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
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
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
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
3D Structure of the Orion
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.
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.
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
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.
2.1. The ionization layer is modeled by
varying opacity with distance from the
polygonal model's surface.
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
2.2. A distance field for a smooth surface,
and with turbulence to deform the surface.
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.
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.
2.4. Voxelization of a volume scene graph
evaluates the scene graph's functions
for each 3D location on a grid.
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
2.6. The volume rendered model and a close-up
showing some of the proplyds and shock
fronts in the nebula's center.
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
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.
||March - December, 1999
||D. Nadeau, J. Genetti - SDSC
C. Emmart, E. Wesselak - Hayden Planetarium
B. O'Dell - Rice University
||Polygonal surfaces, Hubble images, generated volume data
||Volume modeling and rendering
||3 GBytes of volume data (after generation)
||Volume Scene Graph Toolkit (prototype)
VISTA Volume Renderer (prototype)
||Modeled on SGI Origin
Intermediate renders on SGI Origin, Tera MTA, IBM SP, Sun HPC
Final renders on IBM teraflops SP