THE SUN The sun is of great importance to us. It is our main source of energy. Without it, no living thing can thrive our planet. Do you know that the sun is only one of the approximately 100 billion stars that make up the Milky Way Galaxy? How was the sun formed? Will the sun die What are the parts of the sun? What are its features? You will know the answers if you read through this chapter.

Evolution of the sun The evolution of the sun can be summarized in

the following phases.

Condensation Phase The sun like any other star was formed from

the condensation of interstellar gas and dust composed mainly of hydrogen. Shock waves or disturbances triggered the condensation of gas and dust to form a small gas cloud called Protostar. Once condensation starts, hydrogen atoms combine to form hydrogen molecules. This process further accelerates condensation because of the greater mass of the hydrogen molecules.

Contraction Phase The protostar falls to its center because

of its own gravitational pull. It has been estimated that a protostar collapses in about 10 million years. A protostar rotates with a velocity that becomes greater as the body becomes smaller.

As the protostar collapses, its potential energy is converted into heat and light energies. When the surface temperature reaches 2,000 to 3,000 K, the temperature at its center is as high as 100,000 K.

Convection Phase Because of the great difference in

temperature between the central and outer parts of the protostar, the interior becomes unstable. Convection occurs. In this process, the hotter inner part is transported outward and the cooler outer part moves into the inner region. Contraction continues. During this this stage, luminosity decreases.

Radiative Phase After thousands of years (for a high-

mass star) or millions of years (for a low-mass star), convection stops at the center. Instead, energy is transported outward from the center by radiation. This stage is called Radiative Phase. It occurs rapidly at a particular time. During this period, luminosity stays nearly the same. However, the temperature of the sun increases.

Nuclear Fusion Phase Finally when the temperature

becomes high, nuclear reaction occurs. The type of reaction depends upon the temperature at the center. At temperatures between 5 and 15 million degrees, hydrogen nuclei fuse to form helium. The nuclear fusion is called proton-proton cycle. The sun follows these reactions.

Stable Main Sequence Phase In this phase, the inward pressure balances the

outward pressure. The inward pressure is due to gravitation. The outward pressure is due to high gas and radiation pressure which is caused by the exceedingly high temperature in the center. At this stage, contraction stops and the star becomes stable and constant in size.

A star spends most of its lifetime at this stage. Stars of different masses existing in this condition are referred to as stable main-sequence stars burning hydrogen to helium. Hot stars give a blue luminous color while cool stars give a faint red color. Since sun is intermediate in temperature, it gives a yellow color. A yellow star remains a main-sequence star for about 10 billion years.

Death of the sun Red Giant Stage A star like our sun becomes a red

giant at its last phase of its life. Hydrogen fusion reactions become less efficient in the core region. With the gravitational collapse of the core, the fusion occurs in the shell surrounding the core. This greatly increases the luminosity of the star and it expands. The outer layer then cools off and becomes a red giant.

White Dwarf Stage A star as massive as the sun or

smaller will slowly shrink to a size as big as Earth called a white dwarf. At this stage, the star becomes cooler and dimmer until it dies as a Black Dwarf. Cooling of a White dwarf is very slow. It takes around 10 billion years to cool down to a temperature 3,000 K. There are many black dwarfs scattered in the universe.