In the simplest terms, nebula are interstellar clouds that are often the beginnings of stars.  This was the most curious sunrise I've  witnessed - 'Nebula' seemed like an appropriate title, being so much about clouds and beginnings.  This is from Cadillac Mountain in Acadia National Park, Maine, a popular location to watch the sunrise, being the easternmost and highest altitude location in the U.S. 

The entire mountain was completely socked in by fog and low-lying clouds.  We were certain we weren't going to see much.  We weren't alone on the mountain this particular morning, some people gave up and left shortly after 'sunrise.'  Perhaps ten minutes later, with the sun slightly higher on the horizon, there was a brief, very brief, break in the clouds.  The sun broke through, providing a backlight to the fog and clouds that surrounded us.  And this is what we saw.  It lasted all of two minutes, maybe three - we had been there waiting for two hours.

It's difficult sometimes to wait and wait, then wonder if you should wait a little longer, hard to be 'ready' for that special moment when nothing is happening.  But this sort of thing has happened to me often enough that I have learned to be more patient, to pay attention - because you just never know.