| 单词 | sun |
| 释义 | sun The star at the centre of the solar system. A typical main-sequence dwarf star (see Hertzsprung-Russell diagram; stellar evolution), the sun is some 149 600 000 km from earth. It has a diameter of about 1 392 000 km and a mass of 1.9 × 1030 kg. Hydrogen and helium are the primary constituents (about 75% hydrogen, 25% helium), with less than 1% of heavier elements. In the central core, some 400 000 km in diameter, hydrogen is converted into helium by thermonuclear reactions, which generate vast quantities of energy. This energy is radiated into space and provides the earth with all the light and heat necessary to have created and maintained life (see solar constant). The surface of the sun, the photosphere, forms the boundary between its opaque interior and its transparent atmosphere. It is here that sunspots occur. Above the photosphere is the chromosphere and above this the corona, which extends tenuously into interplanetary space. See also solar wind |
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