另外，去年有其他天文学家报告称在一个更远的星系中发现了较大的金属丰度梯度，但那个星系不是螺旋星系，所以它与银河系的关系就没有那么明确了。
In addition, last year other astronomers reported a steep metallicity gradient in an even farther galaxy, but that galaxy is not a spiral, so its relevance to the Milky Way is less clear.
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。
Different theories predict how this metallicity gradient changes over billions of years.
如果天文学家能够观测到十亿光年外的螺旋星云的金属丰度梯度，我们就可以知道亿万年之前的梯度有多大，因此也就可以知道它是如何随时间变化的。
If astronomers could observe metallicity gradients in spiral galaxies billions of light-years away, we could see how steep the gradients were billions of years ago and thus how they change over time.
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