Most of the energy generated in the protostar stage comes from the gravitational energy of the cloud , just like a ball will drop with increasing speed from your hand if you release it 在原恒星阶段，星体的能量主要来自星际云引力塌缩时所释放的位能，犹如树上的苹果会向下掉一样。
One day a visitor from the city came to a small rural area to drive around the country roads , see how the farms looked , and perhaps to see how farmers earned their living . the city man saw a farmer in his yard holding a pig up in his hands , andlifting it so that pig could eat apples from an apple tree . the city man said to the farmer , " i see that your pig likes apples , butisn ' t that quite a waste of time ? 一天,有一个城市里的游客来到一个小乡村,在乡间路上开着车,想看看农庄是什么样子,也想看看农夫怎样种田过日子.这为城里人看见一位农夫在宅后的草地上,手中抱着一头猪,并把它举得高高的,好让它能够吃到树上的苹果.城里人对农夫说: "我看你的猪挺喜欢吃苹果的,但是,这不是很浪费时间吗?
