When a star similar to the sun fuses most of its hydrogen into helium, then the helium into carbon, the star becomes unstable and puffs out into a red giant.

当一颗类日恒星大部分的氢聚变为氦，氦又聚变成碳的时候，它就变得不再稳定，而膨胀成一颗红巨星。

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Metallicity is a measure of the relative abundance in a star of elements other than hydrogen and helium.

金属丰度是对恒星内除氢和氦之外元素的相对丰度的度量(天文学中比氦重的元素都叫金属。——译者注)。

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The star used to be like the sun, a "main sequence" star that burns hydrogen by nuclear fusion, producing helium.

这颗恒星曾和太阳一样，是颗“主序星”(“main sequence”)。它通过核聚变燃烧氢，从而产生氦。

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One shows the remains of a dying star four thousand light years away which has thrown off a cloud of glowing hydrogen gas.

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Acting alone, gravity would cause stars to collapse completely, but as long as energy is produced at their centers by nuclear fusion (the joining of atomic nuclei to form new elements, as in a hydrogen bomb), the star is heated and puffed up.

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Red giants are created when a star exhausts its supply of hydrogen at its core, with the result that the inner layer contracts and the outer layers expand, forming a redder and much larger star.

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Red giant refers to a late phase in a star's life when it has begun to exhaust its hydrogen fuel.

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