Stellar synthesis

S2-4 Helium burning as additional process for nucleogenesis 19S4 Slow neutron absorption added to stellar reactions 195S-7 Comprehensive theory of stellar synthesis of all elements in observed cosmic abundances 196S 2.7 K radiation detected... 

Introduction.—The identification of technetium in stars has been confirmed, thus establishing that stellar synthesis of this element is occurring. Recent developments in the analytical chemistries of technetium and rhenium have been reviewed, as has the extraction of rhenium from hydrochloric acid solutions a text describing the analytical chemistry of technetium and other man-made elements has been published. ... 

Klement 43 in the seventh subgroup of the periodic system, technetium, is the lowest atomic number radioelement. Stable, non-radioactive isotopes do not exist according to Mattauch s rule. Technetium isotopes can be produced artificially by nuclear processes. Long-lived isotopes are Tc (2.6 10 a), Tc (4.2 10 a) and Tc (2.1 10- a). The spectroscopic discovery of technetium in several fixed stars provided the first proof of stellar synthesis of heavy nuclides. Traces of Tc occur in the earth s crust where they arise mainly from spontaneous fission of... 

Schematic representation uf the main features of the curve of cosmic abundances shown in Fig. 1.1, labelled according tn the various stellar reactions considered to be re.sponsible for the synthesis of the elements. (After E. M. Burbidge et...

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

In the last few years, there has been great progress in the more a-priori method of evolutionary population synthesis , pioneered by Tinsley (1968), using the same stellar libraries as in the empirical fitting, and calculating the spectral evolution of single stellar populations (SSP) as a function of age and metallicity. One problem... 

Blue and UV light from stellar populations is dominated by young, massive stars which also contribute the lion s share of the Z-elements ( metals ). Evolutionary synthesis computations show that, between the Lyman limit (912 A) and 2000 A or so, the spectrum expressed as Lv is more or less flat (Fig. 12.3). This leads to some rather simple considerations because Iv retains its value (and flatness) when the light is redshifted (see Eq. 12.1), because the degradation in energy is simply taken up by the factor dv on integration (Lilly Cowie 1987). 

As indicated in Table 1, statins, which block cholesterol biosynthesis by inhibition of hepatic HMGCoA reductase, have been used extensively to reduce LDL-C levels. At most therapeutic doses, statins marginally increase HDL levels by 5-10% [3,16]. The HDL elevation observed with statins has been highly variable and not easily extrapolated from the effects on LDL. A recent study (STELLAR) demonstrated increased HDL elevation with the use of rosuvastatin compared to simvastatin, pravastatin or atorvastatin (10% vs. 2-6%) [16,24], Although the mechanism of HDL elevation by statins is not clearly understood, it is proposed that statins enhance hepatic apoA-I synthesis [25] and decrease apoB-containing lipoproteins [26]. A number of clinical trials have demonstrated that statins reduce the risk of major coronary events. However, it is not clear if the statin-induced rise in HDL levels is an independent contributor to the reduced risk of coronary events. The observed small increase in HDL and adverse side effect profile related to liver function abnormalities and muscle toxicity limits the use of statins as monotherapy for HDL elevation [27],... 

The remainder of the Fowler article is of great interest, but in detail that is beyond the scope of this encyclopedia. Topics covered by Fowler include early research on element synthesis stellar reaction rates from laboratory cross sections hydrogen burning in main-sequence stars and the solar neutrino problem synthesis of l3C and, 60 and... 

TECHNETIUM. [CAS 7440-26-8J. Chemical element symbol Tc, at. no. 43, at. wt. 98.906, penodic table group 7, mp 2l 72 ">C, bp 4877 TT, does not occur in nature. The present location of technetium in die penodic table was vacant for many years, during which time several claims to having found the element were made, but never confirmed. One such claimant termed the element masurium. Technetium has been detected in certain stars and this discovery mnst be resolved with current theories of stellar evolution and element synthesis. 

Big Bang nucleosynthesis is responsible for the synthesis of hydrogen and helium and some of the 7Li. (Stellar nucleosynthesis in main sequence stars transforms about 7% of the hydrogen into 4He.) However, neither stellar nucleosynthesis or Big Bang nucleosynthesis can produce the observed abundances of Li, Be, and B. Consequently, the abundances of Li, Be, and B are suppressed by a factor of 106 relative to the abundances of the neighboring elements (Fig. 12.2). 

Extrapolation of the hem lines to Z/N = 1 defines another recognizable periodic classification of the elements, inverse to the observed arrangement at Z/N = t. The inversion is interpreted in the sense that the wave-mechanical ground-state electronic configuration of the atoms, with sublevels / < d < p < s, is the opposite of the familiar s < p < d < f. This type of inversion is known to be effected under conditions of extremely high pressure [52]. It is inferred that such pressures occur in regions of high space-time curvature, such as the interior of massive stellar objects, a plausible site for nuclear synthesis. 

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