The mass of a helium atom is less than the combined mass of two hydrogen atoms.

一个氦原子的质量会小于两个氢原子的总质量。

A helium nucleus is a tiny bit lighter than the combined mass of the 4 protons it is made from.

氦原子核虽然由四个质子形成但重量却比四个质子轻一点。

If their combined mass is more than 2.8 times that of the Sun they'll collapse to form a black hole.

如果它们的总质量超过太阳的2.8倍，它们就会坍缩形成黑洞。