This open cluster is particularly ancient, estimated to be nearly one billion years old.
这个疏散星团尤其古老，据测算有十亿岁了。
NGC 2818 is one of very few planetary nebulae in our galaxy located within an open cluster.
NGC2818是银河系中少数几个位于疏散星团内部的行星状星云之一。
Hence, it is rare that an open cluster survives long enough for one of its members to form a planetary nebula.
因此，一个疏散星团很少能够存活到它内部的某个成员变成行星状星云。
It is called an open cluster because its stars are loosely bound and have started to spread out.
它被称为疏散星团，因为它的成员恒星结构松散而且还有恒星在扩散离开星团。
It is called an open cluster because its stars are loosely bound and have started to spread out.
它被称为疏散星团，因为它的成员恒星结构松散而且还有恒星在扩散离开星团。
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