10.3 Life Cycle of Stars

Notes

• A nebula (gas and dust) collapses into a protostar; when the core reaches about 10 million K, hydrogen fusion starts and a main-sequence star is born.
• Fusion balances gravity. When core hydrogen runs out, the star expands into a red giant (or supergiant).
• Low and medium mass stars (like the Sun): shed outer layers as a planetary nebula, leaving a white dwarf.
• High mass stars: fuse heavier elements up to iron, then collapse in a supernova, leaving a neutron star or black hole.
• Massive stars live shorter lives: millions of years versus about 10 billion for the Sun.