Stars luminosity

The observed terminal velocities of O-stars provide interesting information about the evolution of massive stars. This is shown in Fig. 5, which is taken from Garmany and Conti (1985) and displays vM vs. Tef for a sample of O-stars (luminosity classes between V and III) in the Galaxy, LMC and SMC. Two striking facts can be read off from Fig. 5 ... 

Brunhilde, of her own volition, displays figure 5.2, which shows the same kind of information as figure 5.1, with an emphasis on stars luminosities in comparison to the Sun s luminosity. 

In this chapter we also discussed the Cepheid variables, stars whose periods (the time for one cycle of dimness and brightness) are proportional to the stars luminosity. Using the distance modulus formula, this luminosity can be used to estimate interstellar and intergalactic distances. The American astronomer Henrietta Leavitt (1868-1921) discovered the relationship between period and luminosity in Cepheid variables (figure 7.9). I think of her on par with the... 

LBVs. In plots relating the N/O mass fraction to the He mass fraction, Esteban et al. (1992) find that SE nebulae lie close to the stellar evolution tracks of Maeder (1990) for initial masses 25 - 40 M0, which become WN stars after a red supergiant (RSG) phase. This is consistent with the initial masses estimated from the star luminosities (Esteban et al. 1993). 

Figure 1.2 The Hertzsprung-Russell diagram for stars with known luminosities and spectra.

Henrietta Leavitt, Harvard College Observatory, 1912, established the relation period-luminosity of Cepheid variable stars. Being a woman i.e. confined to the tedious and ridiculously paid work of examining photographic plates for the benefit of an all-male establishment, she would not be awarded credit for her milestone discovery... 

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. 

Fig. 10 from our study of a large sample of stars in M5 (Cohen, Briley Stetson 2002) (not reproduced here due to limits of length) is typical of the GCs studied thus far in such detail. It shows a strong anti-correlation between C and N abundances, i.e. conversion of C into N, with strong-to-star variations in derived C and N abundances seen at all luminosities probed. (This sample contains mostly stars at the base of the RGB and just below the main sequence turnoff,... 

Since most (if not all) low-metallicity objects that are currently observed in the halo are not in the AGB phase, material enriched in carbon and the s-process elements is assumed to have accreted from the companion AGB stars, which have already evolved to faint white dwarfs, to the surface of the surviving companion. This scenario is the same as that applied to classical CH stars [4], Unfortunately, long-term radial velocity monitoring has been obtained for only a limited number of objects a clear binarity signature has been established for six objects in our sample to date. However, there exists additional support for the mass-accretion scenario for the Ba-rich CEMP stars. Fig. lb shows [C/H] as a function of luminosity roughly estimated from the effective temperature... 

Fig. 1. Various color-magnitude diagrams for NGC 1851 obtained with uvby filters at the Danish 1.54m telescope on La Silla. Seven stars in our sample have previous low-resolution spectroscopy from [2] which classified them into CN strong (open squares) and CN normal (plusses) groups. Note how the CN strong stars stand out clearly from the cluster sequences when using filter combinations involving the u and v filters. We see from the lower righthand panel that the RGB stars in this cluster also show a large scatter in the mi index at a fixed luminosity - this is the only cluster in our sample of 20 which show mi scatter. This points to very large C variations (larger than for other clusters). Could this be related to the bimodality of the cluster horizontal branch ...

The age at which Li depletion occurs increases with decreasing mass (and Li-burning temperatures are never reached for M < 0.06 M0). As luminosity, L oc M2 for PMS stars, the luminosity at which complete Li depletion takes place is therefore a sensitive function of age between about 10 and 200 Myr [6]. This relationship depends little on ingredients of the PMS models such as the treatments of convection and interior radiative opacities because the stars are... 

Since the absolute luminosities of the stars are unknown, log g has been taken as a first order indicator of the luminosity, and we could show that generally speaking the unmixed stars belong to the low RGB and the mixed stars are on the Horizontal Branch or above (Fig. 3-a). 

Scl is a close companion of the Milky Way, at a distance of 72 5 kpc [7], with a low total (dynamical) mass, (1.4 0.6) x 107Mq [8], and modest luminosity, My = —10.7 0.5, and central surface brightness, Soy = 23.5 0.5 mag/arcsec2 [9] with no HI gas [10]. CMD analysis, including the oldest Main Sequence turnoffs, has determined that this galaxy is predominantly old and that the entire star formation history can have lasted only a few Gyr [11]. 

The chemical analysis has revealed that rather low C/O ratios are found in metal-poor extragalactic carbon stars, as found for galactic carbon stars of the solar vicinity. Furthermore, the three analyzed stars show similar s-elements enhancements [ls/Fe]=0.8-1.3 and [hs/Fe]=l.l-1.7. This leads to new constraints for evolutionary models. For instance, the derived C/O and 13C/12C ratios are lower than model predictions at low metallicity. On the contrary, theoretical predictions of neutrons exposures for the production of the s-elements are compatible with observations (see Fig. 1). Finally, from their known distances, we have estimated the luminosities and masses of the three stars. It results that SMC-B30 and Sgr-C3 are most probably intrinsic carbon stars while Sgr-Cl could be extrinsic. 

The accretion history of a parent galaxy is constructed using a semi-analytical code. The full phase-space evolution during each accretion event is then followed separately with numerical simulations [2]. Star-formation and chemical evolution models are implemented within each satellite. The star formation prescription matches the number and luminosity of present-day galaxies in the Local Group, whereas the chemical evolution model takes into account the metal enrichment of successive stellar populations as well as feedback processes. Below we present results of a sample of four such simulated galaxy halos, denoted as Halos HI, H2, H3 and H4. 

The uncertainty in the age of pre main sequence stars is therefore of the order of the thermal timescale at the luminosity of D-burning smaller than a few times 105 yr for normal T Tauri, and larger than 106 yr for very low mass stars and brown dwarfs (BD). In fact, comparing observations spanning a wide range of masses we could even constrain the models, for example we can ascertain whether the Stahler et al. (1986) picture of collapse is valid also in the BD regime, or... 

The luminosity of a star, L, is the total rate at which energy is radiated by the black body over all wavelengths. Assuming that the star is a sphere, the total surface area is given by A = 4ttR2, where R is the radius of the star, then the luminosity of the star is given by ... 

Now that we have a simple model for the continuum spectrum of the stars based around the Planck curve, the temperature and the luminosity, we can make some observations and classifications of the stars. There are some constellations that dominate the night sky in both the northern and southern hemispheres and even a casual look should inspire wonder. Star hopping in the night sky should lead to the simplest observation not all stars have the same colour. A high-quality photograph of the constellation of Orion (see page 2 of the colour plate section) shows stars... 

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