| {{U}} (21) {{/U}} summary of the physical and chemical {{U}}(22) {{/U}} of life must begin, not on the Earth, but {{U}}(23) {{/U}} the Sun; in fact, {{U}}(24) {{/U}} the Sun's very center. {{U}}(25) {{/U}} is here that is to be found the {{U}}(26) {{/U}} of the energy that the Sun constantly {{U}}(27) {{/U}} space {{U}}(28) {{/U}} light and heat. This energy is liberated at the center of the Sun as billions upon billions of nuclei hydrogen atoms {{U}}(29) {{/U}} each other and {{U}}(30) {{/U}} to form nuclei of helium, and, {{U}}(31) {{/U}} doing so, {{U}}(32) {{/U}} some of the energy that is stored in the nuclei of atoms. The output of light and heat of the Sun {{U}}(33) {{/U}} that some 600 million tons of hydrogen {{U}}(34) {{/U}} helium in the Sun every second. This the Sun {{U}}(35) {{/U}} for several thousands of millions of years. |