Galactic evolution

Studies of galactic evolution have focused on the comparison between the atomic (HI) and molecular (H2) gas properties and star formation rates as a function of environment, luminosity, and galaxy type. The general conclusions from these studies are as follows  

The sum of the galactic atomic and molecular masses range from 10 to 5 x 10 Mq. 

The ratio of molecular to atomic gas mass Mn /Afui decreases as a function of morphological type for 

The ratio of total neutral gas mass to dynamical mass 

The key word in modern theory is evolution . The impressive consistency of the astro-nuclear view of the heavens has established the idea of an evolution of nuclear species which has the same significance for astrophysics as the evolution of living species for biology, it is itself preceded by an evolution of particle or corpuscular species, which would have been very short, lasting iess than i second. This process was of a quite crucial nature in determining the components available to buM up atoms, that is, those stable particles, protons and neutrons, that serve as the buMing-blocks, and the forces that bind them together. 

Note that I do not say infinitely small , for there are things smaller than atomic nuclei, namely elementary particles. There are also things larger than the astronomical scale of stars and galaxies that concerns us here. 

This conclusion eliminates Gamow s proposal according to which the temperature and pressure at the very beginning of the Universe would have been sufficient to produce elements beyond lithium. The hard reality of observations thus put paid to one of the most beautiful theories, attributing a common origin to all the elements. 

The trail followed by each star is inked onto the parchment of the HR diagram according to its mass and, to a lesser extent, its metalhcily at birth. Each position on a particular evolutionary trail corresponds to a specific cycle of thermonuclear fusion. Every star follows a different path and at a different rate. 

The construction of nuclear species in massive stars reaches its apotheosis in the explosion of supernovas. 

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It is remarkable that chemical abundances are a valid, and relatively robust, tracer of galactic evolution it is worth considering why this is possible. 

In my talk at this meeting I actually avoided discussion of 3He, until it was forced upon me during the question session. In part, this was because this subject was ably covered in Tom Bania s talk, in Dana Balser s poster, and in Bob Rood s conference summary. In part, however, it was because, in my opinion, for both observational and theoretical reasons 3He has more to teach us about stellar and Galactic evolution than about BBN. 3He is a less sensitive baryometer than is D since (D/H)BBN oc 7-1 6, while (3He/H)BBN oc r] 0-6. Even more important... 

We have learned many things about Gamow s tralphium since 1949. A personal selection includes (1) the abundance of 3He has not changed significantly over 14 Gyr of Galactic evolution, which is remarkable (2) it has not changed not... 

Fig. 4. The Magellanic stream, reminding us that continuing inflow of partially enriched gas is a significant factor in Galactic evolution.

For purposes of comparison with stellar abundances, it is useful to have the relative contributions of s- and r-processes to the various elements (as opposed to nuclides) in the Solar System, because in most cases only element abundances without isotopic ratios are available from stellar spectroscopy. At the same time, elements formed in one process may often be expected to vary by similar factors in the course of stellar and Galactic evolution, but to be found in differing ratios to elements formed in another process. Relative contributions are listed for some key elements in Table 6.3. 

Stars drive galactic evolution. At the end of their existence, they inseminate space with the products of their nuclear alchemy. Then, in dark clouds, sheltered from ravaging photons, molecules build up. Stars and planets are constantly being born in the cold of space. 

Given the available data, galactic evolution can be divided into two eras, the halo era (metallicity between one ten-thousandth and one-tenth of solar values) and the disk era. The corresponding epochs are 1 and 10 billion years long, respectively. Concerning the history of the halo, two consecutive evolutionary... 

The study of galactic evolution thus comprises two aspects. One is of an observational nature, wherein abundances are assessed in stars of various generations, by means of spectral analysis. The other is purely theoretical, involving numerical simulation of the chemical evolution of the interstellar medium in which stars are born. 

Rather unexpectedly, a striking similarity is observed between proportions of r-process elements measured in a handful of ancient stars and the Sun. The hallmark of the r process thus appears very early on, indicating the operation of a rapid and efficient process in the very first stages of galactic evolution. 

These are therefore the main hues of research into galactic evolution. However, stellar archaeology is still in its early stages. In future years it will obtain a signihcant boost from the VLT and its high-quality spectrographic observations, to become eventually a major branch of astronomy. 

It is known that the oxygen abundance in the interstellar medium increases all the time this nucleus is produced by type 11 supernovas which, one after the other, also contribute their iron production to the Galaxy (Fig. 8.7). The pO mechanism is thus likely to grow in importance as the Galaxy evolves. In other words, clues to the Op mechanism should be sought in the early phases of galactic evolution, that is, in halo stars. The fact remains that the two mechanisms induce different evolution in beryllium and boron as a function of oxygen. 

The great cycle of inspiration and expiration of matter, passing through condensation, nucleosynthesis, ejection and back to condensation, serves as a bellows to fire today s studies of galactic evolution. We owe this understanding to Fred Hoyle, present at every stage in the edification of our astro-nuclear doctrine. 

The model assumes that evolution takes place in a closed system, with successive generations of stars being bom into the interstellar medium. At each generation, a fraction of the gas is transformed into metals and returned to the interstellar medium. Gases imprisoned in stars of low mass and compact residues play no further role in galactic evolution. In this model, metaUicity is bound to increase as time goes by. And so the arrow of galactic time is defined. Evolution will continue until no further gas is available to form new stars. 

Apart from the lifetime of stars as a function of their initial mass, the galactic evolution kit contains the following three items ... 

The whole art in the study of galactic evolution is to put forward a model that relates to available data, taking their volume and accuracy into account. Consequently, the evolutionary model must itself be conceived in an evolutionary way. 

The calculation is in principle quite straightforward, but the parameters are uncertain, as we mentioned earlier. For this reason, the theory of galactic evolution should not be viewed as a mature theory, but rather as a realistic scenario. Running the history of the Milky Way up to the present epoch, we can follow through the behaviour of gas, stars and metals, which are dialectically related. From this great material adventure, the following trends are singled out ... 

The Caabundance has been studied in stars by optical spectra. Because 40 Ca dominates the spectra of Ca in stars, and especially because 4°Ca is a primary alpha nucleus, the galactic evolution of 4°Ca is the same as the observed galactic evolution of calcium elemental abundance. As nucleosynthesis progressed in the galaxy the Ca/H abundance... 

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