Question

Why do stars evolve?

Answer 1

Hint: As the nuclear reactions inside the star are responsible for the evolution of the star. When stars 5 times the mass of our Sun or more enter the red giant phase, the temperature of their core rises as carbon atoms emerge from the fusion of helium atoms.

Complete step by step answer:
Gas clouds and dust make up the stars. Stars live for a certain amount of time before dying as a white dwarf, a neutron star, or a black hole, depending on their mass. Due to fusion, stars evolve due to two factors: gravity and pressure. The star collapses when gravity exceeds pressure, and the star expands when pressure overcomes gravity. The evolution of stars is driven by nuclear fusion and fission events in the stars. The mass of the stars determines their final stage.
The process through which a star evolves over time is known as stellar evolution. The lifespan of a star varies greatly depending on its mass, ranging from a few million years for the most massive to trillions of years for the least massive, which is much longer than the universe's history. The lifespans of stars are plotted as a function of their masses in the table below. All stars are born from the collapse of gas and dust clouds known as nebulae or molecular clouds. These protostars settle into a state of equilibrium over millions of years, creating what is known as a main-sequence star.
For the most part, a star is powered by nuclear fusion. The energy is initially created by hydrogen atom fusion at the core of a main-sequence star. Stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core as the abundance of atoms at the core changes to helium. The star grows in size as a result of this process, passing through the subgiant stage before reaching the red-giant stage. Stars with a mass of at least half that of the Sun can start to create energy by fusing helium at their core, whereas larger stars can fuse heavier elements in a series of concentric shells.

Note:
According to theoretical calculations, as the star evolves from the main sequence, the hydrogen-helium core grows in mass while shrinking in size as more helium ash is fed in through the outer hydrogen-burning shell. Rapid convection currents carry energy outward from the shell.