Stellar light

Dust grains act like stones in the desert. They accumulate heat and restore it to the medium in the form of infrared radiation. They are intermediaries between light from stars and interstellar gas, for they absorb stellar photons in a most efficient manner. This is why these clouds appear so dark in photographs. In fact, they shine in the infrared. The dust strewn across the Galaxy trades the big money of stellar light for the small change of the infrared. 

The temperature of dust grains is a measure of the equilibrium between heating by absorption of stellar light and re-emission in the far IR. In free... 

Figure 3. The spectral energy distributions of various galaxy components (starlight, warm dust, and active nuclei) at the IRAC wavelengths. The IRAC bandpasses are shown at the bottom of the figure. Warm dust emitting in the PAH lines, and active nuclei, both appear red in the IRAC bandpasses, while stellar light appears blue. The presence of CO absorption in the 4.5 pm bandpass also results in cool, late-type stars having a bluer color [3.6] — [4.5] than earlier-type stars like Vega.

In the case of giant H II regions, where the observing slit encompasses stellar light, one must first correct for the stellar absorption in the hydrogen lines. This can be done in an iterative procedure, as outlined for example by Izotov et al. (1994). 

Figure 17.2 shows the relative abundance of the elements of the universe and of the earth. The abundances are approximate, as a consequence of die difficulties in their assessment and limitations of experimental techniques. The abundances in the universe (based on spectral measurements on stars and interstellar matter) are used as a refinement of data obtained for the solar system. Stellar light is divided in spectral classes depending on the surface temperature of the star, see Fig. 17.1. The various classes (Harvard Spectral Classification) show lines of the elements as listed below in approximately decreasing intensity ...

Different strategies exist to characterize a planet s atmosphere direct detection resolves the planet and star individually, and transmission as well as secondary eclipse measurements subtract the stellar light fi om a combined star-planet detection. For directly imaged planets, in the visible part of the spectrum, we observe the starlight, reflected off the planet in the thermal IR we observe the planet s own emitted thermal flux. An Earth-like, temperate planet is a very faint, small object close to a very bright and large object, its parent star. 

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... 

Big efforts have been devoted in the last years to the study of light elements abundances. Definitively their importance is strongly related to cosmology as well as to stellar structure and evolution. In fact hints on the primordial nucleosynthesis can be achieved from Li, Be and B primordial abundances. Moreover these studies can be a precious tool for testing and understanding the inner stellar structure, especially for what regards the mixing processes in stellar envelopes [11-... 

The THM has been recently applied to several reactions whose cross section is crucial for the study of light element abundance in stellar environments. In particular the reactions 6Li(p, a)3He, 7Li(p, a)4He, 9Be(p, a)6Li and 11B(p, a)sBe were studied and the corresponding bare nucleus cross sections were measured. An exhaustive discussion of the experimental results is reported in references [4-7] respectively. 

In the next sections I will present the main results from these recent Li and Be datasets. In particular, I will address the following questions 1. What are the timescales of Li depletion 2. Is the Sun typical 3. What parameter drives Li depletion at old ages 4. Do stars deplete Be at the same rate as Li Whereas I will mostly focus on stars with masses close to solar, I mention in passing that light element depletion is also strongly dependent on stellar mass. 

Abstract. The observations of light elements (Lithium and Beryllium) in Globular Cluster (GC) stars are reviewed. Light element observations in GC are very powerful tracers of mixing processes in the stellar interior and shed new light on the GC formation history. 

Ten years after the first ESO Workshop on light elements, and five years after the IAU in conference in Natal (Brazil) on the same subject, we felt that it was time to have a new conference on this topic a wealth of new observational data have become available from the 8-meter telescopes and in particular from the VLT, and, at the same time, in the last years new theoretical roads have also been inaugurated in the interpretation of the stellar data. It soon became clear, on the other hand, that in order to understand the evolution of the fragile Li and Be in stars and in the Galaxy, the whole problem of internal mixing in stars must be addressed and understood first, and therefore a large number of elements must be investigated, in many different environments. 

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. 

Stellar evolution has consequences in the development of luminosity and colours of stellar populations, as well as chemical enrichment. Boissier and Prantzos (1999) have produced a more-or-less classical model of the evolution of the Milky Way, making a detailed study of this aspect, known as chemo-photometric evolution , using an IMF similar to the Kroupa-Scalo function in Chapter 7 this detail is significant because the Salpeter(O.l) function often used has a smaller contribution from stars of around solar mass which dominate the light at late times. The chemical evolution results are combined with metallicity-dependent stellar isochrones, synthetic stellar spectra by Lejeune et al. (1997) and a detailed treatment of extinction by dust. Some of their results are shown in Fig. 8.39. 

The light elements (D to B, apart from 4He) have such fragile nuclei (see Table 9.1) that they tend to be destroyed, rather than created, in thermonuclear burning, although certain special processes can lead to stellar production of 3 He, 7Li and nB. 

Table 9.1. Destruction of light nuclei in stellar interiors...

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). 

The existence of outsized hydrogen atoms was inferred early on from the observation that 33 terms of the Balmer series could be observed in stellar spectra, compared to only 12 in the laboratory [58]. More recently [59] Rydberg atoms have been produced by exciting an atomic beam with laser light. When the outer electron of an atom is excited into a very high energy level, it enters a spatially extended orbital which is far outside the orbitals... 

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