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 the Sun 【M1】_________

constantly pours out into the space as light and heat. This energy is 【M2】_________

liberated at the center of the Sun as billions on billions of nuclei of 【M3】_________

hydrogen atoms collide with each other and fuse together to form nuclei

of helium, and, in doing so, releases some of the energy that is stored in 【M4】_________

the nuclei of atoms. The output of light and heat of the Sun requires that

some 600 million tons of hydrogen are converted into helium in the Sun 【M5】_________

every second. This the Sun has been done for several thousands of 【M6】_________

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 【M7】_________

is absorbed by atoms inside the Sun, to be reemitted at slight longer 【M8】_________

wavelengths. This radiation, in its turn, 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, eventually becoming light. At

this stage, it has reached that we call the solar surface, and can escape 【M9】_________

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 【M10】________

passing unhindered through interplanetary space, it hits the Earth.

【M1】