In conversation on this subject (as I recall it), I asked Pickering about certain other faint stars, not on my list, mentioning in particular 40 Eridani B. This piece of apparently routine work proved very fruitfulâit led to the discovery that all the stars of very faint absolute magnitude were of spectral class M. With characteristic kindness, he had volunteered to have the spectra observed for all the starsâincluding comparison starsâwhich had been observed in the observations for stellar parallax which Hinks and I made at Cambridge, and I discussed. I was visiting my friend and generous benefactor, Prof. Stars of very low mass will not be able to fuse helium, hence, a helium white dwarf may form by mass loss in binary systems. If the mass of the progenitor is between 8 and 10.5 solar masses ( M â), the core temperature will be sufficient to fuse carbon but not neon, in which case an oxygenâneonâmagnesium white dwarf may form. Usually, white dwarfs are composed of carbon and oxygen. After such a star sheds its outer layers and forms a planetary nebula, it will leave behind a core, which is the remnant white dwarf.
If a red giant has insufficient mass to generate the core temperatures required to fuse carbon (around 1 billion K), an inert mass of carbon and oxygen will build up at its center.
After the hydrogen-fusing period of a main-sequence star of low or medium mass ends, such a star will expand to a red giant during which it fuses helium to carbon and oxygen in its core by the triple-alpha process. This includes over 97% of the other stars in the Milky Way. White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star, that of about 10 solar masses.
0 kommentar(er)
