Tài liệu tham khảo WikiPedia: NGC_6426 http://cseligman.com/text/atlas/ngc64.htm#6412 http://gclusters.altervista.org/cluster_4.php?ggc=... http://spider.seds.org/ngc/revngcic.cgi?NGC6426 https://ned.ipac.caltech.edu/cgi-bin/objsearch?obj... https://in-the-sky.org/data/object.php?id=NGC6426
