Red giants are evolved from main-sequence stars with masses in the range from about 0.3 M ☉ to around 8 M ☉. When a star initially forms from a collapsing molecular cloud in the interstellar medium it contains primarily hydrogen and helium with trace amounts of “metals” (in stellar structure this simply refers to any element that is not hydrogen or helium i.e. atomic number greater than 2).
If the star s mass when on the main sequence was below approximately 0.4 M ☉ it will never reach the central temperatures necessary to fuse helium. p. 169. It will therefore remain a hydrogen-fusing red giant until it runs out of hydrogen at which point it will become a helium white dwarf. § 4.1 6.1.
A neutron star is the collapsed core of a massive supergiant star which had a total mass of between 10 and 25 solar masses possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white holes quark stars and strange stars) neutron stars are the smallest and densest currently known class of stellar objects.
If the star s mass when on the main sequence was below approximately 0.4 M ☉ it will never reach the central tempera…