Molecular clouds collapse

Giant molecular clouds collapse to form stars and solar systems, with planets and debris left over such as comets and meteorites. Are comets and meteorites the delivery vehicles that enable life to start on many planets and move between the planets as the solar system forms, providing water and molecules to seed life The planets have to be hospitable, however, and that seems to mean wet and... 

Astronomical observations of molecular clouds and young stellar objects provide the basis for our understanding of the early solar system (Cameron, 1995 Alexander et al., 2001). The first stage in this process is when a fragment of an interstellar molecular cloud collapses to form a disk-like nebula, or proto-planetary disk. This process normally takes... 

The overall picture is now clear. As a result of the variety of nuclear processes available to stars, the creation of nearly all of the known isotopes can he accounted for. Once these isotopes are formed in stars, they are released to the interstellar medium upon the star s demise. When they have become part of the ISM, these isotopes are available for capture by other young stars in the early stages of molecular cloud collapse and protostar formation. Surely stars are the ideal example of how recycling can benefit the environment ... 

One useful trick in solving complex kinetic models is called the steady-state approximation. The differential equations for the chemical reaction networks have to be solved in time to understand the variation of the concentrations of the species with time, which is particularly important if the molecular cloud that you are investigating is beginning to collapse. Multiple, coupled differentials can be solved numerically in a fairly straightforward way limited really only by computer power. However, it is useful to consider a time after the reactions have started at which the concentrations of all of the species have settled down and are no longer changing rapidly. This happy equilibrium state of affairs may never happen during the collapse of the cloud but it is a simple approximation to implement and a place to start the analysis. 

Figure 6.2 Formation of the solar system (a) unstable molecular cloud possessing some angular momentum (b) angular moment conservation produces the disc shape under collapse (c) matter accretion forms the planets (d) the mature system of planets seen today evolves after 4 Myr...

Concepts Solar nebula The collapsing giant molecular cloud that leads to the formation of a star, specifically our Sun with its associated debris in the form of meteorites, meteors and comets... 

Cycle of star formation The collapse of a giant molecular cloud forms a star nuclear synthesis within the star produces more elements the star ages and ultimately dies in a supernova event elements are thrown into the interstellar medium to form a giant molecular cloud. 

Jeans mass The mass of giant molecular cloud that is required for it to collapse spontaneously under the force of gravity. 

The life cycle of stars begins in cold dark molecular clouds. A particularly dense region in the cloud may become gravitationally unstable and collapse. As the density increases, the temperature rises due to the conversion of gravitational potential energy to thermal energy. 

In recent years, a new source of information about stellar nucleosynthesis and the history of the elements between their ejection from stars and their incorporation into the solar system has become available. This source is the tiny dust grains that condensed from gas ejected from stars at the end of their lives and that survived unaltered to be incorporated into solar system materials. These presolar grains (Fig. 5.1) originated before the solar system formed and were part of the raw materials for the Sun, the planets, and other solar-system objects. They survived the collapse of the Sun s parent molecular cloud and the formation of the accretion disk and were incorporated essentially unchanged into the parent bodies of the chondritic meteorites. They are found in the fine-grained matrix of the least metamorphosed chondrites and in interplanetary dust particles (IDPs), materials that were not processed by high-temperature events in the solar system. 

Stars form when dense regions of cold molecular clouds undergo gravitational collapse. In dense molecular cloud cores, the temperature is in the order of 10-20 and the gas density... 

Collapse of a gas pocket within the omau molecular cloud star ... 

Because the Sun and planets are believed to have resulted from the gravitational contraction and collapse of a dense cloud core within a molecular cloud, one asks... 

The collapse of rotating molecular cloud cores leads to the formation of massive accretion disks that evolve to more tenuous protoplanetary disks. Disk evolution is driven by a combination of viscous evolution, grain coagulation, photoevaporation, and accretion to the star. The pace of disk evolution can vary substantially, but massive accretion disks are thought to be typical for stars with ages < 1 Myr and lower-mass protoplanetary disks with reduced or no accretion rates are usually 1-8 Myr old. Disks older than 10 Myr are almost exclusively non-accreting debris disks (see Figs. 1.3 and 1.5). 

This describes the final steps of stellar evolution of single stars. Since stars are formed in clusters, the initially unstable region in the molecular cloud has to fragment into many collapsing subunits. How this works is a yet unsolved problem. Additionally, a large fraction of stars are formed with at least one companion star this process is also not well understood. Both problems remain important challenges in our understanding of star formation (McKee Ostriker 2007). 

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