1. 河外星云：extra high vacuum 特高真空 | extragalactic nebula 河外星云 | extragalactic radio astronomy 河外射电天文学

2. 河外星云(星系)：河外背景光 extragalactic background light | 河外星云(星系) extragalactic nebula | 河外新星 extragalactic nova

3. 天河外星云：偏心望远镜 extracentral telescope | 天河外星云 extragalactic nebula | 外推法 extrapolation

noun

1. (astronomy) a collection of star systems
any of the billions of systems each having many stars and nebulae and dust

e.g. `extragalactic nebula' is a former name for `galaxy'

Synonym: galaxy