填空题. A summary of the physical and chemical nature of life must begin, not on the Earth, but in the Sun; in fact, at the Sun's very center. It is here where is to be found the source of the energy that 1 the Sun constantly pours out into the space as light and heat. This 2 energy is liberated at the center of the Sun as billions on billions of 3 nuclei of hydrogen atoms collide with each other and fuse together to form nuclei of helium, and, in doing so, releases some of the energy 4 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 are 5 converted into helium in the Sun every second. This the Sun has been done for several thousands of millions of year. 6 The nuclear energy is released at the Sun's center as high- energy gamma radiation, a form of electromagnetic radiation like light and radio waves, only of very much short wavelength. This 7 gamma radiation is absorbed by atoms inside the Sun, to be reemitted at slight longer wavelengths. This radiation, in its turn, is 8 absorbed and remitted. As the energy filters through the layers of the solar interior, it passes through the x-ray part of the spectrum, and eventually becoming light. At this stage, it has reached that we 9 call the solar surface, and can escape into space, without being absorbed further by solar atoms. A very small fraction of the Sun's light and heat emitted in such directions that, after passing 10 unhindered through interplanetary space, it hits the Earth.