Helium main sequence

Fig. 3.37. Rough locations of chemically evolved and peculiar stars in the HR diagram. Full lines show the ZAMS with stellar masses indicated and evolutionary tracks for 0.8, 3 and 15 Af , the helium main sequence (0.5 to 1.5 A/ ),...

Figure 3 Comparison between observations and evolutionary tracks applicable for extremely helium-rich stars. The helium main sequence (HeMS) is labelled with stellar masses, HL is the Hayashi limit. The hatched line indicates the Eddington limit for pure helium composition. Evolutionary tracks are from Paczynski (1971 dashed lines labelled with M/MQ), Schonberner (1977, M = 0.7 MQ, full drawn line) and Law (1982 M = 1 Mq, dotted line). Stellar symbols are the same as in Fig. 1. 

In fact, setting Mh = 0 leads us to the helium main sequence, which is equivalent to (and roughly parallel with) the hydrogen main sequence. 

Extreme horizontal-branch (EHB) stars are horizontal-branch stars with a hydrogen envelope whose mass is insufficient to sustain hydrogen burning . e. the H-burning shell is inert. They can be thought of as helium main-sequence stars with a very thin hydrogen envelope (0.0001 -C Mh/Mq -C 0.01). 

The sdO classification causes some confusion because class members span a range of 2 dex in surface gravity (and hence in luminosity-to-mass ratio). PG classifications do not demonstrate this, but a finer scheme is more successful [154]. Higher-gravity stars lie close to the extreme horizontal-branch / helium main-sequence. A link with the He-sdB stars seems likely (Fig. 33). 

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). 

In fact, the sun is not a first-generation main-sequence star since spectroscopic evidence shows the presence of many heavier elements thought to be formed in other types of stars and subsequently distributed throughout the galaxy for eventual accretion into later generations of main-sequence stars. In the presence of heavier elements, particularly carbon and nitrogen, a catalytic sequence of nuclear reactions aids the fusion of protons to helium (H. A. Bethe... 

PN nucleus, horizontal-branch and white-dwarf regions. The dotted line shows a schematic main sequence and evolutionary track for Population II, while various dashed lines show roughly the Cepheid instability strip, the transition to surface convection zones and the helium-shell flashing locus for Population I. After Pagel (1977). Copyright by the IAU. Reproduced with kind permission from Kluwer Academic Publishers. 

Each star follows a different path and at a different rate. Ageing stars turn red, except for the most massive, which become violet or even ultraviolet, gradually moving away from the main sequence. Their core temperatures and pressures increase, thereby triggering further nuclear reactions which can build carbon from helium as the stars ascend the giant branch. The construction of nuclear species in massive stars reaches its apotheosis in the explosion of type II supernovas. 

The synthesis of helium follows a somewhat indirectpath. The longest part is the first, because it involves the transformation of a proton into a neutron, and such transmutations proceed via the weak interaction (slow at these temperatures). The leisurely pace of these reactions confers long life upon main-sequence stars. 

Figure 3.10 shows the abundances of hydrogen, helium, and the CNO isotopes as a function of stellar radius in a 1.5 M star at the end of the main sequence. The horizontal axis represents the position within the star (in units of mass, a rather quirky astrophysics convention). The center of the star is on the left and the surface is on the right. The lines on the diagram show the mass fraction of each isotope at each position inside the star. For example, at the surface of the star, most of the mass consists of hydrogen, helium is the next most abundant, followed by 1 , 12C, 14N, and so forth. The starting composition of this hypothetical star, and the composition that remains at the stellar surface, is the same as that of our Sun. 

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