06 The Lives of Stars
Trace how stars form, spend their stable years fusing hydrogen, and evolve toward different endpoints according to their mass.
From Cloud to Star
Stars begin in cold, dense regions of interstellar clouds made mostly of gas and dust. When gravity overcomes the pressure supporting part of a cloud, the region contracts and breaks into denser cores. A core gathers material and heats as it contracts, becoming a . At this stage, it is powered mainly by gravitational contraction, not by hydrogen fusion.
If the core becomes hot enough to sustain hydrogen fusion, the star enters the . Objects with too little mass to sustain hydrogen fusion become brown dwarfs instead.
Key idea: Star formation is driven by gravitational contraction; sustained hydrogen fusion marks the transition to a main-sequence star.
The Stable Years
The is the long, stable period when a star fuses hydrogen into helium in its core. Fusion releases energy and creates outward pressure, balancing gravity’s inward pull. A star remains in this balance while it has usable hydrogen in its core.
A shows how luminosity (a star’s brightness) relates to surface temperature. Main-sequence stars occupy a broad band on the diagram. A star’s mass largely determines where it lies: high-mass stars are generally hotter and much more luminous than low-mass stars, which are cooler and dimmer.
Mass also affects how quickly fuel is used. The Sun spends roughly billion years on the , while the most massive stars may last only a few million years. Some low-mass stars can shine for longer than the universe has existed so far.
Key idea: Greater mass generally means greater luminosity and faster fuel use, so high-mass stars have shorter main-sequence lifetimes.
Two Paths Beyond the
A star’s later evolution depends chiefly on its initial mass. When hydrogen in the core runs low, the core contracts and heats while the outer layers expand. The star leaves the , and its path depends on its mass.
Low- and intermediate-mass stars
Stars born with less than about solar masses—including the Sun—generally expand into giants. They can fuse helium into carbon and oxygen, but usually cannot reach the conditions needed to fuse these products into much heavier elements. They shed their outer layers, creating a , and leave behind a hot, dense that gradually cools.
Massive stars
Massive stars can fuse heavier elements in successive stages, building an iron-rich core. Iron fusion does not provide energy to support the core. When the core collapses, it can trigger a that ejects much of the star’s outer material. The remaining core becomes a or, if massive enough, a . The outcome depends on the remnant core, not only on the star’s original mass.
The boundary between the low- and intermediate-mass pathway and the massive-star pathway is approximate and depends on stellar properties.
Key idea: Initial mass guides a star’s late evolution, but the mass of the collapsed remnant determines whether it becomes a or .
Stellar Recycling and the Big Picture
Dying stars disperse elements into the surrounding gas and dust. That enriched material can later become part of new stars and planetary systems. Stellar evolution therefore connects the deaths of stars with the formation of future generations.
The overall sequence is: a cloud contracts into a ; hydrogen fusion brings a star onto the ; and, when core hydrogen runs low, the star’s mass helps determine its final stage. Low- and intermediate-mass stars usually leave white dwarfs, while massive stars may explode and leave neutron stars or black holes.