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The Sun


The sun is the most important object to Earth. Without the sun, life could not exist. There would be no heat, and all of the oceans would be frozen. There would be no light, and all plants would die. There is almost nothing more important to Earth than the sun.

The sun is an averaged-sized type G star and is middle-aged at about five billion years. Yet within our home solar system, the sun contains more than 99% of all matter. As for its size, about 915 Jupiters could fit in side of it, as could about 1,206,885 Earths.


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.


The sun creates its energy the same way all other stars do, through a process known as nuclear fusion, which joins two atoms of hydrogen to create a helium atom, the excess energy being radiated off into space.

pp Chain for Fusion in StarsIn the picture to the right, two protons join together to form a deuterium nucleus, which is also known as "heavy water." A positron and a neutrino are released as by-products. The deuterium nucleus is bombarded by another proton, creating a helium-3 nucleus. The by-product of this is a photon in the form of a gamma ray (a very high-energy form of light). Then, the helium-3 nucleus in bombarded by another helium-3 nucleus, creating a normal helium-4 nucleus. The by-product of this are two protons, which are free to start the whole process over again. The positron will be destroyed and form another gamma ray; the energy from this in the form of gamma rays is radiated out of sun's core.

Every second, the sun converts 500 million metric tons of hydrogen to helium. Due to the process 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.

The Light We See

The light that is currently reaching the Earth was generated in the sun about 100,000 years ago. It takes that long to get to the surface of the sun because the sun is so dense, and so the escaping energy has a much harder time escaping. This is analogous to walking down a hallway that was crowded with people. You couldn't just run right on through, for you would be shoved back by people and be bounced around. A photon of light encounters this same resistance when it tries to escape from the sun's core. The other people are the atoms that make up the sun and other photons generated by nuclear fusion, the process of which is stated above.

When the light formed in the sun finally makes it to the surface, it zooms away at 300,000,000 meters per second (186,000 miles per second), making it to Earth in about 8 minutes and 26 seconds.

The light from the sun is made up of many colors, called the visible spectrum, and many shorter and longer wavelengths of light. These other wavelengths are invisible to humans, but they can be measured with special detectors. The diagram below represents the electromagnetic spectrum, with the scale being in Hz - oscillations per second.

ElectroMagnetic Spectrum

These other wavelengths consist of Infrared (IR), Ultraviolet (UV), Micro, Radio, X, and Gamma. (IR light rays can be produced by specially-modified light bulbs, and are used in many places that sell food.) IR rays heat up matter. Our atmosphere acts as an "infrared shield," and keeps this light from reaching the surface. UV light has become an increasing concern over the past few years. It is a form of radiation, and the hole in the ozone layer is allowing some of the normally blocked UV light to get through. UV light causes tans, sunburns, and skin cancer. Micro waves are put to use in most people's kitchens in the aptly named "microwave." They are used to heat foods quickly, and are more effective at doing so than IR. Radio waves are used in a whole branch of astronomy, for they can penetrate clouds of gas and dust that visible light can't. They are also used for transmitting radio and television shows - television having a slightly higher frequency. X-rays are a form of radiation that are more powerful than UV, and are normally blocked by our atmosphere. X-rays are mainly used for medical purposes. Since they are a form of higher energy, they can penetrate denser objects than visible light can. Gamma rays are the most energetic form of radiation, and can pass through the human body. In cells, they can cause mutations and other severe damage. Luckily, they are blocked by our atmosphere. If they weren't, human life would be impossible.


Sun's AnatomyThe 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.

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. The temperature in the Core is about 16,000,000 K (28,800,000 °F).

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.Solar EclipseSunspots 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 11,000 K (20,000 °F).

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 stays about the same temperature as the Photosphere, 11,000 K (20,000 °F), but 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 right. This portion of the sun is about 1,700,000 K (3,000,000 °F).


SunspotsSunspots are cooler areas of the sun's Photosphere. They also have very strong magnetic fields, up to 10,000 times that of Earth's, and up to 3000 times the rest of the sun. Sunspots usually occur in pairs, and can be as large as the planet Earth. They are about 5,000 K (8,500 °F), which is 6,400 K (11,500 °F) cooler than the rest of the Photosphere. They have been known since ancient times. They were not found to be caused by strange magnetic fields until 1908, by George Ellery Hale.

Sunspots have an eleven-year activity cycle. They peaked last in 2001, and had their last big dip in 1995. If you own a telescope and a solar filter, you should see a decreasing amount of sunspots right now, with the next peak in 2012. No one yet knows why the sun has this "internal clock."

When sunspots are at their peak, the sun actually becomes brighter. This is because magnetically brighter areas surround each sunspot, more than making up for the dimmer areas.Solar Flare Sequence

Along with the eleven-year sunspot cycle, there appear to be huge lows where sunspot activity is extremely minimal, lasting about a century every 200-300 years. The last one was between 1640 and 1715. There are highs, too, with the last one occurring in the twelfth century.

One way that astronomers know this is from past observation. Also, astronomers can tell how much the magnetism dropped or rose by studying the radioactive carbon-14 preserved in the 8,000-year-old trees.

Solar flares are huge outbursts of solar material, which are several miles long. If we had some way of capturing all the energy emitted in one of the smallest solar flares, we would have enough energy to power the Earth for one million years.

Solar ProminenceA prominence is a huge loop of solar material extending from the sun. Again, these are several miles wide and tall.


The sun, a type G star, will die in about five billion years. The sun will have used up most of its hydrogen, and its core will have become denser than it now is. Because of this pressure, the sun's core will heat up to 100,000,000 K (180,000,000 °F), instead of its normal 10,000,000 K (18,000,000 °F). This will cause the sun to expand so much that it will envelop Mercury and Venus, and also vaporize Earth. The sun will be known as a red giant. Its color will have changed from yellow to orange-red.

The swelling is due to the heat: When any matter gets heated, it expands. The sun's intense heat will be enough to fuse the helium into larger atoms, and as long as its helium supply lasts, it will continue to shine - about 100,000,000 years.

During this time, however, the sun will begin to shrink, due to its lessening mass. When its fuel runs out, it will become a white dwarf - a star that shines because it is hot, not because it is producing energy through fusion.

When the sun runs out of heat, it will be a huge, black, chunk of carbon, floating in space. It will be called a black dwarf - a dead star.

Data for the Sun

Ratio (Sun/Earth)
Mass (1024 kg)
Volume (1012 km3)
Average Radius (km)
Average Density (kg/m3)
Visual Magnitude
Absolute Magnitude
Rotation Rate at Equator (hours)

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