南门二（α Cen、半人马座α）位于天空南方的半人马座，英文名Alpha Centauri或Toliman，虽然肉眼分辨不出来，不过南门二实际上是一个三合星系统，其中一颗恒星是全天空第4明亮的恒星。不过因为其中两颗恒星距离过近，肉眼无法分辨出来，所以它们的综合视星等为-0.27等（超过第3亮的大角星），绝对星等为4.4等。南门二也作为南十字星座最外围的指引而闻名，因为南十字星座的位置太过南边，所以大部分的北半球都看不到。传闻当年郑和下西洋，就是用它来指引方向。

南门二是距离太阳最近的恒星系，只有4.37光年（约277,600天文单位）。比邻星（Proxima Centauri）通常被认为是这个恒星系的成员，距离太阳只有4.24光年。因为南门二距离地球相对较近，所以在关于星际旅行的冒险小说中，理所当然将它当成「第一个停靠港口」，并预测在人**炸时甚至会对这个恒星系进行开发与殖民活动。这些观点通常也在科幻小说与电子游戏中出现。

Alpha Centauri (α Cen), also known as Rigil Kent (/ˈraɪdʒəl ˈkɛnt/) (the "Centaur's Foot") or Toliman, is the closest star system to the Solar System at 4.37 ly (1.34 pc). It consists of three stars: the pair Alpha Centauri A and Alpha Centauri B and a small and faint red dwarf, Alpha Centauri C, better known as Proxima Centauri, that may be gravitationally bound to the other two. (Beta Centauri, or β Centauri, should not be confused with Alpha Centauri B, and is a separate, trinary, system of its own.) To the unaided eye, the two main components appear as a single object of an apparent visual magnitude of −0.27, forming the brightest star in the southern constellation Centaurus and the third-brightest star in the night sky, outshone only by Sirius and Canopus.