1、Extragalactic nebula is invisible to the naked eye.
我們用肉眼是看不到河外星系的。

2、N-Cat Series Combined Extragalactic Radio Source Catalogues
N－Cat系列綜合河外射電源表

3、"The claim that it's extragalactic is kind of a guess," says Pravdo.
“這顆行星來(lái)自外星系的說(shuō)法只是一種猜測(cè)，”普拉夫多說(shuō)。

4、First Epoch VLBI Images of Three Extragalactic Radio Sources
三顆河外射電源第一歷元的VLBI圖像