History of the Sun

The sky is no empty arena and stars are not the only actors. The other player in the cosmic drama is the cloud. 

Still curled up at the heart of the parent cloud, the stellar embryos attract more matter in order to embark upon the visible phase of an object of fixed mass in hydrostatic equilibrium. They then disperse any surrounding matter and begin their own lives as free and independent stars. 

In truth, star formation from molecular clouds is no easy subject to study. This is because the processes involved change the density from 10 g cm to about 1 g cm within a space of only a few tens of millions of years. Only the force of gravity, whose long range plays a key role, is able to produce such staggering compression rates. 

But how is it that these objects simultaneously accumulate and shed matter How can a star form by losing mass It is thought that the solution to this paradox lies in the wind. Material deposited on the star from the encircling disk dangerously increases the speed of rotation. A centrifugal barrier then 

The luminosity then decreased rapidly from 20 to 0.5 Lq, where Lq is the Sun s present luminosity, whilst the surface temperature stabilised at around 4460 K. The Sun looked like an orange. The convective zone was resorbed and covered the star like a blanket. Although just 1% of the mass, it occupies today 30% of the radius  

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One of the most interesting of the geophysics results from radiocarbon dates is the history of the sun. Apparently, it is registered in fluctuations of the cosmic ray intensity. These are fluctuations of rather short duration in terms of the radiocarbon lifetime, perhaps a century or so, and apparently they are caused by variations in the solar wind due to long-term changes in the solar emissions. This idea has been developed in some detail recently by Dr. Lai and his collaborators. It promises to give us a way of watching the history of the sun over tens of thousands of years. This fine structure on the curve of calibration was discovered by Dr. Suess and others. 

Now, apart from the planets, many meteorites were formed, moving in quite different orbits and of quite different chemical composition. In particular, the so-called C-l meteorites composed of carbonaceous chondrites have a composition of elements much closer to that of the Sun. It is proposed (see for example Harder and also Robert in Further Reading) that many of these meteorites collided with very early Earth and became incorporated in it, so that eventually some 15% of Earth came from this material (see Section 1.11). Other planets such as Mars and the Moon could have had similar histories, but the remote planets and Venus are very different. 

Such a measurement can tell us about the chemical evolution of oxygen, such as whether the isotopes differentiated via a thermal cycle in which lighter leO fractionates from the heavier lsO, much as Vostok ice-core oxygen ratios reveal the Earth s prehistoric climate. From this fixed point of the Sun s oxygen ratios, we can then trace the history of water in other planetary bodies since their birth in the solar nebulae through the subsequent cometary bombardment [13]. In NASA s search for water on the Moon, important for the establishment of a future Moon base, such isotopic ratios will determine whether the water is a vast mother lode or just a recent cometary impact residue. 

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. 

It should be emphasized that solar abundance ratios are used here only as a convenient referenoe point. The LMC is known to have a total heavy element abundance that is approximately two to three times less than solar (van Genderen, van Driel, and Greidanus 1986 Dufour 1984). The abundances of Sc, Sr, and Ba in the LMC are not known because of the difficulty in detecting lines of these elements in objects. They are probably not solar however, unless the history of nucleosynthesis in the Large Cloud is completely different from that in our Galaxy, the relative abundances of the s-process elements with respect to each other and to Fe should not differ greatly from those of the sun. 

Belt (Bell et al., 1989) and are presumed to be the origin of the radial dependence of asteroid reflectance classes with increasing distance from the Sun. Once started, the internal heating process would have been enhanced by the exothermic heat from hydration processes. Some of the water must have escaped from the interiors of internally heated asteroids and perhaps a significant proportion of main Belt asteroids showed cometary activity before they turned into what are now considered to be asteroids. Had they been observed early in the history of the solar system all of the hydrated silicate-bearing asteroids would have been considered to be comets. 

Figure 1 The habitable zone (Kasting et al, 1993). Too close to the Sun, a planet s surface is too hot to be habitable too far, it is too cold. Early in the history of the solar system, the Sun was faint and the habitable zone was relatively close 4.5 Ga later, with a brighter Sun, planets formerly habitable are now too hot, and the habitable zone has shifted out. Note that boundaries can shift. By changing its albedo and by altering the greenhouse gas content of the air, the planet can significantly widen the hounds of the hahitahle zone (Lovelock, 1979, 1988).

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