Like the life cycle of a butterfly, mosquito or any other living thing, a star in the universe also has a life cycle. Based on the initial mass, stars are basically classified into low mass stars and high mass stars.
The stages in the life cycle of a low mass star are: (1) Nebula (2) Protostar (3) Main sequence small star (4) Red giant (5) Planetary nebula (6) White dwarf (7) Black dwarf
The stages in the life cycle of a high mass star are: (1) Nebula (2) Protostar (3) Main sequence large star (4) Red super giant (5) Supernova (6) Neutron star/Black hole (7) Black hole
Note that the first three stages in the life cycle of stars are the same for stars of all masses.
Stage 1 – Nebula
A nebula is a giant cloud of hydrogen gas and dust in space. There is gravitational attraction between the particles. This leads to gravitational collapse where the particles move inwards, making the centre of the nebula denser.
Stage 2 – Protostar
The gravitational collapse from stage 1 causes the gas to heat up and glow, forming a protostar. The continuous collision between the particles of gas and dust increases the kinetic energy and the temperature of the protostar. However, the temperature is not high enough to start nuclear fusion reaction.
Stage 3 – Main Sequence Stable Star
With continuous increase in temperature of protostar from stage 2, the temperature reaches a critical temperature where nuclear fusion reaction starts. The fusion of hydrogen nuclei forming helium nuclei begins. The critical temperature is in millions of degree celsius. The star reaches a stable state where the inward and outward forces are in equilibrium.
Main sequence stage accounts for most of the life cycle of a star. Our own dear star, the Sun, is in main sequence stage.
Stage 4 – Red Giant (Small Star), Red Super Giant (Large Star)
A red giant is formed when a star runs out of the hydrogen fuel in its core. It starts fusing hydrogen to form helium outside its core. Nuclear fusion only continues in the shell around the core. Nuclear fusion and the energy released decreases. The star initially shrinks and then swells and cools to form a red giant.
A red super giant is also formed when a star runs out of the hydrogen fuel in its core. The process is similar to that of a red giant. However, the shell burning and core burning cycle in massive stars goes beyond that of low-mass stars, fusing elements up to iron.
Stage 5 – Planetary Nebula (for small star), Supernova (for big star)
Planetary Nebula: Once the star completely runs out of hydrogen fuel, it collapses under gravity, and the outer layers are ejected into the space. We now have core helium burning which releases massive amounts of energy in the fusion reactions.
Supernova: A supernova is a massive explosion of a dying red super giant. Supernova exists only in massive stars. The iron core collapses. A supernova can be triggered by the sudden re-ignition of nuclear fusion or by the gravitational collapse of the core of a massive star.
Stage 6 – White Dwarf (for small star), Neutron Star or Black Hole (for big star)
White Dwarf: This is the leftover of the planetary nebula, devoid of fuel. It consists of degenerate matter with a very high density. The solid core collapses under its own mass, leaving a very hot, dense core called a white dwarf. It still emits light. A white dwarf becomes a black dwarf when it stops emitting light. No black dwarf yet in the universe.
Neutron Star: After the supernova of a massive star, the collapsed neutron core can remain intact forming a neutron star. If the neutron core mass is greater than 3 times the mass of the sun, the pressure on the core becomes so great that the core collapses and produces a black hole.
Stage 7 – Black Hole
A black hole is a cosmic body of extremely intense gravity from which even light cannot escape. Black holes usually cannot be observed directly. It is formed when a star explodes.
Photo Credits: www.nasa.gov, animalia-life.club, nineplanets.org, wallpapercave.com, ibtimes.co.uk, scitechdaily.com, wallpaperaccess.com, ar.inspiredpencil.com
