[求助]关于 og 09 的问题，跪盼nn指点

Astronomers at the Palomar Observatory have discovered a distant supernova explosion,one that they believe is a type previously unknow to science.

请问one后面这里是同位语吧，答案they believe to be of 为什么说省略了that是因为 "one"     is sufficient to intoduce the modifier and the sentence is more fluid without    "that".

那什么情况是不sufficient的呢？

RE.

http://forum.chasedream.com/dispbbs.asp?BoardID=23&ID=59580