According to many online sources "star" is either "citlali" or "citlalin" in Nahuatl. It is currently used as a name, and is sometimes spelled "Citlalli." I'm seeing "citlalin" more often than "citlali" - "citlali" seems to be on baby name sites mostly. I also found at one source that there is a hill named "Uixachtecatl" ("Star Hill"). This bears no resemblance to "citlali(n)" as was found in numerous other sources, so I'm not sure what to do with that, but I thought I'd throw it out there.

That's the easy part. The hard part has been to find a simple depiction of a star illustration or example in a dictionary, and to my knowledge it doesn't exist on the internet. However, I managed to come across a few sample illustrations in which a star is present, and you're welcome to browse those - though for the life of me in some of them I can't tell where the star should be.

A depiction of a star in front of a priest can be seen in "The Essential Codex Mendoza, Volume 2; Volume 4" (by Frances Berdan, Patricia Rieff Anawalt) on page 180: http://books.google.com/books?id=JQeAQZHev0IC&pg=PA182&lpg=PA182&dq=codex+Magliabechiano+star&source=bl&ots=XZtXdK9mwg&sig=hP7hAG_Avux_ehqh5_GdYwOj0BE&hl=en&ei=3wdyS-uvOoKTnQeO8OWKCw&sa=X&oi=book_result&ct=result&resnum=2&ved=0CAkQ6AEwAQ#v=onepage&q=star&f=false

Individual stars also seem to have own deities and illustrations, as can be seen on page 106 in "Codex Fejérváry-Mayer: An Old Mexican Picture Manuscript in the Liverpool Free Public Museums" (by Joseph Florimond Loubat, Augustus Henry Keane): http://books.google.com/books?id=d149AAAAMAAJ&pg=PA151&dq=codex+star&lr=&as_brr=4&cd=2#v=snippet&q=star&f=false

There are also a few codices out there that illustrate certain rituals or cultural phenomena of the Aztec cultures (either drawn by the Aztecs themselves, or by visitors). One such codex is the Codex Borgia, and on page 116 of "Tlacuilolli: Style And Contents Of The Mexican Pictorial Manuscripts With A Catalog Of The Borgia Group" (by Karl Anton Nowotny, George A. Everett, Edward B. Sisson) you can find a "translated" illustration that outlines the presence of a star underneath (or perhaps part of) the Quetzal Flower Tree in the picture below: http://books.google.com/books?id=Y5xFXuYfWaYC&pg=PA57&dq=Codex+Borbonicus&lr=&cd=11#v=onepage&q=star&f=false
http://www.famsi.org/research/graz/borgia/page51.jpg

Page 142 of the same book outlines an illustration showing Tlahuizcalpantecuhtli with a star under the left arm as seen below in a page of the Codex Vaticanus B. In the right hand he is holding an atlatl, which according to Wikipedia is "a tool that uses leverage to achieve greater velocity in dart-throwing, and includes a bearing surface which allows the user to temporarily store energy during the throw."
http://www.famsi.org/research/loubat/Vaticanus%203773/page_82.jpg

I was hoping to find you something simple like this (which is, by the way, the Aztec glyph for the Wind God):
http://www.mexicolore.co.uk/uploadimages/381_02_2.jpg

... but apparently "star" is too rare among small research essays in Google Books, or on Aztec astrology websites. I think to find the proper glyph would take some serious research in a university library. But I hope I've provided you with some insight as to the complexity and beauty that is the Nahuatl writing system. :)