2. Energizing the Sun

A hot, dense core

Fig. .. 

Whole atoms are only found in the Sunís relatively cool, visible layers and they do not exist in most of the Sun. The solar atoms are stripped bare and lose their identity inside the Sun. The temperature is so high, and the particles are moving so fast, that innumerable collisions tear the atoms apart into their sub-atomic ingredients.

The most abundant atom in the Sun is hydrogen. Each hydrogen atom is made up of a nucleus that contains a positively charged proton, and a remote, negatively charged electron that orbits the nucleus. In the Sun, collisions rip the hydrogen atoms apart, and separate the electron from the nucleus. Both the electron and the proton are liberated from their atomic bonds, and are set free to move and wander throughout the solar interior unattached to each other.

At great depths inside the Sun, the pressure of overlying material is enormous, the protons are squeezed tightly together, and the material is very hot and extremely dense. To understand how this works, imagine a hundred mattresses stacked into a pile. The mattresses at the bottom must support those above, so they will be squeezed thin. Those at the top have little weight to carry, and they retain their original thickness. The gas at the center of the Sun is similarly squeezed into a smaller volume by the overlying material, so it becomes hotter and more densely concentrated.

Since we cannot see inside the Sun, or any other star, astronomers use mathematical models to determine the internal structure of stars. The crucial equations can be solved without any knowledge of properties of the star before arrival on the main sequence. They describe nuclear energy generation by hydrogen burning in the central core of the star, hydrostatic equilibrium that balances the outward force of gas pressure and the inward force of gravity, energy transport by radiative diffusion, and an opacity determined from atomic physics calculations.

These equations are integrated over 4.6 billion years, the age of the Sun, to obtain the present luminosity of the Sun, for a star of its mass and radius. This results in a Standard Solar Model that specifies the current mass density, temperature and pressure as a function of depth within the Sun (Fig. 3.1).

The model indicates just how hot and dense it is down at the core of the Sun (Fig. 3.1). At the Sunís center the temperature is 15.6 million degrees. The central density is 151,300 kilograms per cubic meter, or more than 13 times that of solid lead. Yet, the protons are so small that they can move about freely as a gas, even at this high density.

The visible solar disk is relatively cool, at 5780 degrees Kelvin, and extremely rarefied, about ten thousand times less dense than the air we breathe. The pressure of this tenuous gas is less than that beneath the foot of a spider. Just as the pressure on your body increases as you dive deeper and deeper in the sea, so does the pressure increase with greater depth into the Sun. The pressure at the center of the Sun is 233 billion times Earthís air pressure at sea level (Fig. 3.1); all that outward gas pressure is needed to support the mass of the Sun.

When the Sun arrived on the main sequence, at zero age, it was assumed to have a homogeneous chemical composition, with a hydrogen abundance of about 71 percent by mass and helium at about 27 percent by mass. Nuclear fusion reactions convert hydrogen into helium within the Sunís hot, dense core, providing the Sunís energy and explaining its luminosity. The amount of helium in the core of the Sun has therefore increased over the past 4.6 billion years, due to the ongoing synthesis of helium from hydrogen, and the amount of hydrogen in the core has decreased (Fig 3.1).

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Copyright 2010, Professor Kenneth R. Lang, Tufts University