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 which is to be found the source of the energy that the Sun constantly pours 21 out into space light and heat. This energy is librated at the center of 22 the Sun as billions upon billions of nuclei of hydrogen atoms collide with each other and fuse together to form nuclei of helium, and doing 23 so, release out some of the energy that is stored in the nuclei of atoms. 24 The output of light and heat of the Sun requires that some 600 million tons of hydrogen are converted into helium in the Sun every second. 25 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 radiation, a form of electromagnetic radiation like light and radio waves, only of very much short wavelength. This gamma radiation 26 is absorbed by atoms inside the Sun to be reemitted at slight longer 27 wavelengths. This radiation, in its turn is absorbed and reemitted. As the energy filters through the layers of the solar interior, it passes through the X-ray part of the spectrum eventually becoming light. At this stage, it has reached that we call the solar surface, and can escape into space 28 without being absorbed further by solar atoms. A very small fraction of the Sun's light and heat are emitted in such directions that after 29 passing unhindering through interplanetary space, it hits the Earth. 30