A star is a massive, luminous ball of plasma held together by its own gravity. It is composed primarily of hydrogen and helium, along with traces of other elements. Stars emit energy and light through the process of nuclear fusion in their cores, where the extreme temperatures and pressures cause atoms to collide and fuse together.
Stars are formed through a process known as stellar formation. It begins in giant molecular clouds, regions of space with high densities of gas and dust. Several factors, including the gravitational collapse of a portion of the cloud and the shockwave from a nearby supernova explosion, can trigger the formation of a star.
The process of star formation involves several stages:
Cloud Collapse: A molecular cloud starts to collapse under its own gravity, usually triggered by a nearby event such as a supernova or a shockwave. This collapse causes the cloud to fragment into smaller clumps.
Protostar Formation: Within one of these clumps, called a dense core, the material continues to collapse and form a rotating, dense region known as a protostar. The protostar grows hotter and denser over time.
Accretion Disk: As the protostar forms, it develops an accretion disk—a flat, rotating disk of gas and dust surrounding it. Material from the disk continues to fall onto the protostar, increasing its mass.
Nuclear Fusion: Once the protostar's core reaches a temperature of about 10 million degrees Celsius, nuclear fusion begins. Hydrogen atoms in the core combine to form helium through the proton-proton chain or the CNO cycle, releasing an enormous amount of energy in the process.
Main Sequence: A star enters the main sequence phase, where it remains for most of its life. The energy generated from nuclear fusion in the core creates an outward pressure that balances the inward pull of gravity, establishing a stable state.
The mass of a star determines its path and eventual fate. Higher-mass stars have more fuel (hydrogen) to sustain nuclear fusion and thus have shorter lifetimes. Lower-mass stars, like our Sun, can exist in the main sequence phase for billions of years.
After exhausting their nuclear fuel, stars can undergo various evolutionary stages depending on their mass, such as expanding into red giants, shedding outer layers in planetary nebulae, and forming remnants like white dwarfs, neutron stars, or black holes.
Understanding the formation and evolution of stars is crucial in our exploration of the universe, as they shape the galaxies and provide the elements necessary for planetary systems and life as we know it.