Question

Suppose that star formation stops today, everywhere in the universe.

a. At what time t_die will the last stars die out?

b. What will be the scale factor a(t_die) at that time?

Answer 1

Well their is no actual end to a star's life so stars will never die. Most of them will end up being black dwarf stars, a white dwarf star that has cooled down and no longer commits heat or light. The majority of the stars will become black dwarfs, some will become black holes though. So assuming a black dwarf is the end of a star life, we can now determine how long it will take for every star to get to the end.

So lets take the beginning stage of a star's life, a nebula. Now we want to find the youngest nebula because that will be the last star to brcome a black dwarf.

So for a nebula to become a black dwarf would take around 20-30 billion years. Seems like a big number? Well the white dwarf cooling down into a black dwarf takes over 10 billion years!