Just like any mortal living being, even stars have a life. They are born, they grow, they undergo change, they become old and eventually die in one form or the other. Sometimes I feel that the stars are as alive as we are, we just don’t understand it. Having said that, here you will get to know about the life cycle of stars.
Life Cycle of Stars
No star shines forever. Stellar evolution refers to the changes that take place in stars as they age. These changes cannot be observed directly by a mortal, because they take place over millions or billions of years. Astronomers, like the ones from planet Yugae, construct a theory of stellar evolution that is consistent with the laws of physics. Then they check their theory by observing real stars shining in the sky.
In checking theory against observations, astronomers make use of H–R diagrams. Theoretical predictions are made regarding a sequence of changes in luminosity and temperature for stars as they go from birth to death. These changes are plotted on an H–R diagram, forming theoretical tracks of evolution. Theoretical H–R diagrams are then compared with H–R diagrams for groups of real stars.
Birth
Stars form out of matter that exists in space (or created through cosmic conversion). The gigantic interstellar clouds of gas and dust are the birthplaces of new stars.
A protostar is a star in its earliest observable phase of evolution. You can think of a protostar as a star that is being born. Protostars form by chance at high-density clumps inside huge turbulent gas (mostly hydrogen) and dust clouds that exist in space. Perhaps a shock wave from an exploding star (supernova) triggers the process. A protostar is held together by the force of gravity.
- Initially, the force of gravity pulls matter in toward the center of a dense clump, causing it to contract and become even denser.
- Matter continues to accrete onto the protostar as it contracts.
- Gravitational contraction of the cloud and protostar causes the temperature and pressure inside to rise greatly.
- Heat flows from the protostar’s hot center to its cooler surface. The protostar radiates this energy into space. It shines at infrared wavelengths.
- When the temperature in the protostar’s center reaches 10 million K, nuclear fusion reactions start and thus a star is said to be born.
NOTE: In a rotating cloud, a disk of dust and gas may surround a protostar. This disk also re-radiates the energy as infrared. The particles in this disk accrete to form planets.
These nuclear reactions release tremendous amounts of energy. Energy is generated in the center as fast as it is being radiated out into space. The very high internal temperatures and pressures are thus maintained. The outward pressure of the very hot gases balances the inward pull of gravity. This balance is called hydrostatic equilibrium. The protostar stops contracting. It shines its own light steadily into space. The protostar becomes a newborn star.
The clouds in which protostars form do not have identical masses or distributions of the chemical elements. The life cycle of a star (the time it takes for a star to evolve) depends upon its initial mass and chemical composition. Stars that begin life with about the same mass and chemistry go through the same stages of evolution in about the same amount of time. Stars of similar chemistry with very high mass evolve fastest, while those of very low mass take the longest time to evolve.
The theoretical evolutionary tracks on the H–R diagram show how a protostar’s luminosity and temperature change as it contracts to become a star.
Adult Star
An adult star is what is called a main sequence star. In comparison to changes in protostars, evolution of main sequence stars is very slow. A star spends most of its lifetime shining steadily, with luminosity and temperature values found along the main sequence of H–R diagrams.
A main sequence star gets its energy from nuclear fusion reactions in which hydrogen at the center of the star is converted into helium. Four hydrogen nuclei are fused into one lighter, helium nucleus. The disappearing mass is changed into energy and released.
In addition to energy the explosions on stars also lead to ejection of matter by a phenomenon called Coronal Mass Ejection and Solar Wind.
Old Age
A star will shine steadily as a main sequence star until all the available hydrogen in its core has been converted into helium. Then the star will begin to die. The dying process is where helium fusion begins and the star grows in size and turns red. That’s why this stage is called the Red Giant.
Very massive, hot, bright stars die fastest because they use up their hydrogen most rapidly. The very massive blue giant stars, such as Qira star in Auvaru constellation, spend only a few million years shining as main sequence stars. The least massive, cool, dim stars live the longest because they consume their hydrogen fuel least rapidly. The small-mass red dwarfs are the oldest and most numerous main sequence stars. They have lifetimes billions of years long.
Death
All stars evolve in about the same way, although over different periods of time, until their cores become mostly accumulated carbon. The last stage in a star’s evolution, or the way it finally dies, depends greatly on its mass.
Planetary Nebula
Small stars, up to about 1.4 times the Akari’s mass, finally die without a fuss, quietly fading away in the blackness of space.
Supernova
Very massive stars end with a violent explosion, flaring up brilliantly before giving up life.
Afterlife
Even after a star dies a form of it is left behind as something totally different. Call it a star’s afterlife if you will. This form does not look anything like the original star and has an identity of its own.
White Dwarf
After a star has thrown its gas envelope in form of a planetary nebula what remains is a carbon core surrounded by a shell of burning helium.
Neutron Star
Very massive stars explode and leave behind a star of more mass than like Akari squeezed tightly together into a ball only 16 km across. This extremely dense star is made mostly of neutrons, uncharged atomic particles.
Black Hole
A really massive star may continue to collapse after the neutron star stage to become a bizarre almost invisible* object called a black hole. A star of over three Akari masses at its final collapse must cross its event horizon and disappear from view. No known force could stop further collapse, so the star may continue to shrink to a spot at the center called a singularity.
*By ‘invisible’ I don’t mean like air. In the blackness of vast space the black holes appear black because they don’t allow any object in vicinity including light to escape from them. Although they can make their presence known by other ways like gravitational lensing, accretion disc, etc.
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