When the hydrogen used as fuel vanishes, and fusion slows, gravity causes the star to collapse in on itself.

当作为燃料的氢消失，且聚变过程减缓时，重力就导致恒星向内坍缩。

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NASA calls this a gravity assist, and exploits it to save fuel in missions to outer planets such as Jupiter and Saturn.

美国航空航天局称这是一个重力辅助，并利用它来节省执行去木星和土星等外行星的任务时消耗的燃料。

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Lifting heavy spacecraft and fuel out of the Earth's gravity is expensive.

将沉重的航天器和燃料送出地球引力以外费用昂贵。

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The orientation also makes for a much lower center of gravity than a typical fuel-cell car.

CNN: Hydrogen-bombing down the track

Lifting heavy spacecraft and fuel out of the Earth's gravity is expensive.

CNN: Bush: 'It is time for America to take the next steps' in space

Scientists said they launched the satellite from midair to save money, compared with an expendable rocket launched from the ground, because it doesn't take as much fuel to break free of the planet's gravity.

WSJ: NASA NuStar Telescope to Hunt for Black Holes