慧核本身太小，它的引力不足以束缚住这些气体。

The comet nucleus itself is too small to hold on to these gases through gravitation.

youdao

与此同时，还要设法判断慧核（最明亮的部分）的亮度，观察一下慧尾有多远。

At the same time try to judge the brigheness of its nucleus(brightest part), and see how far you can trace its tail .

youdao