Evolution stellar

The four general types of stars (main sequence, white dwarfs, giants and supergiants) provide a classification based on the fundamental observable properties but also suggest an evolution of stars. Astrochemically, the cooler giants and supergiants have many more atomic and molecular species that are the products of the nuclear fusion processes responsible for powering the stars. The nuclear fusion processes allow for the formation of more of the elements in the Periodic Table, especially the heavier elements that dominate life on Earth - principally carbon. 

Big Bang nucleosynthesis produced only H and He atoms with a little Li, from which nuclei the first generation of stars must have formed. Large clouds of H and He when above the Jeans Mass condensed under the influence of gravitational attraction until they reached the temperatures and densities required for a protostar to form, as outlined. Nuclear fusion powers the luminosity of the star and also results in the formation of heavier atomic nuclei. 

The B-N object may be considered to be a zero-age main sequence star that evolves with increasing surface temperature and luminosity at optical wavelengths. The descent from the right-hand upper quarter of the HR diagram, along with what has been called the Hayashi track or birth lines, precedes the entrance onto the 

Nuclear fusion processes derive energy from the formation of low-mass nuclei, which have a different binding energy. Fusion of two nuclear particles produces a new nucleus that is lighter in mass than the masses of the two fusing particles. This mass defect is then interchangeable in energy via Einstein s equation E = me2. Specifically, the formation of an He nucleus from two protons and two neutrons would be expected to have mass  

The nuclear generation processes in stars, however, do not follow simple fusion of nuclei one at a time. There are preferential networks of nuclear reactions called cycles that produce different atomic nuclei, but only some cycles are triggered in heavier stars. 

The birth of stars depends on two essential characteristics of the universe. First, the universe has apparently never been completely homogeneous. As the discovery of the cosmic microwave background anisotropy has confirmed, there are very small differences in the concentration of matter in various parts of the universe. In some regions of space, the density of matter is slightly greater than it is in other regions. Second, the force of gravity acts to attract any two particles anywhere in the universe. 

The evolution of stars is a complex process, hut a simplified overview of the process is as follows  

Stars in the universe today represent most of these life stages. At each stage of its evolution, a star is capable of transforming its existing matter into energy and new elements. 

Stars have two, and only two, fundamental observable properties brightness and color. Astronomers usually prefer the term luminosity rather than the term brightness. An even more precise term is absolute luminosity, which is defined as the brightness a star would have if it were placed at a distance of 10 parsecs from the Sun. The parsec is a unit of measure used in astronomy equal to 3.26 light-years. 

Scientific work can be classified into two general categories collecting and collating data, and drawing conclusions and building theories based on those data. Neither activity can be conducted without the other, and either, in and of itself, is incomplete. One of the great data collectors and collators in the history of astronomy was Annie Jump Cannon, who came to astronomy somewhat late in life, at the age of 31. 

The first generation of stars that formed in this way is called (for historical reasons) population HI stars. They consisted of hydrogen and helium, were massive, had relatively short lifetimes, and are now extinct. The debris from these stars has been dispersed and was incorporated into later generation of stars. 

The second generation of stars, called population II stars, was comprised of hydrogen, helium, and about 1% of the heavier elements such as carbon and oxygen. Finally, there was a third generation of stars, like our sun, called population I stars. These stars consist of hydrogen, helium, and 2-5% of the heavier elements. 

Our sun is expected to spend 7 x 109 more years on the main sequence before becoming a red giant. In a shorter time of 1.1 —1.5 x 109 y, the sun will increase slowly in luminosity by 10%, probably leading to a cessation of life on Earth. (In short, terrestrial life has used up of its allotted time since it began 3.5 x 109 y ago.) 

For massive red giants (M 8 solar masses), one finds they undergo a more spectacular death cycle, with contractions, increases in temperature leading to helium burning, carbon-oxygen burning, silicon burning, and the like with the production of the elements near iron, followed by an explosive end (Fig. 12.8). 

All observations are consistent with a density of these elements and isotopes as measured. So the distribution of the isotopes and elements leads to a value of 3 X 10 kg m for the average density of normal matter in the universe today. 

One has to stress again that no elements more massive than boron have been formed in the Big Bang. In this context we also mention dark matter. If dark matter consists of normal matter i.e. of protons and neutrons, then the density of protons and neutrons in the early universe would have been much higher and the resulting abundances of the light elements and isotopes mentioned would have been different from what we measure. A review about dark matter was given e.g. by Bertone, Hooper, and Silk, 2005 [26]. 

We have seen in this section, that there occurred a short phase in the early universe when density and temperature was high enough for the fusion of hydrogen into heavier elements up to lithium and boron. Hydrogen (protons) was present since protons were formed form quark particles. Neutral hydrogen since the recombination age. However, the second constiment of water molecules, oxygen, was not formed dming these phases. In the next section we will describe the evolution of stars and see how elements heavier than lithium can be synthesized by thermonuclear fusion reactions inside stars. 

The typical chemical composition of a star is demonstrated by giving the values for the Sun in Table 8.1. In this table the elements hydrogen and oxygen are written in boldface style. The two constituents of a water molecule, hydrogen and oxygen are the most abundant and the third most abundant in solar like stars. 

Stars can be classified according to their spectra (see Chap. 9). The most important parameter for the evolution of a star is its mass. 

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The evidence on which this theory of stellar evolution is based comes not only from known nuclear reactions and the relativistic equivalence of mass and energy, but also from the spectroscopic analysis of the light reaching us from the stars. This leads to the spectral classification of stars, which is the cornerstone of modem experimental astrophysics. The spectroscopic analysis of starlight reveals much information about the... 

During the red giant phase of stellar evolution, free neutrons are generated by reactions such as C(a,n) and Ne(a,n) Mg. (The (ot,n) notation signifies a nuclear reaction where an alpha particle combines with the first nucleus and a neutron is ejected to form the second nucleus.) The neutrons, having no charge, can interact with nuclei of any mass at the existing temperatures and can in principle build up the elements to Bi, the heaviest stable element. The steady source of neutrons in the interiors of stable, evolved stars produces what is known as the "s process," the buildup of heavy elements by the slow interaction with a low flux of neutrons. The more rapid "r process" occurs in... 

Open clusters (OCs) are important tools both for stellar and for galactic astrophysics, as tests of stellar evolution theory for low and intermediate mass stars and as tracers of the Galactic disk properties. Since old OCs allow us to probe the lifetime of the Milky Way disk, up to about 10 Gyr ago, they can be used to study the disk evolution with time, and in particular its chemical history. 

Planetary nebulae (PNe) offer the opportunities 1) to study stellar nucleosynthesis in the advanced phases of stellar evolution of stars in the wide mass range - -O. S to Mq and 2) to probe radial and as well horizontal/vertical chemical gradients in spiral galaxies by the time of formation of their progenitors. 

This relationship has been predicted by the stellar evolution theory, as resulting essentially from the so called first and second dredge-up episodes. In Fig. la we see the corresponding predictions by the theory (Marigo, 2001). The sensitivity of the predicted abundances both to the metallicity at the epoch of formation of the progenitors and to their stellar mass in the Main Sequence is evident, making such diagrams powerful tools to trace back the evolutionary history of individual PNe. Uncertainties prevent however us to pursue now this... 

We have observed 3He towards several PNe that have been selected to maximize the likelihood of 3He+ detections. First epoch observations with the GBT are discussed in [3]. Figure 2 shows a 4 a detection towards the PNe J 320 with the VLA. Both of these results are consistent with 3He/H abundances between 10 4 — 10 3 by number and standard stellar evolution models. Observations with Arecibo are planned for winter 2005. Our goal is to be able to make a connection between some of the selection criteria and a high 3He abundance. In this way we can use subsidiary measures, e.g., the N abundance, to estimate what fraction of PNe have preserved their 3He. 

The correlations and anti-correlations among these elements for stars in GCs and the range of variation of each element resemble those of proton-burning. They appear to be independent of stellar evolutionary state, with the exception that enhanced depletion of C and of O is sometimes seen just at the RGB tip. This extra depletion of O just near the RGB tip is seen in our M13 data shown (see also Sneden et al 2004). Metal poor halo field stars, however, show no evidence for O burning (Gratton et al 2000) or Na enhancement. The variations seen in the field stars are much closer to those predicted by classical stellar evolution that those seen in the GC stars. 

M. Asplund, N. Grevesse, A. Jacques Sauval The solar chemical composition . In Cosmic abundances as records of stellar evolution and nucleosynthesis, ed. by F.N. Bash, T.G. Barnes (ASP San Francisco 2005), in press... 

McWilliam, Smecker-Hane, Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis ASP Conference Series, Ed. Bash and Barnes, (2005)... 

Abstract. We have performed the chemical analysis of extragalactic carbon stars from VLT/UVES spectra. The derived individual abundances of metals and s-elements as well as the well known distance of the selected stars in the Small Magellanic Cloud and the Sagittarius dwarf galaxies permit us to test current models of stellar evolution and nucleosynthesis during the Asymptotic Giant Branch phase in low metallicity environments. 

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. 

We have now to go one step further and to build stellar evolution models where the transport of angular momentum will be followed self-consistently under the action of meridional circulation, shear turbulence, and internal gravity waves. In this path some important aspects still need to be clarified Can we better describe the excitation mechanisms and evaluate in a more reliable way the quantitative properties of the wave spectra What is the direct contribution of 1GW to the transport of chemicals, especially in the dynamical shear layer produced just below the convective envelope by the wave-mean flow interaction What is the influence of the Coriolis force on IGW How do 1GW interact with a magnetic field Work is in progress in this direction. 

How much the ages of young PMS object depend on the starting stellar evolution model Baraffe et al. (2002) show that the first million year(s) are very uncertain for low mass stars (the statement refers to masses O.IMq or smaller), as they... 

Practically all sophisticated stellar evolution models predict the existence of processes altering photospheric abundances on long timescales (see e.g. Pinson-neault, these proceedings). For example, Richard et al. [6] predict iron abundances in turnoff stars of NGC 6397 to be lower by 0.2 dex than in red giants. 

Recent research now concentrates on the more physical models involving theories of rotation. The long-term aim of these attempts are to provide fully self-consistent models which include stellar evolution, rotation, transport of angular momentum and of chemical species. The key players in this field are P. Denis-senkov [11,12] and C. Charbonnel and coworkers (for their approach, see the contributions by Charbonnel and Palacios in this volume). Both groups employ the theoretical description of rotation by Zahn and Maeder [27,19]. 

D.D. Clayton Principles of stellar evolution and nucleosynthesis, 2nd edn. (Univ. of Chicago Press, Chicago 1983)... 

Red giant stars, both in the field and in globular clusters, present abundance anomalies that can not be explained by standard stellar evolution models. Some of these peculiarities, such as the decline of 12C/13C, and that of Li and 12C surface abundances for stars more luminous than the bump, clearly point towards the existence of extra-mixing processes at play inside the stars, the nature of which remains unclear. Rotation has often been invoked as a possible source for mixing inside Red Giant Branch (RGB) stars ([8], [1], [2]). In this framework, we present the first fully consistent computations of rotating low mass and low metallicity stars from the Zero Age Main Sequence (ZAMS) to the upper RGB. 

The matter that made up the solar nebula from which the solar system was formed already was the product of stellar birth, aging and death, yet the Sun is 4.5 billion years old and will perhaps live to be 8 billion years but the Universe is thought to be 15 billion years old (15 Gyr) suggesting that perhaps we are only in the second cycle of star evolution. It is possible, however, that the massive clouds of H atoms, formed in the close proximity of the early Universe, rapidly formed super-heavy stars that had much shorter lifetimes and entered the supernova phase quickly. Too much speculation becomes worrying but the presence of different elements in stars and the subsequent understanding of stellar evolution is supported by the observations of atomic and molecular spectra within the light coming from the photosphere of stars. 

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