Cycle of star formation

The interstellar medium is thus a chemically diverse medium fed nearly all of the chemical elements by supernova explosions. Conditions in the interstellar medium produce a cocktail of molecules that ultimately find themselves back on the surface of planets during the formation of the new star and solar system. Does the interstellar medium seed life with molecules from space The nature of interstellar medium chemistry might then add credibility to the formation of life in many places within the Universe and act as a panspermia model for the origins of life. 

The plot of luminosity versus temperature for all stars, resulting in the main sequence, red giants and white dwarfs. Stellar evolution leads to mass - dependent birth lines onto the main sequence 

The birth of a protostar and its life as a pre-main-sequence star, its descent to the main sequence and death, starting with a red giant leading to planetary nebula and ending in white and black dwarfs. This sequence varies with mass 

Observation of the Balmer series in the H atom implies population of the n = 2 level of the H atom and the temperature of the local environment 

Observation of a splitting in the spectrum of an atom associated with the magnetic field of a star 

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Figure 4.15 The cycle of star formation. Best resolution available. (Reproduced by courtesy of NASA)...

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. 

The most abundant elements (Fig. 2.2) up to Fe are multiples of He ( C, O, Mg, Si, etc.). During the red giant phase of stellar evolution, free neutrons are generated which can interact with all nuclei and build up all the heavy elements up to Bi all nuclides with the atomic number > 84 are radioactive. Recently (2003) it has been found that even ° Bi decays, but extremely slowly (Ti/2 = 1.9 10 yr). The build-up of elements of every known stable isotope depends on different conditions of density and temperature. Thus, the production process required cycles of star formation, element formation in stellar cores, and ejection of matter to produce a gas enriched with heavy elements from which new generations of stars could form. The synthesis of material and subsequent mixing of dust and gas between stars produced the solar mix of elements in the proportions that are called cosmic abundance (Fig. 2.2 and cf Table 2.13). 

If these values are typical, even a young cloud should contain appreciable amounts of carbon cycled through solar nebulae. Abundances of interstellar molecules relative to CO are at least 2 orders of magnitude lower than yields in FTT syntheses (Gammon, 1978). It appears that only a moderate degree of star formation and CO processing would suffice to account for the interstellar molecules. 

Another type of parameter, to which the model can be sensitive, is the choice in the initial composition of species (initial conditions). Since the chemistry is not at steady-state in most objects, the chemical composition predicted by the model will depend on the assumed initial conditions. There does not exist yet any model able to follow the chemical composition of the gas and dust during a complete cycle of evolution starting with material ejected from stars and ending with the collapse of clouds to form new stars, mainly because the evolution between different stages of star formation (e.g., diffuse to dense clouds, proto-steUar envelopes to proto-planetary disks) is not fuUy... 

In effect, stars return between about 50% and 90% of their initial mass by winds or by explosive mass ejection. Figure 2.3 gives an overview of the relative contributions of the different stellar types to the total mass replenishment. The mass returned by the stars becomes part of the ISM and serves as raw material for the formation of the next stellar generations. In this way part of the baryonic matter in a galaxy is continuously cycled between stars and the interstellar matter. Only the very-low-mass stars (initial masses < 0.8 M0) are not involved in this matter cycle because they have lifetimes exceeding the present age of the Universe and have not yet evolved very much. Some fraction of the matter therefore accumulates in very-low-mass stars and in stellar remnants, but at least part of this loss from the matter cycle... 

The cycle of birth and death of stars that is initiated by population III stars constantly increases the abundance of heavy elements in the interstellar medium, a crucial prerequisite for terrestrial (rocky) planet formation and subsequently for the origin of life (75). Metals dispersed in the interstellar gas or incorporated into micron-sized dust particles and molecules like CO and water, have the ability to cool the interstellar gas much more efficiently than molecular hydrogen does for population III stars. These elements and molecules are also excited through atomic and molecular collisions and their return to lower lying energy levels releases energy via far-infrared and sub-millimeter radiation below... 

This cycle repeats itself many times over, and each time the temperature b higher such that increasingly heavier nuclei can be made to fuse together. The essential detaib of Hoyles scheme are shown in table 10.1 for a star of approximately 25 solar masses, although his calculations extended to various types of stars. In this way, different elements could be formed at different stages in the course of a star s hfe, culminating with the formation of the most stable nuclei of them all, those of iron. 

In pursuing a research problem as complex as this, it was felt desirable to have encouragement from a leading astronomer. The statements by Opik (13) in The Oscillating Universe seem to fulfill this requirement. He compares the formation of new stars to meteorological phenomena—a cycle like that of precipitation and evaporation of water on... 

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