在几百亿年甚至几千亿年之后，白矮星最终冷却形成黑矮星，后者不再散发任何能量。
Eventually—over tens or even hundreds of billions of years—a white dwarf cools until it becomes a black dwarf, which emits no energy.
宇宙中这些最古老的光子，被认为在宇宙大爆炸之后已存在大约38万年，后者最终冷却以便开始形成原子。
These oldest photons in the universe are thought to have been generated about 380,000 years after the Big Bang, when matter was finally cool enough to start forming atoms.
最终的结果是一股持续的气体流，开始是星系间的热气体，结束时是一股被称为“冷却流”的少量冷气体，落入星系中央。
The net result is a continuous flow of gas, starting as hot gases in intergalactic space and ending as a drizzle of cool gas called a "cooling flow," falling into the central galaxy.
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