Here’s a quick way to create a “nebula” effect without any code at all:

Create a default particle system from the menu in Unity’s UI

Delete just the “Particle Animator” component

Check the “One Shot” option in the emitter settings

Now you have a static set of particles. The min & max emission controls how many particles there are (you should probably set them both to the same value, unless you want a random value somewhere between the two). The default particle textures are already fairly suitable as they are have a fuzzy glow look. Try these values for a very quick nebula-like effect:

Min size: 0.1

Max size: 0.7

Min & Max emission : 200

You might want to set one of the ellipsoid size dimensions smaller than the others, to give it a sort of oval shape instead of spherical. And of course you could use some custom particle textures, colour variations, and adjust the size to fit your game.

When I tried this, I found that having two particle systems in the same location worked really well, where the first particle system serves to show the larger blobby white areas, and the second particle system has the size values turned right down so that the particles are tiny specks, and the emission values turned right up (to 2000 or so).

The settings for the first particle system are:

Min Size: 0.3

Max Size: 3.5

And for the second particle system:

Min Size: 0.01

Max Size: 0.03

And apply these settings to both systems:

Emit: On

One Shot: On

Simulate in world space: off

Min Emission: 2000

Max Emission: 2000

Ellipsoid: X:8, Y:4, Z:8

Then add the second system as a child of the first system (by dragging the 2nd onto the 1st in the hierarchy). Because they are not set to simulate in world space, you can then just rotate the parent particle system and the whole nebula will rotate including the tiny dots in the child particle system.

(until I sort out proper hosting for my unity files on this blog, you’ll have to make do with pretty pictures and video!)