van Maanen, A. (1916). “Preliminary evidence of internal motion in the spiral nebula Messier 101.”. 《The Astrophysical Journal》 44: 210. doi:10.1086/142287. ISSN0004-637X.
van Maanen, A. (1916). “Preliminary evidence of internal motion in the spiral nebula Messier 101.”. 《The Astrophysical Journal》 44: 210. doi:10.1086/142287. ISSN0004-637X.