So I just made what appeared to be a Nebula, and I'm not quite sure how. Either way, I'm very amazed by it!
Here's what I did in steps, and I want to know if anyone can determine what caused the nebula to form when I unpaused the simulation:
1. Loaded a new star in a blank project
2. Changed stars mass to 2 times that of the sun
3. Radius changed to 902020 km instead of 702020 km
4. Clicked zero velocity
When I then played the simulator, the star disappeared. What came out was a cloud of green and blue, which expanded rather quickly before losing its colour. Here's a screenshot (the circle in the middle indicated Barnard's Star, which I placed after in an attempt to re-enact the nebula- to no avail.)