Sky & Telescope April 1981 [antikvár]

The Milky Way as depicted by Knut Lundmark, from photographs by Solon I. Bailey and Frank E. Ross. This version has been redrawn by Martin and Tatjana Keskiila. It shows the entire Milky Way, with the direction to the galactic center in the middle of the map. The Small and Large Magellanic Clouds are the little patches in the lower right quarter. CourtesyLund University Observatory, Sweden.Early Phases of Star FormationBart J. Bok, Steward Obser\>atory, University of ArizonaDuring the past decade the quest to understand star formation has become one of the most active endeavors in Milky Way research. Thirty years ago, many of us who were working on problems concerning our galaxy felt that the time was ripe to tackle the questions of star formation; it had become clear that the required processes would probably be at work in the interstellar medium, where several varieties of gas and dust clouds seemed then and still seem on the verge of collapse into protostars. In 1947 we held a mini-symposium on the subject, the results of which were published in Harvard Observatory Monograph No. 7 (1948); it contained papers by L. Spitzer, F. Whipple and myself. But it was difficult to make headway, for we were limited to trying out little bits of theory supported only by optical observations from the blue-violet region of the spectrum to the nearest infrared.All of this has changed during the past 15 years or so. Radio astronomy has given us access to over 50 varieties of interstellar molecules. Infrared astronomy has blossomed forth. Equipment for the range of wavelengths between one and 10 microns, in the near infrared, is by now fully ready to discover and observe young stars embedded in dense obscuring clouds, and searches at two microns are proving especially effective. In the far infrared (100 to 1000 microns), studies from the Kuiper Airborne Observatory are revealing much that is new about the youngest protostars as well as dust and gas clouds ready for collapse.There is now a firm bridge connecting studies in the farthest infrared with ones at the shortest radio wavelengths. These284 Sky and Telescope. April. !981studies, combined with spectrographic observations of objects that were out of reach 10 years ago, are giving us a fascinatingly complete picture of the physical conditions and processes at work inside the clouds of gas and dust. We suspect these clouds of harboring protostars and, in several cases, we have found some very young stars.Optical, radio, and infrared astronomers are participating in the gathering of observational evidence. Theoretical astrophysicists are now presenting observation ally based models for clouds of gas and dust that seem to be either in a state of collapse or on the verge of it. We know distances and linear dimensions of many of these clouds. Optically, their masses can be estimated from the dust content; by radio these data can be derived from studies of carbon monoxide, ammonia, and formaldehyde. From radio observations we can also deduce the molecular hydrogen (Hj) content of each cloud, this material being the principal constituent.A few general comments are in order. We must remember that in practically all of the suggested processes of star formation, the initial step is the concentration of matter into individual clouds of atoms and molecules (species of hydrogen head the list). Many of these clouds have small but not insignificant admixtures of cosmic dust. Some of these units are probably huge complexes. For example, we shall see that giant molecular clouds may have masses equivalent to several hundred thousand Suns. But we also observe smaller units: globules, for example, some with masses no greater than 20 Suns. All that these units have in common is that they should collapse, mostly by self-gravitation with possibly a push from outside pressures.REFLECTIONS ON THEORYThere is one outside influence that may well play an important role in triggering gravitational collapse. It is the force exerted upon a cloud and its incipient units when the spiral density wave of our galaxy (and its associated shock wave) passes through a particular part of the interstellar medium.The Lin-Shu density-wave theory came into being during the early 1960's when C. C. Lin and F. H. Shu (MIT and Harvard) suggested that the spiral structure of our own and other galaxies could best be explained as an effect produced by a hydro-dynamic density wave passing through the interstellar medium. Wherever the wave passes, the gas density and pressure increase by up to eight to 10 times their original values, and the piling up that results may well produce condensed units of interstellar gas and dust. The density wave literally "triggers" star formation.However, the density wave cannot be expected to compress strongly every unit that exists in its path. I view its effect as highly selective; its supposed power is exerted only in a few places, but with great efficiency when it comes through with full force. We note that the clouds which presumably contain protostars, clusters, and associations often come in bunches and generally are not distributed uniformly along a spiral arm.In the detailed descriptions of the five distinct principal processes that are likely important for star formation, we shall be concerned mostly with the gravitational collapse of individual clouds or cloudlets. During the past 10 to 15 years, observers dealing with star formation have been