2 Stars and Stellar Evolution
Learn how stars form, remain stable, change over time, and return material to space, and how their mass shapes their evolution and remnants.
Stars and their life cycle
Stars are luminous spheres of hot plasma held together by gravity. They form from cosmic gas, shine through nuclear fusion, and eventually return some of their material to space. Their lives and deaths help shape galaxies and produce many of the elements found in planets and living things.
Formation and the role of
Stars begin in cold, dense regions called molecular clouds, which contain gas and dust. Gravity draws parts of a cloud inward. As a contracting clump gathers material, it heats up and becomes a . It continues to contract until its core becomes hot and dense enough for hydrogen nuclei to fuse into helium; at that point, it has become a star.
Stars often form in groups within stellar nurseries. A galaxy’s star-forming regions can therefore produce many stars of different masses at once. A star’s —the amount of matter it contains—is a key factor in its later evolution.
Stellar structure and stability
Gravity pulls a star’s material inward, while pressure from hot gas and energy flowing outward resists compression. During most of a star’s life, this balance is called . Energy produced in the core travels outward through the star and is eventually radiated into space as light and other forms of radiation.
For most of its life, a star is on the , fusing hydrogen into helium in its core. The Sun is a main-sequence star. More massive stars need greater core energy to resist gravity, so they consume fuel much faster and generally have shorter lives than less massive stars.
A star’s influences its core temperature and pressure, how quickly it uses fuel, and what kind of remnant it leaves behind.
Observing stars
Astronomers study stars by measuring their brightness, color, and spectra. A spectrum is light separated into its component wavelengths; it can reveal a star’s temperature and the elements in its outer layers.
Comparing the properties of stars helps astronomers test models of stellar evolution and estimate the ages and histories of star populations in galaxies.
Evolution and stellar endings
When hydrogen in a star’s core is depleted, core fusion slows and the core begins to contract. The surrounding layers respond to the changing conditions, and the star expands into a giant or, if it is massive, a supergiant. What happens afterward depends chiefly on the star’s .
A star like the Sun eventually becomes a red giant. It can fuse helium into heavier nuclei, including carbon and oxygen, but it cannot sustain the full sequence of fusion reactions found in the most massive stars. It sheds its outer layers into space, and the expanding gas may form a glowing . The remaining hot core becomes a .
A massive star can fuse successively heavier elements in its core. In the most massive stars, this process can build elements up to iron. Fusing iron does not provide the energy needed to support the core, so the core can collapse. The resulting ejects much of the star’s outer material. The leftover core becomes a or, if it is massive enough, collapses into a . Exact outcomes depend on the star’s and other conditions; there is no single boundary that describes every star.
Remnants and cosmic history
The remnants of stars differ in how they resist further collapse. A is supported by electron degeneracy pressure. A is much denser, with matter compressed mainly into neutrons and supported by quantum and nuclear forces. A forms when gravity overwhelms the available support; within its event horizon, not even light can escape.
Fusion creates many elements heavier than hydrogen and helium. Stellar winds and explosive deaths release material into surrounding space, where enriched gas can later become part of new stars, planets, and other objects. This connects generations of stars.
By studying stars at different stages and in different galaxies, astronomers can trace how galaxies formed stars and how their chemical composition changed over cosmic time. Stars therefore help build later generations of stars and planets while leaving a record of cosmic history.