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 that is to be found the source of the energy that the Sun constantly pours out into space as light and heat. This energy is librated at the center of the Sun as billions upon billions of nuclei of hydrogen atoms collide with each other and fuse together to form nuclei of helium, and in doing so, release some of the energy that is stored in the nuclei of atoms. The output of light and heat of the Sun requires that some 600 million tons of hydrogen be converted into helium in the Sun every second. This the Sun has been doing for several thousands of millions of year.

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 shorter wavelength. This gamma radiation is absorbed by atoms inside the Sun to be reemitted at slightly longer 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 what we call the solar surface, and can escape into space without being absorbed further by solar atoms. A very small fraction of the Sun's light and heat is emitted in such directions that after passing unhindered through interplanetary space, it hits the Earth.

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作文地带知识拓展:

• 英语（English）属于印欧语系日耳曼语族西日耳曼语支，最早被中世纪的英国使用，并因其广阔的殖民地而成为世界使用面积最广的语言。英国人的祖先盎格鲁部落是后来迁移到大不列颠岛地区的日耳曼部落之一，称为英格兰。这两个名字都来自波罗地海半岛的Anglia。该语言与弗里斯兰语和下撒克森语密切相关，其词汇受到其他日耳曼语族语言的影响，尤其是北欧语（北日耳曼语），并在很大程度上由拉丁文和法文撰写。

  英语的英文释义：

  English 名词 uk  /ˈɪŋ.ɡlɪʃ/ us  /ˈɪŋ.ɡlɪʃ/

  the language that is spoken in the UK, the US, and in many other countries

  英语

  American/British English

  美式／英式英语

  Do you speak English?

  你会说英语吗？

  English   形容词 adjective   uk  /ˈɪŋ.ɡlɪʃ/ us  /ˈɪŋ.ɡlɪʃ/

  in or relating to the English language

  英语的

  an English teacher

  英语老师

  relating to or from England

  英格兰的;英国的

  English films/food/people

  英国电影／食品／人

  English law

  英格兰法律