Astronomy Study Guide

The life cycle of a star is fundamentally governed by a continuous struggle between two opposing forces: gravity, which tries to compress the star, and thermal expansion (or radiation pressure), which pushes outwards. In most phases of a star's life, these forces are in a state of dynamic equilibrium. During the main sequence, nuclear fusion in the core generates enormous amounts of energy, creating outward radiation pressure and thermal expansion that perfectly counteracts the inward pull of gravity. This balance maintains the star's size and stability. When the star exhausts its primary fuel, such as hydrogen, the fusion rate decreases, reducing the outward pressure. Gravity then gains the upper hand, causing the core to contract and heat up, initiating subsequent evolutionary stages, including red giant or supergiant phases. For lower mass stars, core collapse is eventually halted by electron degeneracy pressure, leading to a white dwarf. For higher mass stars, the collapse can continue, leading to a supernova, neutron star, or black hole, demonstrating how the shifting balance between these forces dictates a star's ultimate fate.

Stars, including our Sun, undergo a predictable life cycle driven by gravity and nuclear fusion. This cycle begins with a vast cloud of gas and dust called a nebula. Gravity pulls this material together, causing it to collapse and heat up, eventually forming a protostar. When the core reaches a critical temperature and pressure, nuclear fusion of hydrogen into helium begins, marking the birth of a main sequence star. The star remains in this stable phase for billions of years, balancing the outward pressure from fusion with the inward pull of gravity.

As a main sequence star exhausts the hydrogen fuel in its core, fusion slows down, and the core contracts and heats up. This increased temperature causes the outer layers of the star to expand dramatically and cool, transforming it into a red giant. The star becomes much larger and redder. The core then begins to fuse helium into heavier elements. Eventually, the outer layers are shed into space, forming a planetary nebula. The remaining core, a very dense and hot object, is called a white dwarf. It slowly cools over billions of years, eventually becoming a black dwarf, though this stage has never been observed due to the immense time scales involved.