银河系位于一个较小的星系团——本星系群——之内。在本星系群中，银河系和仙女星系可算是两个质量最大的成员星系。长期以来，人们普遍认为仙女星系是本星系群中的“老大”，银河系只是“老二”，科普文章、甚至基础性专业教科书中都会这么说。然而，实际情况远不是那么简单。

Which is the most massive member in the Local Group，the Milky Way galaxy or Andromeda galaxy? The dynamical mass of the Milky Way galaxy or Andromeda galaxy can be estimated in different ways. Up to now, however, the unanimity has not yet been reached for the above problem.