A Star Is Born

Star formation begins in molecular clouds. Recall from chapter 2 that a molecular cloud is a large mass of hydrogen gas that is cool enough for the hydrogen to exist in the form of molecules (H2). The 

Two properties of molecular clouds make them ideal locations for star formation. First, the density of hydrogen molecules within a molecular cloud is relatively high. Second, the temperature of a molecular cloud (about 10-20 K) is very low. These two properties mean that the hydrogen molecules that make up a cloud are more likely to he strongly attracted to each other than they are to be dispersed into space. 

In some cases, the gravitational attraction between molecules in a cloud is strong enough in and of itself to cause the cloud to begin collapsing onto itself. In other cases, some external event may initiate or accelerate that collapse. For example, the explosion of a neighboring star can produce a shock wave of sufficient intensity to force a molecular cloud to collapse suddenly. Also, two clouds may collide with each other, causing contraction of them both. 

Second, as the cloud begins to contract, it also begins to rotate. During rotation, some of the gas contained in the cloud is thrown outward, away from the center, where it forms a thin disk of material around the central core of the cloud. The more the cloud collapses, the faster it rotates and the more material it ejects into the surrounding disk. The disk that surrounds the cloud contains particles that may themselves coalesce to form small bodies (planets) that revolve around the young star. 

A contracting molecular cloud is not equally dense throughout. As in the interstellar medium, some regions of a cloud are more dense than others. Thus, when the cloud contracts, it does not 

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Keckermann, Bartholomaeus. Systemaphysicum (Hannover Joannes Stockelius, 1623). Kehr, Dave. A Star Is Born, New York Times, November 18, 2001, sec. 2, pp. 1, 26. Kenseth.Joy. The Age of the Marvelous (Hanover, N.H. Dartmouth College, 1991). 

A star is born from a cloud of gas and dust called a nebula. When conditions are right, gravitational forces collapse the cloud, and its core density and temperature rise until nuclear fusion com-... 

Theory doesn t tell us what initial Li a star has, only what depletion it suffers. An accurate estimate of the initial Li abundance is therefore a pre-requisite before observations and models can be compared. The Sun is a unique exception, where we know the present abundance, A(Li) = 1.1 0.1 (where A(Li)= log[AT(Li)/AT(H)] + 12) and the initial abundance of A(Li)= 3.34 is obtained from meteorites. For recently born stars, the initial Li abundance is estimated from photospheric measurements in young T-Tauri stars, or from the hotter F stars of slightly older clusters, where theory suggests that no Li depletion can yet have taken place. Results vary from 3.0 < A(Li) < 3.4, somewhat dependent on assumed atmospheres, NLTE corrections and TeS scales [23,33]. It is of course quite possible that the initial Li, like Fe abundances in the solar neighbourhood, shows some cosmic scatter. Present observations certainly cannot rule this out, leading to about a 0.2 dex systematic uncertainty when comparing observations with Li depletion predictions. 

The futnre is likely to be no less rewarding. Astronomers have not let success go to their heads and their thirst for discovery and knowledge has remained intact. They are certainly not so naive as to believe that the stars have delivered up all their secrets. The most ancient stars, those born before the Galaxy had assumed its present form, have now become a subject of intense interest. The next goal on the distant horizon is a complete picture of chemical evolution in space. In this context, it is quite clear that the early stages of this evolution are the least well understood. The end of the road is not yet in sight. 

Assuming that the initial mass function is invariable, we may calculate the average production of the various star generations, born with the same metaUicity, and estimate their contribution to the evolution of the galaxy (see Appendix 4). The abundances produced by a whole population are not as discontinuous and irregular as those shown in the table of individual yields (Table A4.1). This is because the latter are averaged over the mass distribution. 

On the profit side of the account, we carry over all nuclei ejected by stars at the end of their lives. This includes all those stars born in earlier epochs and entering the throes of death precisely at the time t in question. The exact amounts of nuclei depend of course on the mass of the dying star. Thus a star of mass M which dies at time t was born x years before, where x is the mass-dependent lifetime. For example, the nuclear donation, that is to say, the nuclear return on investment, from a star weighing in at 20 solar masses is made 10 million years after its birth, when it explodes. The return from a type la supernova occurs much later, at least 100 million years after the formation of a stellar couple with explosive vocation in which one of the members will eventually become a white dwarf. Even more extreme is the delivery date for stars with similar mass to the Sun. Those which formed at the beginning of the Galaxy are only just opening up... 

They said that a comb polymer with very short branches would not fulfil condition 1, and a star polymer would not fulfil condition 2 unless the branches were very much longer than the critical chain length but polymers having relatively few but long branches, especially ramified or tree-like structures, should fulfil these conditions. Subsequent work, summarised in Section 5, has largely borne out their predictions, though no quantitative treatment is yet available. 

Planetary systems are now generally believed to be by-products of the process of star formation. Star formation, therefore, is the natural starting point for discussions of planet formation. Almost all stars are born as members of stellar clusters that, in turn, are bom in molecular clouds. Formation of isolated stars seems to be possible according to observations, but this is a rare process. Whether the Sun and its associated planetary system formed in isolation or as member of a cluster is not known some indications hint to formation in a cluster (see Hester Desch 2005 Gounelle Meibom 2008 and Chapter 9, this volume). 

The continued addition of matter increases the density and temperature of the core until H2 begins to dissociate. The dissociation consumes heat, which holds temperature approximately constant, i.e. the heat capacity becomes very high and y - 1. The stability condition y >4/3 becomes violated and a new collapse of the core ensues. The core collapses until all H2 is dissociated and the H finally becomes ionized. The temperature then increases again with further contraction and the second core is formed that approaches stellar density. The second collapse phase is short and lasts for a solar-type star of the order of 103 years. By this event a protostellar embryo is born, which continues to grow in mass by collecting the remaining material from its environment. 

Spirit-Body is mixed with the Soul and the Spirit and raised seven times into the Air, through which the Spirit-Body is truly exalted and is impregnated with the heavenly influences until the materia sublimates itself totally clearly, like a star. This is the true exaltation of our Stone, our in the Air born-child, which as soon as it returns again into its seed containing water, coagulates with the Soul and the Spirit into a red-gold sap which is our Elixir. 

If the compact object is a black hole (BH) some parameters scale with its mass. BHs in quasars have about 10s solar masses and may result when huge quantities of gas collapse into the central region of a new born galaxy, whereas BHs in microquasars contain only a few times the mass of the sun ( 10M ), and may be the remains of a star after a supernova explosion. Since the mass of the microquasar is about 7 orders of magnitude smaller than that of a quasar, phenomena taking place in time scales of years in quasars can be studied in time scales of minutes in microquasars. 

Dense molecular clouds, after further contraction, are the places where stars are born. The observation of protostars, stars still embedded in their placental cloud, is a probe of the presence of ices in the clouds the almost black-body continuum emitted from the young object is absorbed by grains whose temperature changes as a function of the distance from the object. These observations, which are mainly obtained by IR spectroscopy, may reveal the evolution of ices due to thermal and/or energetic (e.g. interaction with UV photons and/or stellar particle winds and cosmic rays) processing (e.g. Cox and Kessler [6]). 

This very simple description applies to those stars which evolve as single stars or as members of a wide binary system which do not interact. It is increasingly clear that a large fraction of stars are born in binary or multiple systems in which two stars exchange material at some point during their evolution. The possibilities of what can happen thereafter are too numerous to be able to cover here, but some of the more bizarre possibilities will be considered later. 

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