Our Sun is an ordinary star, just one of the 200 billion stars in the Milky Way Galaxy. It lies 28,000 light years from the center of the Galaxy, in one of the spiral arms.
Although it is only a medium-sized star, the Sun is enormous compared to the planets in the Solar System. Its diameter is more than a hundred times the Earth's diameter.
The Sun's mass is about 2,000,000,000,000,000,000,000 tons: that is about 99 percent of the mass of the whole Solar System. About three quarters of it is hydrogen and the rest mostly helium.
In the core of the Sun, the weight of all the gas pushing down makes it hot and dense enough for hydrogen to be turned into helium by a process called nuclear fusion. Some of the hydrogen is converted entirely ito energy. The temperature here is around 59 million degrees F.
Energy gradually flows outwards towards the surface of the Sun. In some layers, currents of hot gas rise upwards. They share their heat with the cooler gas above and then sink down again. This process is called convection.
The yellow surface of the Sun we see is called the photosphere. Its temperature is about 10,000�F. The Sun looks yellow because of its particular temperature: cooler stars are red and hotter stars white.
Many dramatic events take place on the surface of the Sun. Together these are called 'solar activity'. Solar activity increases and decreases over a cycle lasting about 11 years.
Sunspots are one form of solar activity. These dark blotches can last several months but most disappear in 10 days or less. Sunspots normally occur in groups; individual spots can measure thousands of miles across. Sunspots look darker only because they are 1,500 degrees cooler than the rest of the photosphere. Seen alone against a dark sky, a sunspot would be as bright as the full Moon. The cooling effect is caused by the strong magnetic field in a sunspot.
Solar flares are another kind of solar acitivity. These often occur near sunspots. In a flare, a huge amount of energy is released in just a few minutes and temperatures can go up to more than 200 million� F. Flares produce bursts of X-rays, radio waves and clouds of atomic particles that stream into space. When these particles enter the Earth's atmosphere they cause bright lights in the sky called the northern and southern lights (the aurora) sometimes seen in the night sky, usually in the polar regions. They can also upset the Earth's own magnetism and affect long-distance radio communications. The atmosphere protects us from the harmful effects of solar X-rays but, when astronauts were visiting the Moon, a careful watch was kept for flares. Astronauts have to return to their spacecraft for protection as the Moon has no atmosphere.
Solar Eclipses
Although the Sun is 400 times larger than the Moon, it appears the same size in the sky because it is 400 times further away.
When the Moon lies directly between the Earth and the Sun, it blocks the Sun's light and, from some parts of the Earth, a total solar eclipse is visible. During a total eclipse you cannot see the Sun and everything is dark.
A partial eclipse occurs when the Moon does not quite cross directly in front of the Sun. Part of the Sun is visible so there is still some light.
THe distances between the Earth, Moon, and Sun vary to some extent because the Moon's orbit around the Earth, and the Earth's orbit around the Sun are elliptical, and not perfect circles. This is called annular eclipse from the Latin word annulus, meaning 'ring'.
During a total eclipse of the Sun, the Moon blocks out blinding light of the Sun's surface and the faint, outer parts of the Sun become visible. The solar corona appears as a milky white halo around the Sun during total eclipse. The corona extends outwards for many times the Sun's diameter. It contains particles at temperatures of millions of degrees.
The Sun's Future
When will the Sun exhaust its supply? Scientists estimate that the Sun's total lifetime is about 10 billion years. About half of this has gone by already. When the supply of hydrogen in its core does begin to run out, the Sun will gradually expand to be about a hundred times bigger than it is now. It will engulf the Earth in the process. The Sun's surface temperature will fall and the color will change to red. The Sun will become a type of star known as a Red Giant. Ultimately, this red giant will shrink again to become a white dwarf, about the size of the Earth.
Did you know?... Every second, the Sun uses up 400 million tons of hydrogen completely and changes it into energy.