Stars are of fundamental scientific significance. Few things in science are more basic than the way stars shine, or in astronomy than the way they form. Do astronomers really understand the origin of stars? Can we describe the specific formative stages in the changing galactic matter that eventually produce stars? What about the evidence—is there any experimental or observational support for our ideas? Questions like these are now under intellectual scrutiny at observatories around the world. The answers aren’t yet crystal clear, but remarkable progress has been made in the past few decades. As it stands now, our knowledge of star formation is a robust combination of theoretical insights and observed facts.
Before we launch into this long epoch, take a good look (Figure 3.1) at one of the most stunning, nearby star clusters—the Pleiades, some 400 light-years away. Not only shining much energy, those stars also radiate sheer beauty. But just how much do we know about stars?
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FIGURE 3.1 – The Pleiades star cluster (popularly known as the Seven Sisters) actually houses nearly a hundred stars, all ~400 light-years distant. (AURA) |
The STELLAR EPOCH offers a better description of matter on scales smaller than galaxies than does the GALACTIC EPOCH on scales larger than galaxies. In other words, we know much more about the origin and evolution of stars than we do about galaxies. Gravitational instabilities, invoked with partial success for galaxies, can be modeled more effectively to understand the formation of stars within those galaxies, regardless of how the galaxies themselves arose. Much as was the case in the early Universe, chance mixes with necessity to affect change. At the outset, random fluctuations often occur at various parts of large gas clouds within any already-formed galaxy. Although such chancy fluctuations alone prove insufficient to cluster huge amounts of matter into galaxies, calculations imply that the process should work much better—and quicker—to assemble smaller clumps of matter into stars. Swirling eddies of loose gas in interstellar space are cooler and denser than those of the primordial fireball, hence are well suited to collect enough matter to mold individual stars or groups of stars, after which they contract, heat, and eventually ignite their nuclear fires.
Astrophysicists have built intricate models of the stages through which gas clouds evolve to become genuine stars. These models, like those of the early Universe, are essentially “number-crunching experiments” performed on powerful, high-speed computers. But here, in the STELLAR EPOCH, we have much data to test the models. The computational factors include mass, heat, rotation, magnetism, elemental abundances, and a few other physical conditions typifying a changing interstellar cloud. These factors resemble the ingredients of an elaborate recipe, yet in this case the recipe is mathematical that teems with symbolic equations. And as is true for any new recipe, although the types of ingredients are known, the amounts of each are often uncertain.
Huge computer programs, built during the past 20 years and containing as much as a million lines of code, enable theorists to use trial-and-error routines for this multifaceted problem of star formation. Though computers do nothing more than calculate numbers rapidly, they can do this basic task more agreeably than humans, adjusting and readjusting the many ingredients to best match the theoretical predictions of the models with the observational findings of actual stars on the sky.
The accuracy of these models is presently mediocre, for it’s tricky to take that 3rd step of the scientific method and test them experimentally. To stress an oft-repeated quandary, no one has ever seen an interstellar cloud or a genuine star parade through all of its evolutionary paces. The lifetime of a human being, or even the duration of our civilization, is very much shorter than the time for a cloud to contract and form a star. Since about 30 million years (or about a million human generations) are needed to concoct a star such as our Sun, no one person can realistically expect to observe any celestial object proceed through its full pageant of star birth.
The stellar models are not without observational support, however. Telescopic monitoring of various gas clouds at many stages of their evolutionary trek helps to refine our knowledge of star formation. Modern technology enables astronomers to peek at interstellar clouds and nascent stars for hints and clues about their embryonic development. Studies of invisible radio and infrared radiation emitted by cool, tenuous galactic regions have proved especially useful, though we are still learning to grope in the dark where young stars emerge; like the mammals, the bright stars incubate in total darkness. By studying numerous interstellar clouds, often at unrelated places along the Milky Way, we can now piece together an observational understanding of many key stages of prestellar evolution.
Current efforts of astronomers and astrophysicists resemble those of anthropologists and archaeologists, who unearth bones and artifacts at many unrelated locales strewn across our planet’s surface. Not having lived at the time of our ancient ancestors, social scholars sift the scattered rubble and ponder the myriad remains, trying to decipher how all of it can be pieced together into an overall mosaic of human evolution. Likewise, space scientists observe a panoply of celestial objects in many disparate parts of our Galaxy, seeking to fathom how each one fits into the larger scheme of stellar evolution. The terrestrial bones and extraterrestrial objects are much like segments of a puzzle. The picture becomes clear only when each piece is found, identified, and fitted properly relative to all the others.