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 41. ______ out into space light and heat. This energy is librated at the center of 42. ______ 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 43. ______ so, release out some of the energy that is stored in the nuclei of atoms. 44. ______ 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. 45. ______ 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 46. ______ is absorbed by atoms inside the Sun to be reemitted at slight longer 47. ______ 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 48. ______ 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 49. ______ passing unhindering through interplanetary space, it hits the Earth. 50. ______