A friend is working on an online astronomy course and asked me this question based on a part of the lesson:
Is there a way to visually confirm a DSO is either a planetary nebula or a galaxy? What test can you perform using the commonly available tools in your visual toolbox?
The example given is Hoag's Object (a galaxy) https://apod.nasa.go...d/ap040815.html vs. a similar ringed planetary. Example: http://annesastronomynews.com/photo-gallery-ii/nebulae-clouds/the-fine-ring-nebula/
I know one can definitively test for one type but the other is not definitive as it is a process of elimination which cannot lead to a definitive answer.
I'm curious to read your thoughts...