Normal Stars

Nearly all types of normal (i.e., noncollapsed) single stars generate X-ray emission at some level. Stellar X- 

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Ba (La) and Eu abundances have a linear relation for stars whose metallicities are below -2.75. At higher metallicities the relation is still linear, but the fit shows different slope. Since Barium stars are rich in s-elements, that relation is different relative to normal stars with similar metallicities. 

According to the Fig. 1, Ba is more overabundant than La in Barium stars relative to normal stars with similar metallicities. No dependence on luminosity classes was found in the s/r behavior among those Barium stars. 

It should be noted that at least three of the Ba-rich stars in our sample exhibit no clear variation in their radial velocities over the last 8-10 years. Either their periods are quite long, or the mass-accretion scenario may not apply to these objects. Further investigation of the binarity for these objects is clearly required. Ba-normal stars The other nine objects in our sample have relatively low Ba abundances ( — 1.0 < [Ba/Fe] < —0.5). These values are typical in metal-deficient stars that show no carbon excess ([C/Fe]< +0.5), hence the scenario of carbon enrichment by AGB stars cannot be simply applied to these stars. 

Fig. 1. A comparison of the 3990-4050 A regions for the nitrogen rich stars B 32 and B 04 in NGC 330, the nitrogen normal star AV304, and the Be star B 12. The Hel lines at 4009 and 4026 A are marked together with the NII lines at 3995, 4041 and 4043 A. Note the strength of the NII line at 3994 A in B12, which is significantly weaker than in the other stars. The element shows under-abundances of —0.35 relative to AV 304.

Only one or two first stellar generations containing normal stars plus very massive stars included in the best model of PM04, produce negligible effects on the subsequent photo-chemical evolution, when either the yields of HW02 or those of UN are adopted. Therefore, these models are acceptable and we cannot assess Pop III existence nor disproved it. 

The epoch and mode of galaxy formation are not well known, but both quasars and star-forming galaxies are known with redshifts up to about 7, corresponding to an era when the expanding Universe was only 1/8 of its present size, and the emission-line spectra of quasars indicate a large heavy-element abundance (solar or more Hamann Ferland 1999), suggesting prior stellar activity. The first stars, on the other hand, known as Population IIP, would have been devoid of metals whether they differed from normal stars in other basic characteristics, notably their mass distribution, is not known, since no completely metal-free stars have been... 

A scenario imagined by Zwicky in 1938 was for a long time the only explanation of the phenomenon. According to this view, a supernova marks the transformation of a normal star into a neutron star, drawing its energy from gravitational collapse. This led astronomers to think that the death of a star was the transition from luminous perfection to a kind of dark perfection. 

Fig. 7.7. Stellar duo. The presence of a companion star can considerably perturb a star s evolution. Hence, mass transfer by accretion transforms a rather dull white dwarf into an erupting nova or a type la supernova. As an example, let us follow the life of a star with mass between 4 and 9 Mq and its little sister star with mass between 0.9 and 3 M , separated by a distance of between 1500 and 30000 Rq (where Rq is the solar radius). In childhood, the system is calm. The big star evolves more quickly than the small one, however, a universal feature of stellar evolution. It soon becomes an asymptotic giant, sweeping the companion star with its winds, and then a white dwarf. The oxygen- and carbon-built white dwarf shares an envelope with its partner and together they evolve beneath this cloak as one and the same star. The result is a pair comprising a white dwarf with mass between 0.9 and 1.2 M and a normal star with mass between 0.9 and 3 M , still evolving on the main sequence. The two components are separated by a distance of some 40-400 Rq, corresponding to a period of revolution of 30-800 days. The second star swells up and becomes a red giant. This is a boon for the dwarf. It captures the matter so generously donated. However, it cannot absorb it A tremendous wind is generated and, in the end, a cataclysmic explosion ensues. (After Nomoto et al. 2001.)...

Abstract Although once it was thought that main-sequence stars are remarkably homogeneous with respect to their chemical composition, the upper main-sequence stars (30000 > Te > 7000) show a variaety of chemically peculiar stars besides the so-called normal stars. Those include the Am, Ap, A Bootis, He-deficient, and He-rich stars. This review summarizes the current data, which are necessary to construct and test the theoretical models of these stars. In the second half of the review we concentrate on Li. In the lower main-sequecnce stars abundances of Li have been determined in hundreds of stars. Some of the remarkable results are ... 

The inner boundary radius R is the radius of the neutron star for X-ray bursts and is zero in tfie normal stars. 

Brunhilde, display Wolf-Rayet stars. Figures 3.1 through 3.3 are displayed. This wind from the star becomes so thick that it totally obscures the star. This means that when we look at a Wolf-Rayet star, we re really just seeing this thick wind. The wind carries about the same mass of the Earth into space each year. The star is losing so much mass that it doesn t live as long as a normal star. Very... 

Stellar nucleosynthesis No 21B is produced by thermonuclear reactions in stellar interiors. It is instead destroyed if heated above several million degrees. This means that the only boron that exists in normal stars is that inherited when the star formed from the interstellar gas, and, moreover, that stellar boron is limited to the outer layers of the atmospheres of the stars. So its natural source must be found elsewhere. 

The sun is a relatively normal star of intermediate mass, temperature and luminosity. It is therefore very reasonable to assume that it once passed through the same three phases of protostellar evolution observed for other stars. This assumption allows us to draw upon ultraviolet observations of young stars to represent the probable ultraviolet spectrum of the young sun. 

Fusion of hydrogen isotopes in normal stars prodnces He, which reacts fnr-ther to produce all of the heavier elements in a series of fnsion reactions called nucleosynthesis. 

Today, the concept of chemical evolution of the Galaxy - old stars are metal-poor and young stars are metal-rich - is embedded so firmly in our thinking that it may be hard to imagine a time when it was supposed that all normal stars had a very similar ( cosmic ) composition. Yet, this was once the accepted idea or prejudice. The first real challenge... 

Lack of an abundance estimate for a trace element like boron has no effect on the accuracy of the abundance analysis for other elements but merely restricts astrophysical interpretations involving B. On the other hand, helium is an abundant elements with effects on the atmospheric structure and through this on the derived abundances of other elements. Although rarely stated explicitly, abundance analyses of cool stars are dependent on an assumption about the He/H ratio the assumption enters both into the model atmosphere and synthetic spectrum calculations. For normal stars, ignorance about the He/H ratio is mitigated by the fact that the He/H ratio is surely constrained within tight limits (Y = 0.24 to 0.26, see above). 

One frontier for current exploration concerns the nucleosynthesis occurring in the Universe s dark ages between the time the cosmic microwave photons were set free and systems containing normal stars had formed. In this connection, I close with an extensive quotation from Fred Hoyle s Frontiers of Astronomy ... 

Pegasi (Michel Mayor and Didier Queloz) Mayor and Queloz detect a planet orbiting another normal star, the first extrasolar planet (exoplanet) to be found. As of June, 2009, 353 exoplanets were known. 

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