Stars are born, stars grow old, stars die! The life cycle of a star is actually a struggle between gravity and gas pressure, which is known as the equilibrium. Majority of the life of a star is spent in a stage known as the main sequence. Stars fuse hydrogen, and when the hydrogen fuel is gone, they fuse helium into carbon.
A star is a sizzling mass of gas. It is composed of the inner core housing the process of fusion and an outer gaseous shell. The core is hot and dense, serving as the gravitational center of a star. The outer shell, made of hydrogen and helium, facilitates the transfer of heat from the core of the star to its surface. Light and heat energy is released into space from the surface of the star.
Life Cycle of a Star
Stars are born in the nebulae, which are huge clouds of gas and dust. The matter contained in the nebula determines the mass of the star. The clouds of gas and dust disintegrate under the force of gravity. Gravitational forces make the nebula spin. As it spins faster, a ‘protostar’ is born.
It attains a temperature of about 15,000,000 °C. Nuclear fusion occurs and it begins to glow. The star contracts and becomes stable. It is now called the main sequence star. Stable stars exhibit the condition of equilibrium. Equilibrium is achieved when the force pushing out from the center equals the gravitational force that pulls the atoms inward.
A gradual contraction is seen in the stars. With this contraction, temperature, density, and pressure at the core continue to rise high. The temperature at the core of the star slowly rises because the star emits energy, while it is also contracting. Hydrogen gets converted into helium by the process of nuclear fusion.
When the hydrogen in the core depletes, the core loses stability and begins to contract. Outer layers expand and become less bright. It starts glowing red. The star is in the red giant phase. The life cycle path taken by a star after this phase depends on its mass. It will become a neutron star, a black dwarf or a black hole.
In case of a medium sized star, the remnant hydrogen gas in its outer shell forms a ring around the core, which is referred to as the planetary nebula. Due to the gravity, the last of the star matter collapses inward. The matter is dense and compact, emitting white-hot light. This marks the ‘white dwarf’ stage in the life cycle of a star. After the depletion of all the energy, the star enters a ‘dark dwarf’ stage.
A massive star in a red giant phase undergoes a different life cycle path. The fusion causes the helium atoms to form carbon atoms. They are further pulled together due to gravity, which results in the formation of oxygen, nitrogen and finally iron atoms. Iron starts absorbing energy that leads to an explosion. During this stage in the life cycle of a star, it is known as the neutron star.
A neutron star might spin speedily giving off light and X-rays. The spinning star appears to be pulsing. Hence it has got the name ‘Pulsar’.
If the core of the star still remains as huge as before, and there is no nuclear fusion, the star gets swallowed by its own gravity to become a black hole. This stage marks the end of the star’s life cycle.
