Comparing with the open cluster their stars are older.

和疏散星团相比，星一般较老。

Most of the V is formed by an open cluster of stars, called the Hyades.

这个V字型大致是由一串星星组成的，它们叫毕星云。

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。

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。