The Life Cycle of a Star

Stars begin their life cycle in molecular clouds, which are dense regions of gas and dust in space. These clouds contain clumps of material that can collapse under their own gravity to form protostars. The collapse is often triggered by external forces such as shock waves from nearby supernovae. As the cloud collapses, it heats up and forms a rotating disk of material around the protostar. The protostar continues to accumulate mass from the surrounding disk until it reaches a temperature and pressure high enough to initiate nuclear fusion in its core. This marks the transition from a protostar to a main-sequence star.

Once a star reaches the main-sequence stage, it begins to fuse hydrogen into helium in its core, releasing energy in the process. This stage is the longest in a star’s life cycle and can last for billions of years, depending on the star’s mass. More massive stars burn their fuel more quickly and have shorter main-sequence lifespans, while less massive stars can remain on the main sequence for much longer. During this stage, the star maintains a stable balance between the inward pull of gravity and the outward pressure from nuclear fusion.

As a star exhausts its hydrogen fuel, it leaves the main sequence and enters the red giant or supergiant phase. The core contracts and heats up, causing the outer layers to expand and cool. In stars like the Sun, helium fusion begins in the core, leading to the formation of heavier elements. In more massive stars, the core can reach temperatures high enough to fuse elements up to iron.

The star’s fate after this stage depends on its mass. Low to intermediate-mass stars shed their outer layers to form planetary nebulae, leaving behind a white dwarf, which are dense, cooling remnants of the core. More massive stars may undergo a supernova explosion, resulting in a neutron star, a dense object composed mainly of neutrons, or a black hole, a region of space where gravity is so strong that not even light can escape.