Seite - 99 - in Water, Energy, and Environment - A Primer

Energyfromourmedium-sizedstar isabundantandrenewable, and is theprincipal factor thathasenabledandshaped lifeonour planet.OurSunbelongs to the classofdwarf yellowstarswhose members aremore numerous than those of any other class. The energy radiated into space by the Sun is fueled by a fusion reaction in the Sun’s central core where the temperature is estimated to be about 10 million degrees Celsius. At this temperature the corresponding motion of matter is so violent that all atoms andmolecules are reduced to fast-moving atomic nuclei and stripped electrons, collectively known as a plasma. The nuclei collide frequently and energetically, producing fusion reactions of the type that occur in thermonuclear explosions. While about two-thirds of the elements found on Earth have been shown tobepresent in theSun, themost abundant element is hydrogen, constituting about 80% of the Sun’s mass – approximately 2 trillion trillion million kilograms. When hydrogen nuclei collide in the Sun’s core they fuse and create helium; roughly 20% of the Sun’s mass is in the form of helium. Also created in each fusion reaction are two neutrinos, high-energy particles with no net electrical charge that escape into outer space, andhigh-energygamma radiation that interacts strongly with the matter that surrounds the Sun’s core. As this radiation streams outward from the core it collides with and transfers energy to nuclei and electrons, heating the mass of the Sun so that it achieves a surface temperature of several thousand degrees Celsius. The energy distribution of the radiation emitted by this surface is fairly close to that of a classical ‘black body’ (i.e., a perfect emitter of radiation) at a temperature of 5500°C,withmuch of the energy radiated in the visible portion of the electromagnetic spectrum. Energy is also emitted in the infrared, ultraviolet, and x-ray portions of the spectrum (seeFigure 8.1). The Sun radiates energy uniformly in all directions, and at a distance of 93 million miles the Earth’s disc intercepts only 4 Renewableenergy 99