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Oddly enough, though, other high-pressure processes actually facilitate the star-formation process. Imagine the following scenario. The USS Voyager is cruising along in the Delta Quadrant, through a relatively dense region of the ISM where there are as many as 10 particles per cubic centimeter (a density 10 times higher than average).

Suddenly Voyager starts to get some disturbing readings from an uncharted yet very bright star 30 light-years off the bow. The crew fears the star is headed for a supernova explosion and flies off at Warp 9 in the other direction for fear of being blown apart. The crew also enters the coordinates of this region into the Federation data banks and makes a note that it should be explored again, about 100,000 years into the future. Why?

Ultimately the large amount of rapidly moving supernova debris will sweep up the surrounding ISM like a snowplow. Thanks to this snowplowing, a moderately dense region that was destined to disperse can be compressed to the point that it becomes self-gravitating. In fact, many types of outward pressure, including that from H II (ionized hydrogen) regions around newly formed hot O- and -B-type stars, can squeeze gas with star-forming potential into actual star-forming gas. This kind of triggered star formation is sometimes called sequential star formation because the H II regions and supernovae formed in one region of a molecular cloud can compress a neighboring region, thereby triggering a new wave of star formation.

One supernova can sweep up far more mass than it contained on its own before it exploded. For example, if a 10-solar-mass star goes off in a region with an average gas density of 10 hydrogen atoms per cubic centimeter and the shock wave expands out to a distance of 60 light-years, the shock wave would sweep up about 8,000 solar masses of material. Now that is one heck of a recycling truck! Supernova explosions are much more important in the collection phase of the recycling process than in any other.

The figure at the bottom of the facing page shows infrared emission from many dust clouds. Dust and gas are well mixed in molecular clouds, so this dust image serves as an excellent (albeit indirect) map of molecular clouds. Notice that the clouds are all arranged roughly in a circle on the sky. It is hypothesized that at least one supernova went off somewhere near the middle of this circle and swept up all the surrounding gas. The swept-up clouds are now all potential sites of star formation. The most massive stars formed in the new clouds could in turn explode as supernovae in the future, blowing apart their parent clouds but sweeping up many new potentially star-forming ones in the process. This cycle can continue until all the virgin gas and recyclables

recycling under pressure. Bright blue in this Hubble Space Telescope image and "heat" detected by the Infrared Observatory (shown above, right, in false ( attest to the copious conversion of molei clouds into stars — a consequence of the lision of NGC 4038 and NGC 4039, the galaxies that comprise the Antennae in Co Courtesy François Schweizer; Laurent Vig and the ISOCAM team; and Brad Whitmo Schweizer, and NASA, respectively.

put back into the ISM by the cycle used up (that is, converted into nonr clable material like brown dwarfs, dwarfs, neutron stars, or black holes until they are thrown so far away rejoin the general interstellar — or intergalactic — medium.

While the supernova example is very dramatic, the most common form of triggered or sequential star formation is that driven by the winds and high pressure around massive (O- and B-type) stars. These massive stars produce some photons that are energetic enough to dissociate H2 and ionize the resulting hydrogen atoms, creating an H II region. H II regions, the nearest of which appear as bright nebulae in the night sky, harbor very hot gas. Often the pressure at the

edge of an H II region is substantially higher than that in the surrounding ISM, and this pressure difference ultimately compresses gas near the edge of the ionized region. In fact, one often sees regions of new star formation very near the edges of the H II regions created by hot, young, massive stars.

Someday the Milky Way galaxy itself will probably collide and merge with one of the other members of the Local Group, perhaps the great Andromeda Galaxy (M31). When this happens, our galaxy will experience a burst of star formation more dramatic than any supernova can trigger. The famed "Antennae" pair of interacting galaxies (NGC 4038 and NGC 4039) is currently evincing a great burst of such collision-induced star formation. Again, the star formation is triggered by perturbing clouds that were on the verge of collapsing anyway. In the Antennae, it is as if two neighboring communities have been forced to merge their already near-capacity "dumps" for recyclable goods, and the resulting mess compelled them to build a better recycling plant!

Perhaps we should think of the process of triggering star formation as analogous to environmentalists or benevolent politicians, facilitating the creation of recycling plants where otherwise none might exist. This analogy is good in that many communities (molecular clouds) seem to set up production plants (star-forming cores) all on their own, without any extreme "triggering" influences, as the forces of long-term common sense (gravity) triumph over low-level community resistance (kinetic pressure).

signpost of the collection process? The North Celestial Pole loop, as this structure is called, spans an impressive 20°. Might it have been hollowed out by several nearly simultaneous supernovae? Those exploding stars could have pushed surrounding material outward, creating concentrations of gas and dust that may someday give birth to new generations of stars. This false-color image depicts far-infrared emission from dust grains and was made possible by the Infrared Astronomical Satellite, which mapped the entire sky in 1983. Courtesy the author.

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