old nebula的意思|示意

Since open star clusters disperse after only a few hundred million years, this one must be exceptionally old to have one of its member stars evolve to the planetary nebula stage.

因为在仅仅数百万年之后，疏散星团开始消散，可能因为这个星团异常的年老，其内的一颗恒星已经演变到了行星状星云阶段。

The Crab pulsar, the best-known example of a bright pulsar wind nebula, is about 950 years old.

蟹状星云脉冲星是脉冲星风星云中最为著名的一个，但它已经950岁了。

Also known as Abell 21, this Medusa is an old planetary nebula some 1, 500 light-years away in the constellation Gemini.

也被称作阿贝尔21，水母星云是双子座中一个1500光年远的年老行星状星云。