{cite book}}: Invalid |ref=harv (help)Bertin, G.; Lin, C.-C. (1996). Spiral Structure in Galaxies: a Density Wave Theory. MIT Press. ISBN 0-262-02396-2. {{cite book}}: Invalid |ref=harv (help)Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton University Press. ISBN 0-691-00402-1. OCLC 39108765.Dickinson, T. (2004). The Universe and Beyond (4th ed.). Firefly Books. ISBN 1-55297-901-6. OCLC 55596414.Heidarzadeh, T. (2008). A History of Physical Theories of Comets, from Aristotle to Whipple. Springer. ISBN 1-4020-8322-X. {{cite book}}: Invalid |ref=harv (help)Kepple, G. R.; Sanner, G. W. (1998). The Night Sky Observer's Guide, Volume 1. Willmann-Bell. ISBN 0-943396-58-1. {{cite book}}: Invalid |ref=harv (help)Merritt, D. (2013). Dynamics and Evolution of Galactic Nuclei. Princeton University Press. ISBN 9781400846122. {{cite book}}: Invalid |ref=harv (help)Mohamed, M. (2000). Great Muslim Mathematicians. Penerbit UTM. ISBN 983-52-0157-9. OCLC 48759017. {{cite book}}: 