Stellar Evolution: The Life Cycle of Stars

Stars, like people, have a life cycleLife cycle. They are born, they age, and finally they die. But stars take a lot longer than people to go through their cycle of life: our Sun, for example, is a typical, middle-age star and yet is already nearly five billion years old. And just as we must deal with gravity in our everyday lives, gravity is the primary driver of stellar evolutionStellar evolution, and therefore a star’s mass is its most important property governing its structure and evolution. Stars are born when gravity compresses interstellarInterstellar clouds of dustDust and gas to temperatures and densities high enough to sustain the thermonuclearThermonuclear fusionFusion of hydrogen into heliumHelium, converting some of the original mass into energy in accordance with Einstein’sEinstein, Albert famous equation, E = mc2 = . Half the stars in the Universe, those having no more than half the mass of the Sun, end their lives as burned-out, EarthEarth-size heliumHelium white dwarfsWhite dwarf. The stronger gravity of more massive stars compresses their central regions to the higher temperatures required to fuse the heavier elements necessary for building the stuff of life before ejecting most of this “stardustStardust,” either quiescently as stellar windsStellar winds and planetary nebulae or explosively as supernovaeSupernova and kilonovaeKilonova, into the surrounding interstellarInterstellar medium—a star’s loss but life’s gain—leaving behind a stellar graveyard littered with white dwarfsWhite dwarf, superdense neutronNeutron stars the size of a small city observed as pulsarsPulsar, each having a mass comparable to the Sun, and black holesBlack hole, the ultimate death star in which gravity is totally triumphant and so strong that not even light can escape them. The properties of each are discussed further in relation to our cosmic connectionCosmic connection.