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 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. ______