The Sun

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The night sky is full of stars, so distant they are mere points of light. The sun is a star that, by the gravitational effects of its mass, dominates the planetary system that includes the earth, and we are much closer to it than to any other star. Along with the other planets, the Earth orbits the Sun. The Sun is a ball of glowing gases, three quarters hydrogen and one quarter helium, with traces of other elements.

The Sun provides light and heat, which sustains life on the Earth. The sun furnishes directly or indirectly all of the energy supporting life on earth, because all foods and fuels are derived ultimately from plants using the energy of sunlight.

The sun is an intensely hot, self-luminous body of gases (mainly hydrogen, 70%, and helium, 20%) at the center of the solar system.

Its mean distance from the earth is about one astronomical unit (AU; an AU is defined as the distance a point mass would have to be from the Sun to have a sidereal orbit period equal to the tropical year). The sun is c.865,400 mi (1,392,000 km) in diameter; and its mass 332,000 times, that of the earth. It is the largest object in the solar system and contains approximately 98% of the total solar system mass. One hundred and nine Earths would be required to fit across the Sun's disk, and its volume is about 1,300,000 times that of the earth.

The sun is a medium-size main-sequence star -- average in size and brightness. The Sun is about 4.6 billion years old. Stars are born, live, and die. Our Sun is now in middle age, but in another 5 billion years it will have burned up all its hydrogen fuel. Then it will swell to 100 times bigger than it is now, becoming a red giant. After some millions more years, it will shrink and become a white dwarf, then it will cool down and stop shining forever :(

Structure of the Sun

Its temperature, pressure, and density increase towards the center. At its center, the sun has a density over 100 times that of water, a pressure of over 1 billion atmospheres, and a temperature of about 15,000,000 degrees Kelvin. This temperature is high enough for the occurrence of nuclear fusion reactions, the source of the sun's energy.

The core is about 200,000 mi (400,000km) across. Energy released from the core passes up through the radiative zone, which is nearly 200,000km (300,000km) thick, then passes through the 125,000mi (200,000km) thick convective zone, to the 300mi (500km) thick surface called the photosphere; its temperature is about 6000 degrees Kelvin. (11,000�F). This layer has a mottled appearance due to the turbulent eruptions of energy at the surface.

Above the photosphere is the chromosphere, which consists of hot rarified gases and extends for thousands of miles. Extending outwards from there is the corona a luminous envelope of extremely fine particles surrounding the sun, outside the chromosphere, which emits the solar wind. The solar wind and the Sun's magnetic field form the heliosphere, extending out to the boundaries of the solar system. During an eclipse of the sun, the chromosphere and the corona can be observed.