How can I create a static "particle cloud" within a fixed volume?

Ok, so, the title of this question is relatively abstract, but what I would actually like to do is pretty specific. Yesterday, I asked in a post over at graphic design, how I could recreate the following illustration in Inkscape, in particular the "particle cloud" that represents the cometary nuclei:

I already got some excellent solutions, but now I've tasted blood and would like to do a "proper" simulation of the cloud in 3D. At this point, I'd like to say that I'm completely new to blender. I literally installed it today, but I tried my best. Here is what I've done so far.

My approach

My basic idea was to hollow out a sphere to make the rough shape, and then somehow fill that shape with a point cloud (but maybe this is already the wrong approach). To hollow out the sphere, I thought I could use the Boolean Difference modifier between a solid sphere and a void object (e.g. as described in this video, to end up with something as described here). Because the void is radially symmetric, I thought maybe I could create it from a 2D mesh that traces the edge of the void and is then spun around 360°, similar to one of those folding lampions that you can buy.

So first, I imported a similar illustration into Blender that would allow me to more easily trace the shape of the void (other references can e.g. also be found here or here). Next, I added two Circle meshes, and scaled them to roughly fit the outer and inner edges of the cloud shape. Then I tabbed into Edit Mode, deleted all the vertices of the left half of the inner circle, and manually repositioned the lower group of the remaining vertices to roughly retrace the lower edge of the upper void:

Then I used the Spin tool to spin the 2D mesh around 360° to create the 3D shape of the upper void. Next, I duplicated the shape, mirrored it on the z axis, and joined the two shapes. At this point however, I already run into a problem : there is a small gap in the center where the two shapes meet, and I have no idea how I can close it.

In any case, next, I selected the outer 2D circle mesh, again removed the left half of the vertices, and this time spun the remaining ones 235° using the Spin tool to create the outer edge of the particle cloud shape. Now I tried to make it solid via the Solidify modifier, inspired by this post. However, I don't understand how I can make the entire sphere solid without creating some kind of artifacts, so this is basically where I am stuck right now. If I anyway go through with my plan and apply the solidify modifier to the sphere and then add a Boolean Difference modifier, nothing happens. How can I make this work?

Particle simulation

And of course, I'm still missing the major step of filling the shape with a particle cloud once the shape is created. Unfortunately, in this regard I didn't manage to find any relevant tutorials or similar, since the term "particle cloud" seems to have a very specific meaning in 3D modeling (like smoke and such), which, as far as I can tell, is not what I am looking for. So how would I do this, if that is even the right thing to do?

• Hi. For having installed Blender only yesterday, you have already achieved quite a lot. What do you mean by " do a 'proper' simulation of the cloud in 3D"? Is this a static model/image? Or is it an animation? What should be simulated? Commented Mar 22 at 17:44
• 1) About the gap: the Mirror modifier uses the object's origin (=the little orange dot) for mirroring. You either need to move it or the geometry. Or if you want to keep the connection apply the modifier and fill the gap (Bridge Loop). Or increase the merge distance in the modifier settings. 2) the Boolean modifier requires a non-manifold (=watertight) mesh. Also, a clean topology helps. You need to fill the green areas as shown in your 1st screenshot. 3) the point cloud can be created with the 2 Geometry Nodes Mesh To Volume and Distribute Points in Volume. Use Cycles to render/see pts Commented Mar 22 at 17:55
• Hi, thank you! By "proper" simulation I mean an actual 3D model of the cloud filled with "actual" nuclei, i.e. spheres, instead of just "faking" it with various tools in 2D. So I don't mean physics simulations or anything like that. I actually made a little progress today, doing basically exactly what you are suggesting. It's not working 100% yet though, but I will update my question now.
– mapf
Commented Mar 22 at 18:33
• Hi @Blunder, just FYI, when I was about to finish updating my question, Luis Quintana posted his answer, which made parts of the update obsolete. So instead, I've now posted a separate question here about how to make my "void" object manifold.
– mapf
Commented Mar 27 at 13:14

A static particle cloud with the shape of any object can be created through Geometry Nodes with the aid of the Mesh to Volume and Distribute Points in Volume nodes.

Just tweak the Density values to your like.

In the example the instances are spheres, but you can use any other object or the contents of a Collection.

• Ok, I understand now that you have done this in the Geometry Node Editor and what is created is basically a material that you can then add to the object?. I managed to follow your solution now, and it's essentially working. However, my Mesh to Volume has neither an Exterior Band Width nor the Fill Volume option. Does that matter?
– mapf
Commented Mar 22 at 22:11
• Don´t worry about the node options, its just because I stick to version 3.6LTS. Commented Mar 23 at 1:32
• Also, the later image is an example of using "star" objects in a Collection as instances instead of spheres, not a material. Commented Mar 23 at 1:37
• Hi, thanks a lot again and sorry for the late update, but I've been busy with other stuff in the meantime. This is a nice solution, but I still haven't figured out how to make my "void" object manifold. Maybe you have an idea?
– mapf
Commented Mar 27 at 13:15