Tài liệu tham khảo WikiPedia: Thiên_hà_vòng http://ned.ipac.caltech.edu/level5/Sept01/Appleton... http://antwrp.gsfc.nasa.gov/apod/ap100822.html http://www.eso.org/public/images/potw1619a/ http://www.spacetelescope.org/images/potw1310a/
