[填空题]
A {{U}} (36) {{/U}} of the physical and chemical nature
of life must begin, not on the Earth, but in the Sun. In fact, it is at the
Sun’s very center. It is here that is to be found the source of the energy that
the Sun {{U}} (37) {{/U}} pours out into space as light and heat. This
energy is liberated at the center of the Sun as billions upon billions of nuclei
of hydrogen atoms {{U}} (38) {{/U}} with each other and fuse together to
form nuclei of helium (氦). And, in doing so, it {{U}} (39) {{/U}} some of
the energy that is stored in the nuclei of atoms. The output of light and heat
of the Sun requires that some 600 million tons of hydrogen be {{U}} (40)
{{/U}} into helium in the Sun every second. This the Sun has been doing for
several thousands of millions of years.
The nuclear energy is
released at the Sun’s center as high-energy gamma {{U}} (41) {{/U}}, a
form of electro-magnetic radiation like light and radio waves, only of ver