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The Sun
Birth About 5.5 billion years ago, a passing star or galaxy disturbed a calm and placid cloud of gas and dust, called a nebula. The star or galaxy caused the cloud to swirl around, causing small eddies to form. The swirl caused the gas to start to coalesce together in places. Gravity, one of the universe's four fundamental forces, caused more and more gas and dust to gather onto these masses. The masses kept getting bigger and bigger. At this stage, they were called protostars. As gravity caused the material to pile on, it also caused those lumps to condense, which increased their gravity. The condensation caused the pressure in their cores to rise, and their internal heat increased. When the heat reached a temperature of 10,000,000 K (18,000,000° F), nuclear fusion started, and our sun was born. Energy The sun creates its energy the same way all other non-giant
stars do, using the three main processes of hydrogen fusion. The basic process
is to combine light atoms into heavier ones, but the mass of the heavier ones
is slightly less than the sum of the lighter ones. The extra mass is lost as
energy and radiated into space; the energy is in Einstein's equation of The following process is known as the PPI chain and it occurs approximately 69% of the time in the sun:
A second hydrogen fusion process that occurs approximately 30.907% of the time in the sun is called the PPII chain. After line 2 of the PPI chain:
The third - and much more rare - form of fusion occurs only 0.093% of the time in the sun and is called the PPIII chain. After line 1 of the PPII chain:
Together, this looks like:
What does this mean in terms of actual energy? Every second, the sun converts 500 million metric tons of hydrogen to helium. Due to the processes of fusion, 5 million metric tons of excess material is converted into energy in each second. This means that every year, 157,680,000,000,000 metric tons are converted into energy. The material from one second of energy is about 1x1027 (one octillion thousand) watts of energy. On Earth, we receive about 2/1,000,000,000 (two billionths) of that energy, or about 2x1018 (two quintillion) watts. This is enough energy to power 100 average light bulbs for about 5 million years -- longer than humans have been standing upright. Composition
Anatomy
In the picture to the left, the black circles show a separation between the layers. The center of the sun, the Core, is the only part of the sun that actually makes energy. Models predict that the central temperature is approximately 15,710,000 K (28,800,000 °F). The central pressure is 2.477 x 1011 bars, and that the central density is approximately 1.622 x 105 kg/m3. The next layer of the sun is the Radiative Zone, which is where most of the harmful gamma rays bounce around until they become less energetic forms of light. The temperature here is about 5,000,000 K (9,000,000 °F). The layer that is next is called the Convection Zone, where solar material rises and falls due to heating and cooling. The temperature here reaches only 5,800 K (10,000 °F). The next section of the sun is called the Photosphere, which
is actually what you see when you look at the sun. Earth's crust is like the
sun's photosphere. The Photosphere is about 400 km (250 miles) deep. The next layer is the lower part of the sun's atmosphere, the Chromosphere. It is only visible during a total solar eclipse, when the moon blocks the light from the Photosphere. It reaches temperatures of around 30,000 K (54,000 °F), and it is very thick at about 2500 km (1600 miles). The last layer of the sun is called the Corona, and it is the upper layer of the sun's atmosphere. Like the Chromosphere, it is only visible during a total solar eclipse, such as in the picture at the left. This portion of the sun is about 1,700,000 K (3,000,000 °F). Aging and Death The sun will continue to shine as it currently does for approximately 5 billion years more. This is shown by estimating that the sun will stay in its current evolutionary tract until approximately 10% of its hydrogen is converted to helium. Because hydrogen to helium releases approximately 0.007*m*c2 of energy, and given that it releases approximately 3.846 x 1026 J/s:
Thus, the sun has enough mass to last approximately 10.3 billion
years, and it is approximately 5.5 gyrs old now. As the sun ages, it's luminosity steadily increases. Given the Ideal Gas Law (right), as the mean molecular weight ρ increases due to hydrogen to helium conversion, the pressure drops and the density then rises. This whole process acts to maintain hydrostatic equilibrium. As the core then contracts, the energy generation increases, causing the sun to become more luminous. A little-known fact is that the sun is approximately 140% as luminous now as it was when it was born. Eventually, the inner 10% of hydrogen will run out. The helium core is not hot nor dense enough to fuse to create energy, so the outward pressure is stopped. Gravity will contract the sun, and eventually a shell of hydrogen around the helium core will become hot enough to fuse H -> He. This shell will produce more energy than the previous hydrogen core phase did, so the luminosity will rise. Not all of the energy will escape, though, and it will go into expanding the sun. This expansion will result in a surface temperature drop. The sun will be in the subgiant star, and the cooler surface will have changed from yellow to orange-red. This cooling is due to the energy spreading over a larger surface area, so each unit of area receiving less energy. The helium "ash" from the hydrogen fusion in the shell will effectively fall onto the core, which will result in the sun continuing to contract to maintain pressure to hold up the star. Once the mass in the core is approximately 8% of the sun (the Schonberg-Chandrasekhar Limit), the density will be so great that the core will no longer act as a perfect gas, and it will become degenerate. Now the core will be held up by the Pauli Exclusion Principle, AKA it will be supported by electron degeneracy. This phase will still have the hydrogen burning shell, but the sun's outer layers will continue to expand, causing it to cool. This begins the sun's Red Giant Ascent. In this phase, H- can form when neutral H takes on a free electron. Radiation is easily absorbed by H-, and so the outer layers will have a high opacity. This high opacity and high energy generation will lead to convection, where the whole outer envelope will become convective and the material from the core can rise to the surface in a process called "dredge-up". As the outer layers continue to expand in this red giant phase, the ionization drops so there are fewer free electrons and fewer H- ions, leading to an opacity drop. The luminosity will still rise due to the core contracting. In this phase, the luminosity will be approximately 100 times what it is now, the radius between 30-100 times, and the effective temperature will be approximately 4000 K (6740 °F). Examples of this are Arcturus and Aldebaran.
None of the energy from the helium core flash will make it
out of the sun. It will act to revert the core back to an ideal gas state, and
expand it. Thus, the star will have a helium burning core, a hydrogen burning
shell which will provide most of the luminosity, and a large expanding envelope
of outer atmosphere. During the course of the horizontal giant branch, a carbon and oxygen ash core will begin to build up. In a star such as the sun, carbon fusion cannot occur because the temperature and density are too low. Thus, it will contract and heat, heating the layers outside the core. This will cause the new helium shell to start to fuse, and the star will begin to expand again. This repeats the previous process where there is more energy, a higher opacity, convection and a dredge-up phase. The again-expanding photosphere and higher luminosity combine to move the sun into the Asymptotic Giant Branch (AGB), AKA a red supergiant (cross section to the right). In the AGB phase, the sun will undergo periodic instabilities. One cause of this are helium shell flashes. This comes from the inert helium shell continually having mass added to it from the hydrogen shell. Although it will be in a partially degenerate state, when the mass gets too high, it will "ignite" in a flash similar to the first one. This will cause it to drop in luminosity and contract in size, repeating on a timescale of approximately 100,000 years. Instabilities in the outer envelope can cause AGB stars to pulsate on periods of several hundred days. Mass is lost during this phase at a rate of approximately 0.01% the mass of the sun per year in a process that is not well understood.
As the outer layers expand, their density will drop, and would allow future civilizations to view the hot carbon/oxygen (C/O) core that will be left behind. The C/O core will initially be hot at 100,000 K (180,000 °F). However, it will be dead, with no nuclear reactions to power it. It will be a white dwarf. A current example of a white dwarf is the star Sirius B. The outer layers that will form the planetary nebula will shine and become visible to outside observers, as is the nebula to the left of IC 418 (AKA The Spirograph Nebula), as taken by the Hubble Space Telescope. Typically, planetary nebulas are around 0.3 parsecs (1 light-year), expand at a rate of 10-30 km/s, and last only 10,000 years. When the sun runs out of heat, it will be a huge, black, chunk of carbon and oxygen floating in space. It will be called a black dwarf - a dead star. 
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In
the picture to the right, two 
The
solar composition closely reflects the primordial composition of the universe.
It is mostly hydrogen and helium, with a very few other trace elements thrown
in:
The
sun is made up of several layers, which do not have distinct borders separating
them. However, each layer has unique properties, which are vital to the sun's
functions.
Sunspots
occur on the photosphere. This is the place on the sun where the energy created
can finally escape into space. This portion of the sun reaches temperatures of
6,600 K (11,400 °F) at the bottom, and it dies down to 4,400 K (7,500 °F)
at the top. The pressure at the top of the photosphere is approximately 0.868
mb.
In
the core, the temperature continues to rise. When it approaches 100,000,000 K
(180,000,000 °F), helium will begin to fuse into carbon in the triple alpha
process. However, since the core is degenerate, when the temperature rises, the
pressure does not, for degenerate pressure is only a function of the density
(left). Therefore, the core cannot expand and cool, so the energy raises the
temperature, which raises the energy which raises the temperature, etc. When
this actually happens, 1011 times the luminosity of the sun now will
be released in a few seconds in what is known as the Helium Core Flash.
The
sun will now become a Horizontal Branch Star, for as the core expands, it cools
and the energy generation in the hydrogen burning shell will drop; so the luminosity
decreases, the sun will shrink, and the surface temperature rise.
In
the AGB phase, the outer layers of the sun will be greatly extended (possibly
past Earth's orbit) and will not be strongly bound. Mass loss, pulsations, and
a low binding energy of the outer layers can cause them to be released from the
star, turning this phase into the Planetary Nebula.