星系天文学是天文学的一个分支，研究的对象是我们的银河系以外的星系（研究所有不属于银河系天文学的天体），又称河外天文学。

当工作的仪器获得改善，就可以更详细的研究现在只能审视的遥远天体，因此这个分支可以再细分为更有效的近银河系外天文学和远银河系外天文学。前者的成员与对象包括星系、本星系群，距离近得可以详细研究内部的超新星遗迹、星协。后者远得只是可以测量的对象和只有最明亮的部份可以描述或研究。

一些相关的主题如下：

Extragalactic astronomy is the branch of astronomy concerned with objects outside the Milky Way galaxy. In other words, it is the study of all astronomical objects which are not covered by galactic astronomy, the next level of galactic astronomy.

As instrumentation has improved, more distant objects can now be examined in detail. It is therefore useful to sub-divide this branch into Near-Extragalactic Astronomy and Far-Extragalactic Astronomy. The former deals with objects such as the galaxies of the Local Group, which are close enough to allow very detailed analyses of their contents (e.g. supernova remnants, stellar associations). The latter describes the study of objects sufficiently far away that only the brightest phenomena are observable.