On the other hand, a comparatively warm body with a small surface gravity, like Earth's moon, is unable to hold even the heaviest gas and thus lacks an atmosphere.

另一方面，像地球的卫星这种相对温暖、表面引力较小的天体，它们连最重的气体也无法容纳，因此也没有大气层。

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Most astronomers believe that the large concentration of mass at the galaxy's center is a black hole whose gravity is causing the gas to whirl.

大多数天文学家认为，星系中心的大量物质是一个黑洞，它的引力导致气体旋转。

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When the disk begins to coalesce into protoplanets, the gravity of those protoplanets pulls clumps of gas around the disk with them as they orbit the star.

当这个盘状物开始合并成为一组原行星时，它们的引力拉动着盘状物周围气体所凝聚的硬块，一起环绕着恒星作轨道运行。

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Stars, such as our sun, are giant balls of gas held together by gravity.

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The first stars formed when clouds of gas bunched together due not only to the pull of their own gravity, but also that of dark matter.

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The cosmos was born in the Big Bang about 13.7 billion years ago, and the first stars in the universe are thought to have lit up about 100 million years afterward, when gas finally gathered in clumps dense enough to collapse under their own gravity and ignite nuclear fusion.

MSN: We could soon detect the first stars in our universe