As you all know. I'm a huge fan of clouds. I am also a huge space nerd, and NASA fanatic. Nebulae are considered "space clouds", and nebula is the Latin word for cloud or fog. Instead of producing rain,ice, or snow like our clouds; they produce stars. So I thought it might be fun to also study them as well.

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Unlike the clouds in our atmosphere, nebulae are not contained by gravity or an atmosphere. So they extend for thousands to million of light years across. Like our clouds they are formed from gas, dust, debris, and other particles. In a very basic way, a nebula's density determines how many stars it will produce. However, in the early days of astronomy, the word nebula was used to describe any planetary body beyond Earth. Currently, the word nebula is now only used to describe gas and dust clouds.

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There are different categories of nebulae:

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Galaxies: This might be an odd one to include, but astronomers sometimes use the word nebula to describe galaxies. Galaxies can contain, stars, planets, clusters, nebulae, dark matter, and more. We live in one called the Milky Way. The closest other galaxy to the Milky Way is the Andromeda Galaxy which is on a collision course for ours. Both of these galaxies reside in a cluster of galaxies called The Local Group. See an example of a Galaxy: HERE

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Globular Clusters: These are clusters of thousands old stars, that are usually spread out throughout galaxies. They are held together by the gravitational force of galaxy they are contained in, and the stars that are contained within them. The center of the cluster will usually be packed with stars, and the outer edges will usually be more sparse and spread out. See an example of a Globular Cluster: HERE

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Open Clusters: Open Clusters are also sometimes called galactic clusters. They are similar to globular clusters, however they are as concentrated in density, and contain young stars instead of old stars. They are more disperse, and also can contain some light gas clouds. See an example of an Open Cluster: HERE

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Emission Nebula: These types of nebula mostly contain hydrogen, and therefore they emit a red color. As hydrogen emits a red color on the light spectrum. They also contain star nurseries, and stars can be seen forming inside them. They also emit radiation depending on the amount of energy be produced from them. See an example of a Emission Nebula: HERE

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Reflection Nebula: These are clouds of dust that reflect light from nearby stars or galaxies. They also produce stars. Because they reflect light, they most often are seen containing a blue color. They are often found near by emission nebulae as well. See an example of a Reflection Nebula: HERE

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Dark Nebula: These are pillars of dust that are so thick, that they block any light that is emitting behind them. They can stretch for a few hundred light years across. They can also be found with reflection and emission nebulae. See an example of a Dark Nebula: HERE

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Planetary Nebula: These nebulae are created by stars emitting gas clouds at the end of their lives. The shape of of them resemble planets, but they are not. See an example of a Planetary Nebula: HERE

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Supernova Remnants: These nebulae are created after supernova explosions. The leftover gas and dust can stretch for many light years across based on the size of the explosion. See an example of a Supernova Remnants: HERE

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Now that we've talked a bit about the science behind nebulae, let's talk about how to build them in Houdini.

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Building a Nebula in Houdini

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So way back in my second semester of my final year at Humber, I asked my Houdini mentor how to build Nebulae. He did it in about 5 minutes, but he did point out that it was a bit different than building a common cloud. I would post the file that he helped me build, but due to someone else in the classroom attempting to take the file (without asking) from my computer, they crashed my drive and deleted the file in the process. So we're going to have to talk about them instead until my next tutorial comes out. (Congrats! As of July 2019 it has! Go check it out. :) )

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One way to build a nebula is with a POP network, by adding custom forces to your particles you can trail them out in different directions, and then convert them to a volume to get the fluffy gas clouds. You can then add custom colors to the nebula with attribute VOPs, or a color SOP. Then you can add the particles back into the volume for the stars, or add lights where you see fit. This is a pretty good method for complex nebulae.

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If your nebulae are simple, scattering points, deforming your geometry and merging them together, or choosing a basic/custom shape to convert into a volume also works. You can then add a volume VOP, to edit the density of the volume. After you've created the basic structure of your nebula, if it is a closed geometry, you can even consider using the cloud node. Then you have the option of animating it like a regular cloud.

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I would almost always recommend scattering points in the volume and merging them in later as geometry, as these can be used for stars. You can also object merge them into a different geo SOP so you can keep them separate, and reference them later for volume/geometry lights.

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Rendering a Nebula in Houdini

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Once again , we'll be rendering volumes. So we'll be using ray tracing for the scene instead of PBR.

Because Nebulae are clouds, I recommend reading my cloud notes where I talk about how to render them. See the bottom the page for the link.

 

Lighting Nebula in Houdini

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Lighting a nebula can be rather tricky as it is a volume. As well as considering that nebulae exists in space where there is only light from distant stars hitting it, and it's own self illumination.

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I would recommend using either geometry lights or distant lights in your scene. Or a combination of the two. With a geometry light you can import which sections of the geometry from your nebula you would like to convert to a light source. make sure the referenced geometry in the geo light, and the geometry itself have the same material applied.

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The distant light is going to simulate faint light reaching the nebula from other stars, galaxies, etc. Place this where you see fit.

 

Some resources to check out for further reading:

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The Web Nebula: HERE

The Nebula Predictor: HERE

Modeling(physics) Nebular Structure: HERE

Meet the Eagle Nebula: HERE

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