Magrini, Laura; Stanghellini, Letizia; Villaver, Eva (May 2009). «The Planetary Nebula Population of M33 and its Metallicity Gradient: A Look Into the Galaxy's Distant Past». The Astrophysical Journal696 (1): 729-740. Bibcode:2009ApJ...696..729M. S2CID5502295. arXiv:0901.2273.
Skiff, Brian (10 de enero de 1997). «Messier 81 naked-eye». sci.astro.amateur. Archivado desde el original el 1 de marzo de 2021. Consultado el 11 de febrero de 2010.
Magrini, Laura; Stanghellini, Letizia; Villaver, Eva (May 2009). «The Planetary Nebula Population of M33 and its Metallicity Gradient: A Look Into the Galaxy's Distant Past». The Astrophysical Journal696 (1): 729-740. Bibcode:2009ApJ...696..729M. S2CID5502295. arXiv:0901.2273.
Karachentsev, I. D.; Karachentseva, V. E.; Hutchmeier, W. K.; Makarov, D. I. (2004). «A Catalog of Neighboring Galaxies». Astronomical Journal127 (4): 2031-2068. Bibcode:2004AJ....127.2031K.
Karachentsev, I. D.; Kashibadze, O. G. (2006). «Masses of the local group and of the M81 group estimated from distortions in the local velocity field». Astrophysics49 (1): 3-18. Bibcode:2006Ap.....49....3K. S2CID120973010.
Magrini, Laura; Stanghellini, Letizia; Villaver, Eva (May 2009). «The Planetary Nebula Population of M33 and its Metallicity Gradient: A Look Into the Galaxy's Distant Past». The Astrophysical Journal696 (1): 729-740. Bibcode:2009ApJ...696..729M. S2CID5502295. arXiv:0901.2273.
Karachentsev, I. D.; Kashibadze, O. G. (2006). «Masses of the local group and of the M81 group estimated from distortions in the local velocity field». Astrophysics49 (1): 3-18. Bibcode:2006Ap.....49....3K. S2CID120973010.
Skiff, Brian (10 de enero de 1997). «Messier 81 naked-eye». sci.astro.amateur. Archivado desde el original el 1 de marzo de 2021. Consultado el 11 de febrero de 2010.
«Objeto Messier 33». www.messier.seds.org. Archivado desde el original el 29 de junio de 2023. Consultado el 21 de mayo de 2021.