THE SUN

Structure

The Core
The Sun's core is 64% helium surrounded by a shell of fusing hydrogen-35% of the core's mass. All the other elements in the universe compose only 1% of the Sun's core. All of the energy the Sun radiates is created in the core. The energy that the core produces every second from 4.5 million tons (4 million metric tons) of matter raises its temperature to a spectacular 25,000,000° F (14,000,000° C). The nuclear reaction that takes place in the core produces little particles called neutrinos, which are amazingly unreactive to matter. These particles zoom out of the Sun at nearly the speed of light, taking a mere five seconds. The energy that the reaction produces is in the form of gamma rays. Because the Sun's core is so dense; in fact, 10 times denser than silver or iron, and because of the state of the atoms in the core, the photons are not reabsorbed into the core. Instead, they bounce around for 40 million years before leaving.

The Radiative Zone
The radiative zone is a thick layer of highly ionized, very dense gases which are under constant bombardment by the gamma rays from the core. It is about 75% hydrogen and 24% helium. Because most of the atoms here lack electrons, they can't absorb photons for convection to the surface. Most photons just bounce around. Every once in a while a photon will be absorbed. It will later be re-emitted as a photon with less energy; perhaps an X-ray or a UV ray. Eventually, photons along the entire magnetic spectrum exist in the radiative zone. It takes them about 10 million years to escape.

The Convective Zone
In the convective zone, gases are cool enough and enough of a temperature gradient exists that convection can happen. And, although the convective zone has the same composition as the radiative, the atoms of this zone can absorb the photons because they are less ionized. By the time the photons enter the convective zone, they have spent almost fifty million years in transit. In the convective cells, gas absorbs energy from the radiative zone. This energy heats it, and it rises to the next layer, the photosphere, where it dumps its energy off. By dumping off energy, it becomes cooler and it sinks down towards the bottom to pick up more energy.

The Photosphere
The photosphere is, in places, perhaps only 100 miles deep. It is made up of the same elements as the radiative zone, and in the same proportions. The temperature is about 10,000° F (6,000° C.) When the photosphere recieves energy from the convective zone, it radiates it off into space. Hence its name, photosphere, or "light sphere." It is the home of an interesting feature known as the granule.

• Granules completely make up the photoshpere and are the features that recieve and radiate light. They can be thought of as hot bubbling gas cells or giant firestorms twice as big as a hurricane. The average granule lasts for eight to ten minutes. In that time it rises and falls at about .3 miles (.5 kilometers) every second for a total vertical distance of 16 miles, or 26 kilometers.

The Chromosphere
The chromosphere is made up of the same elements as the radiative zone, and in the same proportions. It ranges from 6,000 miles in some places to 10,000 in others. (10,000-16,000 km) It is semi-transparent. It can only be seen with a special lens, or during a solar eclipse, during which it is visible as a thin, ragged, pinkish rim around the moon. Although it is what would be considered a vacuum on earth, it has an intricate structure. It also sports a variety of interesting features.

• Flares are huge eruptions of flame. Each one resembles a gas explosion but has the force of 10 million hydrogen bombs. Solar flares usually release powerful ions from the Sun's magnetic field, resulting in a flash of light and a brilliant aurora borealis.
• Prominences resemble flares but are somewhat different. A prominence is created when the Sun's violent magnetic field hurls large clouds of gas into space. Sometimes these gas clouds are held in the sky as benevolent clouds which resemble earth's cumulus clouds in shape. These floating prominences are known as quiescent prominences. Sometimes the gas clouds are launched from one magnetic point and drawn back to another one nearby, thus forming a loop. These are, appropriately, called loop prominences.
• Spicules , or "little spikes," are thin upwellings of gas. They last about 10 minutes, during which they extend 10,000 miles (16,000 kilometers) from the Sun's surface and attain speeds of up to 12 miles (19 kilometers) per second. They do not spurt up randomly; instead, they surround large features known as supergranules.
• Supergranules are huge cells of upwelling gas. They are about 20,000 miles (30,000 kilometers) across and are held in place by spicules. The average one lasts half a day, then wells up, spreads out, and dissolves.
• Sunspots are cooler, darker spots on the Sun's incredibly hot, bright surface. Their formation is caused by the way the Sun rotates. Initially, the magnetic field lines ran directly from the north magnetic pole to the south magnetic pole. But because the Sun spins faster at the equator, the field lines became distorted, curving towards the direction of the Sun's rotation. Eventually, they had wrapped around the Sun numerous times and were stretched and twisted like rubber bands. The field lines ruptured through the surface at two places; one going out and one coming back in. These two ruptures are sunspots.
  But why are the sunspots dark? The rupture of the field lines means that the Sun's magnetic field is extremely concentrated at the locations of sunspots. The concentration of the magnetic field is so much that at a sunspot, it is thousands of times as powerful as earth's magnetic field. The magnetism blocks some of the energy from leaving the photosphere. Flares and prominences are much more common around sunspots because of the warped magnetic field. Sunspots are in two parts; the dark umbra is surrounded by hotter gases sloping in towards it, the penumbra.

The Corona
The corona is a cloud of gas made of, as is the rest of the Sun, 75% hydrogen, 24% helium, and 1% other elements. It begins about 1,500 miles (2,500 kilometers) from the Sun's surface and extends tremendously far out; even past the earth's orbit. The corona is visible though a special lens, or during a solar eclipse, at what time it as visible as a ring of bright streamers.

• Coronal holes are holes in the corona that occur near the poles. These holes exist because the gases of which the corona is made tend to line up with the Sun's magnetic lines of force.

The Solar Wind
As it does with prominences, the Sun's magnetic field ejects matter through points of magnetic concentration. The Sun ejects almost a ton of matter from around the poles every second.

1. Two protons collide. It takes 7 billion years for the average proton to find another proton because since all protons are repelled by all other protons, they must collide head-on and must be going fast enough to fuse. Naturally, due to the vast number of protons in the Sun, some took only a few seconds to collide and others never will. That is why the entire Sun doesn't just explode.
   The two-proton nucleus is very unstable, so one of the protons immediately decays into a neutron. The decay releases a neutrino and a positron. The positron eventually collides with an electron (each one is the anti-matter equivalent of the other) and they both turn into gamma rays. The neutrino leaves the Sun at nearly light-speed, taking only a few seconds.
2. The new proton-neutron nucleus, also known as a deuterium nucleus, is very reactive, and it takes only a few seconds for it to collide with a proton. This collision releases another gamma ray. The new nucleus has two protons and one neutron.
3. The nucleus takes 400,000 years to find another nucleus just like it. These two collide, forming for a split-instant a ball with four protons and two neutrons. But two protons get knocked off by the force of the collision, leaving a stable helium nucleus with two protons and two neutrons.

History

• Five billion years ago the ancestor of the Sun was a cloud 300 trillion miles (500 trillion kilometers), or 50 light-years, in diameter. It was a near-vacuum. It was so close to a vacuum, in fact, that to equal the number of molecules in a cubic inch of air at sea level, one would need to remove almost a quarter of a cubic mile of the cloud. However, due to its immense size, it weighed as much as a few hundred Suns. Its temperature was -450° F (-270° C), only a few degrees above absolute zero. As it was so thin and so cold, it was very unstable. Any disturbance would cause it to either dissipate or collapse. And, eventually, such a disturbance did occur, and the cloud began to collapse.
• A few thousand years later, the cloud had collapsed in places into balls of matter called globules. These globules were not much denser than the cloud. They were still what would be considered on earth to be vacuums and their temperatures had risen only to -400° F (-240° C). They were only visible as dark circles against the background of illuminated dust. These globules were about 1 trillion miles in diameter and each one had 25 solar masses.
• 100,000 years after the globules began to form, they had shrunk to about 15 billion miles in diameter and each had only a millionth its original volume. Yet still at this size the average globular diameter could swallow two solar systems out to pluto laid end to end. The Sun's core began to be heated by the pressure of all the matter on top of it and it began to stabilize. It was then a protostar.
• After another few thousand years, the diameter of our Sun was about 225 million miles. Temperatures in the core frequently surpassed 100,000° F, or about 60,000° C, hot enough to strip atoms of their electrons, but not yet hot enough to fuse two hydrogen nuclei. The surface temperature was about 3000° F (1500° C), cooler than the Sun's surface today, but it was so large that it emmited many times more light than the Sun.
• Finally, the Sun collapsed to its present size. During the last contraction, it became hot enough in the core that fusion started. This burst of energy made the Sun unstable, with violent convective currents causing variable luminosity, but this too stabilized. The Sun has remained roughly as large and as stable since then.

The Future of the Sun

• The Sun has about 5 billion years worth of hydrogen with which it can produce energy.
• When five billion years are up, fusion in the Sun's core will cease, and gravity will cause the star to collapse. The heat from collapse will exceed the heat from fusion; in fact, it will reach 150,000,000° F (about 80,000,000° C), and the Sun will expand for a few hundred million years, eventually engulfing the planet mercury. As the Sun grows, it will become a red giant. By that time, its surface will be so large that it will emit 500 times as much light as it emits now.
• The core will be hot enough then to fuse helium into carbon and oxygen. This fusion will produce more heat, in fact too much for the new helium-rich core to radiate. In a few hours, the core will explode. The outer layers of the Sun will absorb the blast. The core will lose enough heat to stabilize, and it will begin to collapse. Again it will produce too much heat, and again it will explode, causing the Sun to swell drastically. This process will repeat itself, causing the Sun to grow and shrink many times.
• Eventually, enough carbon will accumulate in the core to prevent the explosions. The heat from helium fusion will cause the outer layers to expand; 30 million years later, the Sun will be so large that it will have swallowed the earth. The outer layers will keep expanding until they escape the core's gravity and float away into space.
• The core will shrink to what is called a white dwarf, a glowing white dot 8,000 miles (13,000 kilometers) in diameter. The white dwarf will still fuse helium, but it will eventually exhaust its reserves and glow solely from the heat caused by the collapse.
• But eventually this too will be exhausted. The Sun will glow no longer. It will be an invisible entity known as a black dwarf.