Gravitational Confinement

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Gravitational Confinement
The hydrogen in the Sun’s core is compressed to very high density, roughly 10 times denser than lead. But the Sun’s core is not solid—it is kept in an ionized, or plasma, state by the high temperature. This combination of high density and high temperature exerts an enormous outward pressure that is about400 billion (4  1011) times larger than the atmospheric pressure at the Earth’s surface.
An inward force must balance this enormous outward pressure in order to prevent the Sun from expanding. Gravity provides this force in the Sun and stars, and it compresses the Sun into the most compact shape possible, a sphere. At each layer inside the sphere there has to be a balance between the outward pressure and the weight of the material above (outside) pressing downward (inward).  The balance between compression due to gravity and outward pressure is called hydrostatic equilibrium. The same effect occurs in the Earth’s atmosphere: The atmospheric pressure at sea level is due to the weight of the air above—this is the combined gravitational force acting on the air molecules. The atmosphere does not collapse to a very thin layer on the ground under the pull of
gravity because the upward pressure of the compressed gas in the lower layers always balances the downward pressure of the upper layers.

 FIGURE 3.3 The main features of the Sun. Energy is released by thermonuclear reactions in the
core and transported outward, first by radiation and then by convection, to the surface, from where
it is radiated.


In some ways the structure of the Sun is similar to that of the Earth, in the sense that it has a very dense core that contains most of the Sun’s mass surrounded by less dense outer layers known as the solar envelope (Figure 3.3). The temperature in the core is about 14 million degrees Celsius but falls quite rapidly to about 8 million degrees Celsius at a quarter of the radius and to less than 4 million degrees Celsius at half the radius. Fusion reactions are very sensitive to temperature and density and take place only in the core. The fusion power density falls to 20% of its central value at 10% of the radius and to zero outside 20% of the radius. Fusion energy is transported outward from the core as heat, first by radiation through the layers known as the radiative zone. But as the radiative zone cools
with increasing distance from the core, it becomes more opaque and radiation becomes less efficient. Energy then begins to move by convection through huge cells of circulating gas several hundred kilometers in diameter in the convective zone.

Finally the energy arrives at the zone that emits the sunlight that we see, the photosphere. This is a comparatively thin layer, only a few hundred kilometers thick, of low-pressure gases with a temperature of 6000°C. The composition, temperature, and pressure of the photosphere are revealed by the spectrum of sunlight. In fact, helium was discovered in 1896 by William Ramsey, who
found features in the solar spectrum that did not belong to any gas known on Earth at that time. The newly discovered element was named helium in honor of Helios, the mythological Greek god of the Sun. Gravity is a very weak force compared to the forces of nuclear physics, and it can confine a hot plasma only when the mass is very large. This is possible in the Sun and stars but not for the much smaller plasmas that we would like to confine on Earth. Also, the fusion power density in the core of the Sun is very low, only 270 watts per cubic meter, compared to the megawatts per cubic meter required for a commercial power plant. Other methods of providing confinement have to be found, as is discussed in later chapters. Advertisement