填空题

A (36) 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 (37) 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 (38) atoms collide with each other and fuse together to form nuclei of helium (氦). And, in doing so, it (39) 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 (40) 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 (41) , a form of electro-magnetic radiation like light and radio waves, only of very much shorter (42) . This gamma radiation is (43) by atoms inside the Sun, to be re-emitted at slightly longer wavelengths. This radiation, in its turn, is absorbed and reemitted. (44) , it passes through the X-ray part of the spectrum (频谱), eventually becoming light. At this stage, it has reached what we call the solar surface and can escape into space, (45) . A very small fraction of the Sun’s light and heat is emitted in such directions (46) .