between rise and set, because most of its rotation arc around you is hidden below the horizon. Anything north of the celestial equator takes more than 12 hours between rising and setting because most of its rotation arc is above the horizon. For observers in the southern hemisphere, the situation is reversed. However, remember, that everybody anywhere on the Earth sees 1/2 of the celestial equator so at the equinox, when the Sun is on the equator, you see 1/2 of its rotation arc around you, and therefore you have 12 hours of daylight and 12 hours of nighttime everyplace on the Earth.

 

Fig. 2: Earth's motion around the Sun, not as simple as I thought

http://www.youtube.com/watch?feature=endscreen&NR=1&v=82p-DYgGFjI

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Hydrostatic Equilibrium of the sun

Definition: Hydrostatic equilibrium occurs when compression, due to gravity, is balanced by a pressure gradient, which creates a force in the opposite direction. In stars, the pressure gradient appears as a result of the huge quantity of thermal energy (which acts outward) created by nuclear fusion reactions. It is gravity and this thermal energy that are in equilibrium.

It's a bit like blowing a balloon up, the inward pressure is counteracted by the external pressure of the atmosphere. In addition, when we consider stars, this means that the larger the mass of the star, the higher the temperature must be to achieve this balance. Larger stars will use up their supply of hydrogen more quickly and live a shorter life.( http://wiki.answers.com/Q/What_is_hydrostatic_equilibrium)

Consider a spherical shell (of thickness dr, and area dA) at a distance r from the center of the sun: The internal pressure provides an opposing support force against the gravitational force on a mass shell. Assume the Sun to be a spherical gas cloud with density ρ(r). Consider a differential mass shell of this sphere with radius r (Fig. 3). Then differential mass dM is:

Fig. 3: The structure of the sun adjusts until the gravitational "pull" towards its center is just balanced by the "push" of the gas pressure outward. Fortunately, this results in a very stable state, called hydrostatic equilibrium. (http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/suninterior.htm).

The differential gravitational force is:

Here M = M(r) is the total mass contained within the sphere of radius r, and is given by the integration

 

Pressure is equal to force divided by area:

 

Now dividing by dr to get the equation of hydrostatic equilibrium:

The equation of hydrostatic equilibrium shows how the pressure in the sun changes to balance gravitational collapse.

The Equation of State For an ideal gas is:

Stars are held together by gravity. Gravity tries to compress everything to the center. What holds an ordinary star up and prevents total collapse is thermal and radiation pressure (Fig. 3 and Fig. 4). The thermal and radiation pressure tries to expand the star layers outward to infinity.


Fig. 4-a: Hydrostatic equilibrium: gravity compression is balanced by pressure outward.


Fig. 4-b: Greater gravity compresses the gas, making it denser and hotter, so the outward pressure increases.

In any given layer of a star, there is a balance between the thermal pressure (outward) and the weight of the material above pressing downward (inward). This balance is called hydrostatic equilibrium. A star is like a balloon. In a balloon the gas inside the balloon pushes outward and the elastic material supplies just enough inward compression to balance the gas pressure. In a star the star's internal gravity supplies the inward compression. Gravity compresses the star into the most compact shape possible: a sphere. Stars are round because gravity attracts everything in an object to the center. Hydrostatic equilibrium also explains why the Earth's atmosphere does not collapse to a very thin layer on the ground and how the tires on your car or bicycle are able to support the weight of your vehicle. (http://www.astronomynotes.com/starsun/s7.htm)

The stability of the sun

The sun is a million miles wide ball of nuclear furnace, in which the outward force of the fusion heat balances the protracted force of its own gravity. Its output of light and heat remain uniformly constant, as well as its ability to maintain a powerful magnetic force seen within storms and patterns on its surface. The most obvious pattern of variation is the 11-year sunspot cycle. However, even this cyclical variation isn't constant; some cycles are very active, while other cycles seem quite below the average.

The massive gravitational force and extreme magnetic force of the sun is held in a state of relative equilibrium by the vast amounts of energy generated by nuclear fusion within the core. The sun operates in an balanced state between the inward gravitational contraction and outward pressure generated by nuclear fusion.

When the hydrogen fuel begins to be exhausted, helium "ash" collecting in the core will temporarily overcome the gravitational contraction and the sun will expand into a red giant. This is expected to begin fairly gradually some time between three and four billion years from now. (http://wiki.answers.com/Q/What_keeps_the_sun_stable).

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The Final Stages of the Evolution of a Sun-like Star

After the red giant phase, low mass stars follow a different evolutionary path than more massive stars. For this reason, we are going to first consider what happens to low mass (less than 8 times the mass of the Sun) stars as they progress past the red giant phase. To really study and understand stellar evolution in detail, you would want to subdivide stars more finely. That is, you would want to separately consider the evolution of stars of 0.1, 0.5, 1.0, 1.5, 2.0, 3.0, 5.0, and 8.0 solar masses, for example, and you would find differences between each. We are going to continue using a solar mass star as our example for low mass stellar evolution, but you should realize that the details of the evolution of stars of 0.5 solar masses or 5.0 solar masses deviate from the general description presented below.

During the red giant phase of a star's lifetime, the core is not in equilibrium. All of the fusion is occurring in a shell outside of the helium core, so there is no energy generation or outward radiation pressure to support the helium core. For this reason, the core of the star continues to collapse during the red giant phase. Collapse means an increase in temperature and density in the core. In many low mass stars (from about 0.5 - 3.0 solar masses), the core can be compressed to the point that it becomes a degenerate gas. This has important consequences in stellar evolution, so I will briefly describe what this means.

The gas inside stars is a soup of atomic nuclei and free electrons. If you compress a gas of this type to a high enough density, you have to use two of the laws of quantum mechanics to describe its behavior. These say:

1.                 Like electrons bound in an atom, the free electrons can only have certain energies that you can represent as energy levels similar to the energy level diagrams we used in our study of the Bohr model of the atom.

2.                 No two identical electrons can be found in the same energy level (the Pauli Exclusion Principal). Electrons can have two different spins, which each have a slightly different energy, so you can have two and only two electrons per energy level, one with spin up, the other with spin down.

The net effect of these two quantum mechanical effects is that when the gas has been compressed to the point where many of the lower energy levels have been filled, it begins to resist compression. Even though the physical state is still that of a gas, it is harder to compress a degenerate gas than solid steel! (https://www.e-education.psu.edu/astro801/content/l6_p3.html).

The present Sun is right in the middle of its age as a main-sequence star (4.5 billion years).

5.2 Postsolar evolution stages (Reference : [2]).

5.2.1 Red Giant

As the Sun ages, helium collects in its center. After a lifetime of 9 billion years as main-sequence star, approximately 10% of the hydrogen in the Sun's core will have been converted into helium and nuclear fusion reactions will cease producing energy. The equilibrium between the total pressure force directed outwards and the gravitational force directed towards the centre of the Sun will be disturbed. The core of the Sun starts slowly collapsing under its own gravitational attraction. Fusion moves outward to a shell surrounding the core, where hydrogen-rich material is still present. The gravitational energy from the collapse will be converted into heat causing the shell to burn vigorously and so the Sun's outer layers to swell immensely. The surface is now far removed from the central energy source, cools and appears to glow red. The Sun now evolves into the stage of a red giant. For a few hundred million years, the expansion of the outer solar layers will continue, and the Sun will engulf the planet Mercury. The temperature on Venus and Earth will rise tremendously. Hydrogen fusion in the shell continues to deposit helium "ash" onto the core, which becomes even hotter and more massive.

In the Sun's core nuclear fusion of helium into carbon and oxygen will start to trigger even further the expansion of its outer layers. The helium-rich core is unable to lose heat fast enough and becomes unstable. In a very short time of few hours the core gets too hot and is forced to expand explosively. Outer layers of the Sun will absorb the core explosion but the core will no longer be able to produce energy by thermonuclear burning. Helium fusion then continues in a shell and the structure of the Sun would look like an onion: An outer, hydrogen-fusion layer and an inner, helium-fusion layer which surrounds an inert core of carbon and oxygen.

The old Sun may repeat the cycle of shrinking and swelling several times. In this stage of evolution the Sun is called an asymptotic giant branch star. Finally enough carbon will accumulate in the core to prevent the core explosion. Helium-shell burning will add heat to the outer layers of the Sun, mainly containing hydrogen and helium. The asymptotic giant Sun will generate eventually an intense wind that begins to carry off its outer envelope. The precise mechanism behind this phenomenon is not yet well understood. The Sun will expand a final time and after about 30 million years it will swallow Venus and Earth, outer layers will keep expanding outward and as much as half of the Sun's mass gets lost into space.

5.2.2 White Dwarf

The solar core keeps shrinking and because it is not able anymore to produce radiation by fusion the further evolution of this configuration is governed by gravitation. All matter will collapse into a small body about the size of the Earth. Thus, the Sun will have become a white dwarf, this is a dense-matter configuration, having radiated away the energy of its collapse. Then the white dwarf rapidly begins to cool.

What is a white dwarf star ? ([3])

A white dwarf is the final stage of the evolution of a star that is between .07 and 1.4 solar masses. White dwarfs are supported by electron degeneracy and they are found to the lower left of the main sequence of the HR (Hertsprung Russel) diagram. White dwarfs represent a stable phase () in which stars of less than 1.4 solar masses live out the rest of their lives. White dwarf stars got their name because of the white color of the first few that were discovered. They are characterized by a low luminosity, a mass close to that of our sun, and a radius only that of the earth. Because of their large mass and small area these stars are extremely dense and compact objects with average densities approaching up to 1,000,000 times that of water. White dwarfs have low luminosities. Because of this they can be observed only within a few hundred parsecs from the earth ( 1 parsec = 3.26 light years).

Degenerate electrons pressure balances force of gravity and stops contracting ([4])

All stars are burning at some point in their lives but eventually a star stops burning. When the stars stop burning the stars with less than 1.4 solar masses shrink in size. As they shrink they start to grow very faint. But regardless of their color they are called white dwarfs. The value of 1.4 solar masses is known as the Chandrasekhar limit. Chandrasekhar reasoned that something must be holding up material in white dwarfs against gravity, something known as electron degeneracy. When star contracts, electrons get close together and there is a continued increase in their resistance to being pushed even closer. This process is related to pressure. At great densities, pressure from the degenerate electrons is sufficiently great, it balances the force of gravity and the star stops contracting. So electron degeneracy stops the white dwarf form contracting and compresses the gas of the star. What this means is that a white dwarf is incredibly dense. A mass the size of the sun is compressed into a volume only the size of the earth. This is so dense that a teaspoon of white dwarf weighs ten tons.

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A white dwarf represent the final stage, a stable phase () of evolution of a star Like the sun (a star that is between .07 and 1.4 solar masses). The Sun lives out the rest of its life as a dense compact White dwarf; supported by electron degeneracy. Nuclear

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The solar core keeps shrinking and because it is not able anymore to produce radiation by fusion the further evolution of this configuration is governed by gravitation. All matter will collapse into a small body about the size of the Earth. Thus, the Sun will have become a white dwarf, this is a dense-matter configuration, having radiated away the energy of its collapse. Then the white dwarf rapidly begins to cool.

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[1] ) http://www.astronomynotes.com/nakedeye/s5.htm

[2] ) http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter5.html

[3] ) http://www.eg.bucknell.edu/physics/astronomy/as102-spr00/web_pages/web8.html

[4] ) http://www.eg.bucknell.edu/physics/astronomy/as102-spr00/web_pages/web8.html

[5] ) http://www.alwaraq.net/Core/AlwaraqSrv/LisanSrchOneUtf8