Orion Nebula Visualization

What is the Orion Nebula?

Enormous clouds of dust and gas are found throughout the galaxy. One of the closest is the Orion Nebula, shown in Figure 1.1a, which is 1500 light-years from Earth and measures several light-years across. It is visible to the human eye as a fuzzy patch in the constellation of Orion.

The galaxy contains tens of thousands of dark nebulae, so-called because the dust andgas obscure the light of stars behind them. Over time clumps of higher density gas form and grow within some of these, their gravitational attraction drawing matter from the surrounding cloud.

As a clump grows, the weight of layer upon layer of gas builds up, increasing the pressure and temperature at the clump's core. The pressure continues to rise until hydrogen nuclei are packed so tightly together that they fuse, igniting a thermo-nuclear reaction that signals the birth of a star. We see this happening in the Orion Nebula - it is the birthplace of stars.

Hot young stars born within the nebula radiate their energy outward into the surrounding gas. High-energy photons from the stars ionize the atoms of the gas, knocking electrons from their orbits. As these electrons collide with other electrons and slowly return to their former orbits, they emit light. It is this light we see as the nebula's eerie glow. The Orion Nebula is an example of an emission nebula.

Since electrons can reside in atoms only in discrete energy levels, when electrons drop from outer to inner orbits they emit light at discrete wavelengths. By examining the spectra of nebulae, astronomers deduce their chemical content. Most emission nebulae are about 90% hydrogen, with the remainder helium, oxygen, nitrogen, and other elements. Ionization of these gases gives nebulae many of the colors we see in astronomical photographs.

Imagery from the Hubble Space Telescope reveals dozens of stars forming within the Orion Nebula. Wrapped in cocoons of dust and gas, these protostars, or proplyds, often include protoplanetary disks that astronomers believe are planetary systems in the making (Left Picture).

3D Structure of the Orion Nebula

Radiation pressure from the nebula's stars pushes nearby gas away, creating cavities within the nebula's cloud. In the Orion Nebula, four centrally located hot, young stars, called the Trapezium, have hollowed out the core of the nebula. This hollow core has "broken through" the portion of the cloud facing Earth, enabling us to peer inside.

Working with infrared and visible light observations from Hubble and ground-based imagery, C.R. O'Dell and Zheng Wen, of Rice University, USA, derived a 3D model of the inner surface of the hollowed out center of the nebula. Their model shows that the region is a wrinkled, shallow "valley," the surface of which glows from the influence of the young stars above.

The ionizing effects of the trapezium's stars penetrate a limited distance into the nebula. The glow we see is the result of a thin glowing ionization layer atop the valley. Dust in this surface region also reflects starlight, contributing to the total luminosity.

As in any wrinkled surface, portions of the ionization layer that face the Trapezium's stars glow more brightly than do portions that face away. For instance, along the left side of the valley in Figure 1.2, a steep cliff faces the central stars. The cliff face is ionized and glows with a bright yellow light, seen in Hubble imagery as the Bright Bar feature stretching from the mid-left to the lower-right side in Figure 1.1. These lighting effects create complex rippling patterns in the nebula's ionization layer.

Overhanging the far end of the "valley" is a dark cloud called the Dark Bay. The underside of this cloud is illuminated by the nebula's stars, but the upper reaches remain dark - the Trapezium's ionizing effects are blocked by intervening dust and gas. The Dark Bay is visible in Figure 1.1 as the dark region in the upper-left quadrant.

Visualization

To visualize the Orion Nebula, the ionization layer model in Figure 1.2 was extrapolated outward to include the surrounding regions and overhanging Dark Bay. This model was imported as a shape into the Volume Scene Graph Toolkit.

To express the Orion Nebula's polygonal model as a volume, scene graph nodes computed a distance field which encodes the distance from any point in space to the nearest point on the surface. To create a soft, gaseous layer atop the surface, the field's distances were used to vary opacity. Regions near the surface were made semi-transparent, while those inside were made opaque and those far from the surface were made transparent.

Using the distance field, the resulting gaseous layer is smooth and follows the terrain of the polygonal model. To give the layer a rougher, more turbulent look like that observed in real nebulae, the distance field was perturbed by 3D procedural turbulence. Figure 2.2 illustrates the effect on the ubiquitous teapot.

Polygonal models, volume scene graphs, distance fields, and turbulence also were used to model 85 separate proplyds and shock fronts within the nebula. The color, brightness, and turbulence for each of these was tuned to match that seen in high-resolution Hubble imagery.

Figure 2.3. An assortment of proplyds and shock fronts.

Voxelization of the resulting volume scene graph repeatedly evaluates the graph's functions, once for each 3D location in a grid spanning a region of interest. Each evaluation returns an RGB-alpha value that is saved into a voxel in a new volume data set. images of the resulting data set can be created using a volume renderer.

Volumetric models of the nebula and its proplyds and shock fronts were rendered, along with stars, using the VISTA multi-volume perspective ray-casting renderer. Images on the right show the original shaded and textured polygonal model, looking down the "valley" and under the overhang of the Dark Bay. Figure 2.6 shows the volume rendered model

A 2 1/2 minute fly-through animation of the nebula was produced for the Hayden Planetarium's daily show. The planetarium uses seven 1280x1024 video projectors to seamlessly cover the interior of its dome. Animation production, then, computed seven high-resolution images for each frame of the show. The 2 1/2 minute animation required about 31,000 high-resolution images.

Animation frames were rendered using 900+ processors on the San Diego Supercomputer Center's IBM RS/6000 teraflops supercomputer. Running one multi-threaded renderer on each 8-processor node, the frames were computed during a single 12-hour period.

Summary