White Dwarfs

Abstract

It is the intention of this review to explain what white dwarfs are and why it is interesting to study them, and why the H + 2 molecule is of special interest. The evolution, from start to finish, of a star of mass less than about 2 solar masses (M ⊙ ), can roughly be summarized as follows: – A cloud of gas contracts from the interstellar medium until hydrogen ignites at the center and a main sequence (MS) star forms. H is transformed to He and the MS phase continues until H is exhausted in the stellar core. – H continues burning in a shell outside the He core while the core contracts. He “ashes” are added to the core, and a red giant star is formed as the envelope expands. The star evolves up the Red Giant Branch (RGB) (i.e. it becomes more and more luminous and the surface cools). – Towards the end of the RGB phase, mass-loss from the upper layers increases until helium to carbon burning in the core ignites suddenly under degenerate conditions – this is called the Helium Flash (HF). The HF terminates the RGB evolution, and therefore also the mass-loss and the growth of the stellar core. – The star readjusts its structure and the He-core burns steadily on the horizontal branch (HB) (a phase of nearly-constant luminosity) until fuel is exhausted in the He-core. –Then the C/O core contracts anew and the expansion of the envelope, and the growth of the core, during He-shell burning, mimics RGB evolution but relatively little mass is added to the core this time.–The second ascent of the giant branch (the so-called Asymptotic Giant Branch, or AGB) continues with increased mass loss towards the end– Rapid detachment of a considerable fraction of the remaining envelope and the hot core takes place, sometimes observable as the Planetary Nebulae (PN) phase. –The PN is dispersed as the core contracts to a white dwarf (WD).–The WD cools for a long time, as internal kinetic energy and latent heat is released.