星系

例句：

1. 1. The Milky Way is a galaxy that contains billions of stars.
   银河系包含数十亿颗恒星。
2. 2. The discovery of new exoplanets has expanded our understanding of the universe.
   发现新的系外行星扩展了我们对宇宙的理解。
3. 3. The study of galaxies helps us understand the formation and evolution of the universe.
   研究星系有助于我们理解宇宙的形成和演变。