White Dwarf stars

A hot star at the centre is surrounded by a bright shell of gas that has detached itself from the central star. The gas shines by fluorescence. It absorbs ultraviolet light from the small, hot central star (white dwarf) and re-emits it in the form of visible light. The big banknotes (LTV photons) are exchanged for smaller... 

These nebulas are similar in some respects to the Hll regions. The difference is that here the source of ionisation is an ageing star (white dwarf) in its death throes rather than a strapping young blue star. The fluorescent region is both denser and chemically more complex for it includes those atoms expelled from the envelope of the dying star in the form of a stellar wind. 

White dwarfs are less luminous than Main Sequence stars with the same surface temperature, so they must be smaller than those stars. White dwarfs vary considerably in size, hut they tend to have a radius of about 0.01 that of the Sun and luminosities of 0.01 to 0.0001 that of the Sun. 

BLACK hole NEUTRON STAR WHITE DWARF. 

The Ultimate Fate of a Low Massive Star White Dwarfs... 

The evolution of a. star after it leaves the red-giant phase depends to some extent on its mass. If it is not more than about 1.4 M it may contract appreciably again and then enter an oscillatory phase of its life before becoming a white dwarf (p. 7). When core contraction following helium and carbon depletion raises the temperature above I0 K the y-ray.s in the stellar assembly become sufficiently energetic to promote the (endothermic) reaction Ne(y,a) 0. The a-paiticle released can penetrate the coulomb barrier of other neon nuclei to form " Mg in a strongly exothermic reaction ... 

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... 

The four general types of stars (main sequence, white dwarfs, giants and supergiants) provide a classification based on the fundamental observable properties but also suggest an evolution of stars. Astrochemically, the cooler giants and supergiants have many more atomic and molecular species that are the products of the nuclear fusion processes responsible for powering the stars. The nuclear fusion processes allow for the formation of more of the elements in the Periodic Table, especially the heavier elements that dominate life on Earth - principally carbon. 

Low (<1 solar mass) Middle (5-10 solar masses) High (>20 solar masses) Protostar — pre-main sequence — main sequence — red giant — planetary nebula — white dwarf — black dwarf Protostar - main sequence — red giant — planetary nebula or supernova —> white dwarf or neutron star Protostar — main sequence —> supergiant — supernova — neutron star... 

During the discussion of the HR diagram in Section 4.2 it was noted that 92 per cent of all observed stars fall on the main sequence 7.5 per cent are white dwarf... 

The plot of luminosity versus temperature for all stars, resulting in the main sequence, red giants and white dwarfs. Stellar evolution leads to mass - dependent birth lines onto the main sequence... 

Herzprung-Russell diagram A graphical plot of stellar intensity versus photosphere temperature showing that observed stars fall into classes main sequence, red giants, supergiants and white dwarfs. 

White dwarf The compact remnant of a low-mass star that resists further collapse by the internal degenerate electron gas. 

Middle-sized stars, between about 1 and 8 M , undergo complicated mixing processes and mass loss in advanced stages of evolution, culminating in the ejection of a planetary nebula while the core becomes a white dwarf. Such stars are important sources of fresh carbon, nitrogen and heavy elements formed by the slow neutron capture (s-) process (see Chapter 6). Finally, small stars below 1 M have lifetimes comparable to the age of the Universe and contribute little to chemical enrichment or gas recycling and merely serve to lock up material. 

The result of all these processes is that the Sun was bom 4.6 Gyr ago with mass fractions X 0.70, Y 0.28, Z 0.02. These abundances (with perhaps a slightly lower value of Z) are also characteristic of the local ISM and young stars. The material in the solar neighbourhood is about 15 per cent gas (including dust which is about 1 per cent by mass of the gas) and about 85 per cent stars or compact remnants thereof these are white dwarfs (mainly), neutron stars and black holes. 

Novae and symbiotic stars have shells which are excited by extremely hot stars (several x 105 K) like some PNs, but denser, and display overabundances of elements up to Ne or beyond due to thermonuclear processes affecting matter accreted from a companion by a white dwarf. 

Degeneracy, white dwarfs and neutron stars 5.4.1 Introduction... 

White dwarfs are formed hot and gradually cool at nearly constant radius. With increasing mass, the star becomes squashed down until it is highly relativistic and very small. 

This is the Chandrasekhar-Landau limiting mass for white dwarfs, whose actual value (derivable from the theory of polytropic stars see Appendix 4) is... 

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