我们现在的氦源，是地下天然气田中放射元素衰变的副产品，经过数十亿年的累积，逐渐形成的。
Our modern supply of helium has gradually built up over billions of years in subterranean pockets of natural gas as a byproduct of decaying radioactive elements.
研究人员推测数这些碎块经过数十亿年的互相碰撞，柯伊伯型天体磨得更碎，柯伊伯带原始状态不再。
Researchers surmise that over billions of years the debris should collide, grinding the KBO-type objects down to ever smaller pieces that were not part of the original Kuiper Belt population.
这些星系的大小仅相当于银河系的一角，它们经过数十亿年的演化，终成为今天我们看见的成熟的大星系。
The galaxies are a fraction the size of our Milky Way, and so may have evolved over billions of years into the full-grown galaxies seen around us today.
在经过数十亿年之后，我们所处的银河系将一头撞进的邻近的星系仙女座，这样的结局也许遥远的外星人会有幸见到。
In a few billion years our own Milky Way will smash into our neighbor galaxy, Andromeda, and perhaps it will look something like this to the aliens watching from afar.
在经过数十亿年之后，我们所处的银河系将一头撞进的邻近的星系仙女座，这样的结局也许遥远的外星人会有幸见到。
In a few billion years our own Milky Way will smash into our neighbor galaxy, Andromeda, and perhaps it will look something like this to the aliens watching from afar.
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