A star is born when gravity forces a huge volume of mostly hydrogen gas to collapse in on itself.

一颗恒星的诞生是由于重力迫使大量的氢气坍缩。

The gravity force pulls down the runoff from the highest point to the lowest point.

重力将径流从最高点拉到最低点。

The Moon holds that tilt steady. The various gravity forces that are exerted tend to act in a way that holds that steady.

月球一直保持着一个倾角。在万有引力作用下始终保持在一个固定倾角。