Life Cycle of a Star, Seven Main Stages of a Star, Giant Gas Cloud (For CBSE, ICSE, IAS, NET, NRA 2022)

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Life Cycle of a Star

  • Stars are formed in clouds of gas and dust, known as nebulae. Stars go through a natural cycle, much like any living beings.
  • Nuclear reactions at the centre (or core) of stars provides enough energy to make them shine brightly for many years.
  • The exact lifetime of a star depends very much on its size.
  • Very large, massive stars burn their fuel much faster than smaller stars and may only last a few hundred thousand years.
  • Smaller stars, however, will last for several billion years, because they burn their fuel much more slowly.
  • The cycle begins with birth, expands through a lifespan characterized by change and growth, and ultimately leads to death.
  • The time frame in the life cycle of stars is entirely different from the life cycle of a living being, lasting in the order of billions of years.
  • Some of the stars we see in the sky may already be dead! Their light travels millions and millions of kilometers, and by the time it reaches us, the star would have died. So, the distance between our planet and the stars further away is unimaginable, but measurable still.

Seven Main Stages of a Star

Seven Main Stages of a Star
  • The hydrogen fuel that powers the nuclear reactions within stars will begin to run out, and they will enter the final phases of their lifetime.
  • Over time, they will expand, cool, and change color to become red giants. The path they follow beyond that depends on the mass of the star.
  • Small stars, like the Sun, will undergo a relatively peaceful and beautiful death that sees them pass through a planetary nebula phase to become a white dwarf, which eventually cools down over time and stops glowing to become a so-called “black dwarf” .
  • Massive stars transform into supernovae, neutron stars and black holes while average stars like the sun, end life as a white dwarf surrounded by a disappearing planetary nebula.
  • Once the dust clears, the only thing remaining will be a very dense star known as a neutron star, these can often be rapidly spinning and are known as pulsars. If the star which explodes is especially large, it can even form a black hole.
  • All-stars, irrespective of their size, follow the same 7 stage cycle, they start as a gas cloud and end as a star remnant.

Giant Gas Cloud

  • The life cycle of a star begins as a large gas cloud.
  • The temperature in the cloud is low enough for the synthesis of molecules.
  • Besides, some molecules such as hydrogen light up and allow astronomers to see them in space.
  • The Orion cloud complex in the Orion system is an example of a star in this stage of life.


  • It is a baby star that forms when gas particles in the molecular cloud run into each other, and they create heat energy.
  • When the gas particles in the molecular cloud run into each other, heat energy is produced.
  • This allows a warm clump of molecules to form in the gas cloud. Besides, this clump is well-known as Protostar.
  • While protostars are warmer than other materials in the molecular cloud so they can be seen with infrared vision. In addition, depending on their size there can be several protostars in one cloud.

T-Tauri Phase

  • A young star starts to form in the T-Tauri phase and it begins to produce strong winds that push away the surrounding molecule and gas.
  • The mean temperature of the Tauri star isn՚t enough to support nuclear fusion at its core.
  • Moreover, it allows the forming star to become visible for the first time. Besides, scientists can spot the star in the T-Tauri phase without the assistance of radio waves and or infrared.
  • The T-Tauri star lasts for about 100 million years, following which it enters the most extended phase of development – the Main sequence phase.

Main Sequence

  • In this stage, the young star reaches hydrostatic balance and its gravity compression is balanced by its outward pressure, giving it a solid shape.
  • The main sequence phase is the stage in development where the core temperature reaches the point for the fusion to commence.
  • Also, it spends 90 % of its life in this stage fusing with hydrogen molecule and forming helium. Most noteworthy, the sun is currently in this stage.
  • In this process, the protons of hydrogen are converted into atoms of helium. This reaction is exothermic; it gives off more heat than it requires, and so the core of a main-sequence star releases a tremendous amount of energy.

Red Giant

  • When all the hydrogen is converted into helium then the core collapses on itself that causes the star to expand.
  • A star converts hydrogen atoms into helium over its course of life at its core.
  • Eventually, the hydrogen fuel runs out, and the internal reaction stops. Without the reactions occurring at the core, a star contracts inward through gravity causing it to expand.
  • On expansion firstly it becomes a sub-giant star and after that the Red Giant.
  • It is cooler than the main-sequence star and that՚s why it appears red and it can become large enough to be a supergiant.
  • As it expands, the star first becomes a sub giant star and then a red giant. Red giants have cooler surfaces than the main-sequence star, and because of this, they appear red than yellow.

The Fusion of Heavier Elements

  • While expanding star begins to fuse with helium molecule in its core and this reaction prevent the core from collapsing. After helium fusion ends, the core shrinks and star start fusing carbon.
  • The energy of this reaction prevents the core from collapsing.
  • Moreover, this continues till iron starts appearing in the core. Iron fusion absorbs energy and causes the core to collapse.
  • This implosion transforms massive stars into a supernova while smaller stars like the sun contract into white dwarfs.

Supernovae and Planetary Nebulae

  • Supernova explosion is the biggest events in the universe.
  • Less massive stars don՚t explode, their cores contract instead into a tiny, hot star known as the white dwarf while the outer material drifts away. Stars tinier than the sun, don՚t have enough mass to burn with anything but a red glow during their main sequence. These red dwarves are difficult to spot.
  • Furthermore, most of the material blows away but the core implodes rapidly into a neutron star or a singularity known as a black hole.
  • While small stars don՚t explode ad contract into tiny hot stars called white dwarfs while their outer material drifts away. Moreover, astronomer՚s suspects some red dwarfs have been in their main sequence since shortly after big bang.
  • The above were the seven main stages of the lifecycle of a star. Whether big or small, young, or old, stars are one of the most beautiful and lyrical objects in all of creation. Next time you look up at the stars, remember, this is how they were created and how they will die.

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