The Orion Nebula Model
The Orion Nebula Model
| Group Name | orineb |
| Reference | A Three-Dimensional Model of the Orion Nebula,
Wen & O'Dell, The Astrophysical Journal, v438, p784 Image: NASA, ESA, M. Robberto (STScI/ESA), Hubble Space Telescope Orion Treasury Project Team |
| Prepared by | Carter Emmart, Erik Wesselak, Brian Abbott, Ryan Wyatt (AMNH/Hayden) |
| Labels | No |
| Files | orion_nebula.obj, orishocks.obj, oribay.obj, proplyds.obj |
| Dependencies | orineb.sgi |
The Orion Nebula (M42) shows up in the HII regions data group and its associated stars are represented in the Orion Nebula star cluster, the open clusters, and the OB associations, but it receives special treatment here as a three-dimensional model recreated from Hubble Space Telescope observations of the nebula.
The hot, young stars at the center of the Orion Nebula ionize the surrounding gas—ultraviolet radiation strips electrons from their parent atoms—and when the electrons get reabsorbed by other atoms, light is emitted. For this reason, we call such clouds of ionized gas “emission nebulae.” Radiation from the hot stars doesn't ionize all that gas instantaneously; it takes time for the ultraviolet light to penetrate the dense gas clouds in which stars form. The radiation eats into the surrounding cloud, carving out a vast, electrically charged volume of space. Astronomers call the transition region from neutral to ionized gas the “ionization front.” Most of the emission we detect comes from the ionization front.
Furthermore, the emission of light occurs at very specific wavelengths, so astronomers can tune their observations to capture exactly these parts of the electromagnetic spectrum. Although the nebula contains primarily hydrogen and helium, astronomers also study light emitted by oxygen, sodium, sulfur, and other atoms that exist in much smaller quantities. As it turns out, these trace elements allow astronomers to determine many characteristics of the nebula—its density and temperature, for example. Based on the assumption that most of the ionizing radiation comes from a particular star in the nebula (θ1 Orionis C), astronomers have reconstructed the three-dimensional shape of the ionization front.
Most HII regions that we see lie close to the edge of dense clouds of molecular hydrogen (HII regions embedded inside such clouds remain invisible at optical wavelengths). The Orion Nebula has entered what some call the “champagne phase” of an HII region, when the young stars' radiation has heated enough of the surrounding gas for it to expand and burst out of the dense molecular cloud in which it formed. We view the nebula from an angle that allows us to see the “far wall” of the ionization front, in front of which (from our perspective) lie the bright young stars of the Trapezium and the heated gas moving toward us at about 10 kilometers per second. We actually see through the veil of gas to the ionization front, in large part because we are observing the finely-tuned emission from trace elements, which passes through hydrogen and helium without being absorbed.
The ionizing radiation comes primarily from a single star, θ1 Orionis C, which allowed the astronomers Zheng Wen and C. Robert O'Dell to reconstruct the three-dimensional form of the ionization front. Assuming a constant thickness for the emitting layer, one can actually determine its distance from the nebula's brightest star. We know the spectrum of the light coming from the star, and we know how the atoms in the nebula respond to the star's radiation—each atom acting like an electromagnetic tuning fork, ringing with a particular frequency of light. Thus, the shape of a nebula, some 1,500 light years away from us, can be determined through careful observation and a knowledge of the laws of atomic physics.
In the Digital Universe model of the Orion Nebula, we depict the ionization front effectively as a terrain, with a flat Hubble image of the nebula mapped on the undulating surface. In reality, the ionization front has a slight thickness to it—about a third of a light year—but is quite thin compared to the overall size of the nebula, which stretches about ten light years from side to side.
The first American Museum of Natural History space show, Passport to the Universe, used this same model, rendered into a digital movie using advanced volumetric techniques, to voyage through the Orion Nebula. A limitation of the way Partiview displays the model causes parts of it to appear deceptively bright—when the surface folds over on itself, for example, the additive brightness results in a region that appears excessively bright.
Close into the center, near θ1 Orionis C, we see small teardrop-shaped structures with their narrow ends pointing away from the bright star: these are protoplanetary disks (or “proplyds”) of dense gas and dust surrounding young stars. The sides of the proplyds that face θ1 Orionis C form tiny ionization fronts of their own, which end up shielding the far side of the proplyds to form tails. Proplyds near θ1 Orionis C have long, slender tails whereas the ones farther away have short, stubby tails.
The larger formations that one sees farther away from the center of the nebula take on a cup-like shape, with the narrow end pointing away from the nebula's center. These enormous structures are “bow shocks” that delineate the region where high-speed winds from the central star slow from supersonic to subsonic speeds. (There may be no sound in space, but there is a speed of sound—the velocity at which a compression wave naturally travels given the temperature and density of the tenuous medium. “Supersonic” means a speed faster than that; “subsonic” slower.) You can think of an HII region as a sort of tremendous explosion, taking place over millennia, and the bow shocks are part of the outward rush of material.
Both the bow shocks and the proplyds suffer from a problem similar to the aforementioned issue with the surface folding over on itself. When seen with the ionization front model behind them, they appear brighter than normal. Furthermore, the bow shocks consist of a front and back side, which can look slightly odd when viewed from some angles.
Overall, the Orion Model represents a triumph of our understanding of the dynamics and detailed structure of HII regions. Not only can we admire the beauty of the Orion Nebula as seen through a telescope; we can measure particular characteristics of the nebula and reconstruct it in three dimensions.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott