A study of the physical and chemical (1) of life must begin, not on the Earth, (2) in the Sun: in fact, at the Sun’s (3) center. It is here that is to be found the (4) of the energy that the Sun constantly pours out into space (5) light and heat. This energy is (6) at the center of the Sun as billions upon billions of nuclei of hydrogen atoms (7) with each other and fuse together to form nuclei of helium, and, in doing so, release some of the (8) that is stored in the nuclei of the atoms. The output of light and heat of the Sun (9) some 600 million tons of hydrogen to be (10) into helium in the Sun every second. The Sun has been doing (11) 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 (12) like light and radio waves, only (13) a much shorter wave length. This gamma radiation is absorbed by atoms inside the Sun, to be remitted (14) slightly longer wave lengths. This radiation, in its (15) , is absorbed and remitted. As the energy filters through the layers of the solar interior, it passes through the X-ray part of the spectrum, (16) becoming light. At this (17) , it has reached what we call the solar surface, and can escape into space without being absorbed (18) by the solar atoms. A very small (19) of the Sun’s light and heat is emitted in such directions that, after passing a long distance through interplanetary space, it (20) the Earth.