BACKGROUND INFORMATION: ORION'S VIOLENT STAR NURSERY
Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) has lifted the dusty veil of the giant molecular cloud called OMC-1, revealing a chaotic region of star birth where high velocity outflows of dust and gas from massive young stars are reshaping their birthplace.
OMC-1 is in the Orion Nebula, located in the "sword" of the constellation Orion. The nebula harbors the nearest region of our galaxy where high-mass stars are being formed. Most of the objects in this region are invisible in the image taken by the Wide Field and Planetary Camera 2 (WFPC2), because optical light is blocked by the interstellar dust trapped in OMC-1. The infrared light detected by NICMOS penetrates the dust to uncover the very heart of OMC-1. The new image exhibits with unprecedented clarity spectacular features resembling "fingers," "bullets," and "arcs," which were created by newly born stars.
These features provide evidence that star birth is a messy affair. Stars are formed by the gravitational collapse of gas deep inside dense clouds of dusty molecular material. It is now clear that at some stage in their early history stars of all sizes go through a phase of violent mass loss. Massive stars (those with more than 10 times the mass of the Sun) are particularly important, because their bright light and high-speed outflows can have a dramatic impact on nearby gas and can influence the formation of additional stars. Understanding how stars are born and evolve is critical to understanding our universe, because stars are the basic building blocks for galaxies.
Prominent extended features in the image are the long "fingers" radiating outward and the more compact "bullets." These structures are seen most clearly in the light given off by molecules of hydrogen. The hydrogen molecules glow as a result of collisions with ejected material traveling at supersonic velocities, much like a cosmic sonic boom. Ground-based observations show that the fingers probably were formed in the last few hundred years and are traveling toward us at typical speeds of 62 miles per second (or 100 kilometers per second).
The fingers may have been created in those regions of the clumpy interstellar medium with lower densities, allowing the outflows to follow paths of least resistance. The outflows may carve tunnels through these less dense areas, allowing additional material to travel through the same paths. The fingers and bullets also could be dense clumps of material ejected explosively like shrapnel from a central source, trailing "finger-like" wakes. Or, they could have been formed less violently when a series of colliding stellar winds fragmented and propelled material forward.
Other questions remain. Is there more than one outflow source? When did the outflows begin? The NICMOS image, in conjunction with ground-based spectroscopy, will help us answer some of these questions by allowing a detailed analysis of the location, orientation, and morphology of the fingers. The fingers extend beyond the borders of this image to the upper right and lower left, but this picture provides the best view so far of the enigmatic outflow material close to the suspected outflow sources.
The outflows driving the finger-like jets in OMC-1 are not well understood but are thought to arise from winds or jets emanating from one or more young stars in the lower left of the image, southeast of the central bright star called BN, the Becklin-Neugebauer Object. (BN was one of the first infrared source identified by ground-based telescopes and bears the name of its discoverers, Eric Becklin and Gary Neugebauer.) Although BN is the brightest object in the image, it is not a source of the outflow. One of the possible outflow sources, Irc2, is so deeply embedded in a dust cloud that it is invisible even in the NICMOS image. Another source, N, also dust-enshrouded, appears in the NICMOS image as a star located just southeast of BN (at about 7 o'clock). (Irc2 is northeast of N.)
The infrared picture reveals several fascinating extended structures never previously seen. For example, a dark crescent-shaped feature is just above (north of) BN. The crescent suggests that the dark feature is being blasted and/or heated by a source from the south. It is probably a dense clump of material in the outflow's path, acting much like a large boulder obstructing the flow of a river. The dark clump may even condense to form a star if it contains sufficient mass and if it is not disrupted by its slightly older siblings. Other features include two bright "arcs" below (south of) BN which appear orange in this image. They may represent emission from hot interstellar dust, and the arc shapes also could be related to the outflow sources.
The clarity of NICMOS's vision is readily apparent in its detection of least three sets of "twin" stars of similar brightness in these observations. The stars in each pair probably formed together. The projected distance between the closely spaced double stars is about twice the distance between the Sun and Pluto.
The new capability demonstrated by NICMOS in the image of Orion's stellar nursery will be extensively exploited by astronomers in the months ahead, allowing them to identify the origins of the outflow.
