Helium ignition

If we compare the HRD position of the progenitor to evolutionary tracks (keeping in mind that the luminosity is nearly constant after helium ignition) we see that the progenitor had a mass between 15 and 20 M0 (Fig.l). Depending on the inclusion of overshooting in the model computations, the mass at explosion may vary from 12.5 Mg (overshooting) to 17.5 M0... 

The role of such convective dredge-up of matter in the red-giant phase of evolution of 1-IOMq stars is now understood to be an extremely complex process (Busso et al., 1999). On the first ascent of the giant branch (prior to helium ignition), convection can bring the products of CNO cycle burning (e.g., and... 

Very high-mass stars with initial masses greater than 20 M will ignite helium ignition while the star is still a blue supergiant. Their extremely high luminosities lead to a radiation-driven stellar wind and substantial mass-loss. The result is likely a Wolf-Rayet star, followed by a core-collapse supernova. 

In low-mass stars with initial masses less than about 3 M , helium-ignition occurs in a degenerate core - usually when the helium-core mass has reached about 0.48 M . The star subsequently evolves as a horizontal-branch star (Sect. 14). With a sufficiently massive hydrogen envelope (the exact value depends strongly on metallicity), it will evolve up the asymptotic giant branch after core helium exhaustion. Otherwise it will evolve directly to the white-dwarf cooling track. 

Stars less massive than about 8 M0 will avoid the supernova fate. In the 3-8 Mq range, helium ignition occurs in a non-degenerate core, so is not explosive. Core helium burning is associated with a blueward loop through the Cepheid instability strip, after which the star develops a double shell structure and becomes an asymptotic giant branch star. 

EHB and sdB stars pose challenges for stellar evolution theory in how to account (a) for their very low hydrogen-envelope masses and (b) for the range of surface helium abundances. Specifically, what mechanism removes almost the entire hydrogen-envelope from a red-giant star How does it still achieve helium ignition And how should we explain the existence of helium-rich sdB stars ... 

When the mass of the helium-layer accreted into the low-entropy core is sufficiently high, helium ignition will occur at the boundary between the non-degenerate and degenerate material. The star will expand on the short thermal timescale of the envelope ( 103 y) to become a yellow supergiant -a stage which will be described in more detail in Sect. 15.7. 

The evolution of a star formed from the merger of two helium white dwarfs considered a 0.4 M0 helium white dwarf accreting helium at approximately half the Eddington accretion rate (lO 5M0yr 1 [62]). Helium ignites at the core-envelope boundary when 0.067 M0 has been accreted and stars expands to become a yellow supergiant in 103 yr. The accretion is switched off artificially once a pre-selected final mass has been reached, whereupon the star evolves towards the helium main sequence as previously described (Sect. 14.5). 

Fig. 3. Pressure required for ignition of mixtures of acetylene and a diluent gas (air, oxygen, butane, propane, methane, carbon monoxide, ethylene, oil gas, nitrogen, helium, or hydrogen) at room temperature. Initiation fused resistance wire. Container A, 50 mm dia x 305 mm length (73) B,...

The feed composition chosen was 6 mol% n-hexane, 6 mol% ammonia, 12 mol% oxygen and remainder helium, with an overall gas residence time of 2.5 s. Due to the low temperature of n-hexane self-ignition (T 234°C), a relevant contribution of homogeneous, radical reactions was expected. Tests made in the absence of catalyst... 

The reactive metal is pyrophoric in air at elevated temperatures lathe turnings have ignited at 265°C and 140-mesh powder at 135°C [1]. This most extremely toxic metal may be worked under nitrogen, argon, or helium (but not carbon dioxide) at temperatures above 600° C [2],... 

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