The Formation of Stars

Djorgovski is another astrophysicist, like Bechtold and Tyson, whose studies about one phenomenon—the formation and evolution of galaxies—often touch on larger cosmological questions. On the assumption that dark matter constitutes 90 percent or more of the mass in the universe, "large-scale structure is clearly dominated" by it, he said. But galaxies are the components of this large-scale structure, and their structure "is a much more complicated business." Astronomers approach the subject in two ways, he said. Looking at large redshifts, they hope to "come upon a primeval galaxy that will hopefully be a normal example undergoing its first major burst of star formation." Another approach, called paleontocosmology, looks at nearby, older galaxies that can be studied in some detail. Systematic properties may yield scaling laws and correlations that in turn allow strong inferences to be made about the early life and formation of galaxies (Figure 4.4). ''Galaxies are interesting per se," said Djorgovski, but better understanding their evolution could provide astrophysicists "tools to probe the global geometry, or the large-scale velocity field.''

Galaxy formation can be seen as two distinct types of events, said Djorgovski: assembling the mass (which is the business of the dark matter and gravity) and "converting the primordial [hydrogen] gas, once assembled, into stars that shine." The energy for this second step comes from two sources, he explained. "First of all, when you look at galaxies, you find out that they are about 1000 times denser than the large-scale structure surrounding them. That tells you that they must have collapsed out of their surrounding density field by

Figure 4.4 Computer simulation of a density field in the early universe. Shades of gray indicate different densities in a fairly arbitrary way.

about a factor of 10, and that involves dissipation of energy. When you add up how much that is, it comes to about 1059 ergs per galaxy." Once formed, stars engage in nuclear fission to make the helium and heavier metals that come to compose their cores, and they generate about 100 times more energy, 1061 ergs per galaxy.

These events are thought to happen in the era around a redshift of 2, perhaps a little greater, "but we don't know exactly where or when," said Djorgovski, suggesting that the process probably evolves over a considerable period of time. But it will be the emission lines in its spectra that confirm a star in the process of forming, should one be found. Djorgovski showed the scientists some pictures of one candidate "which has the romantic name of 3C 326.1, at a redshift of nearly 2, two-thirds of the way to the Big Bang." Whether this is a primeval galaxy remains a subject for debate, as is the case with many such candidates, because it is associated with intense radio emissions and its signals may be confounded by an active nucleus it contains. "We know about a dozen objects of this sort," said Djorgovski, "but what you really want to find are just ordinary galaxies forming with large redshifts. And that nobody has found so far."