英语语法分析

Three generations of stars may exist based on metallicity.

以金属度划分，恒星有三个世代。

One clue comes from the metallicity of the disc's constituent stars.

一条线索来自于主盘内恒星的金属丰度。

Different theories predict how this metallicity gradient changes over billions of years.

已有几种不同的理论对金属丰度梯度在亿万年间如何变化作了推测。