Gravitational contraction

When 10% of the hydrogen in the core has been consumed gravitational contraction again occurs until at a temperature of 2 X 10 K helium burning (fusion) can occur. This is followed by a similar depletion, contraction and temperature rise until nuclear reactions involving... 

The lithium resonance doublet line X 6707 is fairly easy to observe in cool stars of spectral types F and later, and it has also been detected in diffuse interstellar clouds. There is thus an abundance of data, although in the ISM the estimation of an abundance is complicated by ionization and depletion on to dust grains. The youngest stars (e.g. T Tauri stars that are still in the gravitational contraction phase before reaching the main sequence) have a Li/H ratio that is about the same as the Solar System ratio derived from meteorites, Li/H = 2 x 10-9, which is thus taken as the Population I standard. 

When helium fusion begins, the core of the star is stabilised and a new spherical equilibrium is set up. Gravitational contraction is balanced by the expansive pressure of heat levels maintained by nuclear fusion reactions. Oxygen is produced to the detriment of carbon via the reaction... 

Fusion reactions signify the formation of a true star. The fusion reactions balance the gravitational contraction and an equilibrium... 

In Chap. 1, we introduced the book with a quote from Albert Einstein (Schilpp 1949), which read in part that classical thermodynamics... is the only physical theory of universal content concerning which I am convinced that, within the framework of the applicability of its basic concepts, it will never be overthrown. An important qualification to this statement is the phrase within the framework of the applicability of its basic concepts. The laws of thermodynamics are based on laboratory-scale experiments. To assume that such laws are applicable to the Universe is a big assumption. However, we have no evidence yet that contradicts this assumption on the scales of problems relevant to life. Moreover, there remain vast cosmological questions with no answers and definitely no understanding of implications even if we knew the answers. For instance, does the proton have a very long but finite radioactive half-life Does the neutrino have a very small but finite mass Is the Universe opened or closed with respect to expansion and gravitational contraction Also, the Universe may not be isolated with respect to matter/energy or it could be isolated and cyclical. 

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... 

T Tauri stars very young, low-mass stars, less than 10 million years old and under 3 solar-masses, that are still undergoing gravitational contraction. T Tauri stars represent an intermediate stage between a protostar and a low-mass main-sequence star like the Sun. The prototype for this class of stars is T Tau. 

Beginning of formation of galaxies and stars, gravitational contraction of the stars increase of temperature, thermonuclear reactions (hydrogen burning, helium burning)... 

Further gravitational contraction (depending on the mass of the stars) —> further increase of temperature — carbon burning, oxygen burning, liberation of neutrons by photons —> (n,)>) reactions... 

Gravitational contraction of the stars causes increase of temperature associated with the emission of light and other kinds of electromagnetic radiation. 

The influences of gravitational contraction and thermonuclear reactions depend primarily on the mass of the stars. The phases of contraction and thermonuclear reactions overlap and determine the nucleogenesis as well as the fate of the stars. 

Furthermore, gravitational contraction continues with increasing concentration of and and in stars with masses >3 times that of the sun the temperature and the density rise to about 6 10 K and about 5 10 gcm, respectively. Under these conditions, fusion of two becomes possible and reactions such as... 

With increasing temperature caused by gravitational contraction, more and more excited states of the nuclei are populated, the photon intensities increase and... 

In stars of small mass (<0.1 times the mass of the sun) the energy liberated by gravitational contraction is not sufficient to reach the temperature necessary to start thermonuclear reactions. These stars are directly entering the stage of black dwarfs (black holes). 

Although, in hindsight, the 1932 model of Z protons and N = A — Z) neutrons became obsolete in 1964 (as seen below), it is still the colloquial way of thinking in nuclear physics, and it served to describe the stellar and the major part of the prestellar (primordial) nucleosynthesis. Since geologists pointed out their dissatisfaction with the maximum life-time of 50 million years for our Sun, if powered by gravitational contraction, as first calculated by Lord Kelvin [William Thomson (1824-1907)] one had looked to something like radioactivity, but it was found that the only viable reaction is... 

Consider a star with fairly massive (> 3M0) CO core. Gravitational contraction raises T so that reactions such as... 

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