A Dim Future for Galaxies
Galaxies will produce new generations of stars as long as their stores of hydrogen last. However, this fuel tank is not infinite. As stars continue to process hydrogen into helium and heavier elements, fewer stars will form. The hydrogen supply will dwindle after about 10 trillion to 100 trillion years, and galaxies in the cosmos will gradually fade. The longest-lived stars in the universe, tiny red dwarfs that ration their nuclear fuel, also will shine dimly about that long. Thereafter, the sole denizens of galaxies will be ultracompact objects: cooling white dwarfs, neutron stars, and black holes. Collisions between these bodies may release bursts of light, but by and large, galaxies will be invisible.
Fade to Black
Astronomers who have gazed into the very distant future see a dark universe indeed. About one googol--1 followed by 100 zeros--years from now, the last black holes will evaporate through Hawking radiation. After each black hole shrinks to the size of an atom, it will vanish in a flash of gamma rays that briefly illuminates the darkness (upper right). Then the cosmos will consist of isolated photons, electrons, positrons, and neutrinos (foreground), drifting through space and separated by immense distances comparable to the size of our visible universe today.