The smallest stars will outlive the current age of the universe many times over
A massive, hot O-type star leaves its prime after only a few million years. A tiny red dwarf of a tenth the Sun's mass may keep fusing hydrogen for six to twelve trillion years. Since the universe is only about 13.8 billion years old, no star that small has ever died.
No one can watch a single star age; the changes are too slow even over centuries. Instead astrophysicists survey many stars at different stages and model their interiors on computers. The story starts with a giant molecular cloud, around a hundred light-years across, that collapses and splinters. Each fragment heats as it falls in until it forms a spinning ball of very hot gas, a protostar, cloaked in dust and best seen in infrared; the WISE survey has uncovered many of them.
Mass decides everything. Below roughly 0.08 solar masses, a protostar never gets hot enough to fuse hydrogen and becomes a brown dwarf, glowing faintly and cooling over hundreds of millions of years. Heavier ones reach 10 million kelvin in the core, when hydrogen begins fusing into helium. Outward pressure from that energy balances gravity, and the star settles on the main sequence. The Sun, a mid-sized yellow dwarf, is roughly halfway through a ten-billion-year stay there.
When the core's hydrogen runs out, fusion moves to a shell around it and the star swells through the subgiant stage into a red giant such as Aldebaran or Arcturus. Stars above about half a solar mass can then fuse helium, and heavier stars burn successively heavier elements in nested shells. The smallest red dwarfs skip the giant phase entirely, because their interiors churn from top to bottom, gradually turning almost all their hydrogen into helium.
Endings also split by mass. A Sun-like star eventually sheds its outer layers as a planetary nebula, leaving a dense white dwarf. Stars of about ten solar masses or more can explode as supernovae when their iron cores collapse into a neutron star or black hole.
Source: Stellar evolution
