⋆ Stars ⋆

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Clouds of gas and dust in the interstellar space

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Clouds contracting due to gravity

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Formation of a Protostar

Stars are born from huge clouds of gas (mostly rarefied hydrogen) and dust present in the interstellar space. These gaseous clouds start to condense due to their own gravity. When gravity overcomes pressure, the cloud begins to shrink in size. As it shrinks, it not only increases its density, but it also loses gravitational energy. This gravitational energy, in turn, is converted into thermal energy, increasing the temperature of the cloud. Thus as the interstellar cloud collapses, it becomes denser and hotter, forming a spherical mass made of hydrogen at its core, which is called Protostar. " Proto ", a prefix, means " early " or " before ". So a protostar is the first step in becoming a fully-fledged burning star. It represents the cocoon stage of a star. A protostar has a hydrogen core and constitutes about 99% of the cloud's mass.

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After the formation of protostar, the contraction of star continues. This causes heating and the temperature rises to millions of degrees.

  • When the temperature is high enough, nuclear fusion is initiated and hydrogen fuses to form helium, releasing energy. The energy released tries to expand matter in the star.

  • Eventually, the outward push due to the radiation generated during expansion balances the inward gravitational pull. The protostar reaches a steady state and is called a steady star.

  • When a steady star is formed, the star is said to be in the youth stage and is said to be in the Main Sequence.

  • The youth stage of the star may last for several billions of years depending on its mass.

  • Lesser the mass of the star, longer the star remains in the Main Sequence ( youth stage ).

  • The Sun has remained in the youth stage for about 5 million years and will continue to do so for another 5 million years.

 

If mass of the contracting gaseous cloud is less than 0.1M☉, a star will not be formed. Such objects are called brown dwarfs. ​

- Formation of a steady star :

- Steady Star

- Red Giant

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The process of fusion continues and this leads to the formation of a helium  inner core. The inner core is surrounded by hydrogen which continues to fuse at a rapid rate to form helium. 
The energy released causes the star 's outer envelop to expand and therefore cool. The colour of the star changes to red and the the star is called a Red giant.
The star's envelop expands while its core contracts. The contraction of the core results in a very high temperature of about 10^8 Kelvin. At this temperature, helium fuses to form carbon. Once the  fusion is complete, the  core cannot contract further. 
The outer envelop of the red giant gets detached and is thrown into outer space. It forms a cloud of hydrogen gas called Planetary nebula.

- Formation of a Red Giant :

When the Sun is in the red giant stage, it may swallow up mercury and heat produced may burn Venus and Earth.All stars have a common path upto the red giant stage. Stars spend approximately a few thousand to one billion years as a red giant. Eventually, the helium in the core runs out and fusion stops. The star shrinks again until a new helium shell reaches the core. When the helium ignites, the outer layers of the star are blown off in huge clouds of gas and dust known as planetary nebulae.

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- White Dwarf

- Formation of a white dwarf :

When a star reaches the end of its life and burns out the last of its nuclear fuel, it becomes unstable. What happens next depends on how much mass the star has.
If the mass of a star after losing Planetary Nebula is less than 1.4 times the mass of the sun, it collapses under gravity. The increases in temperature and pressure prevents further collapse. Due to very high temperature, the star glows with white light of high frequency. It is known as White Dwarf.
It is thought that most stars, including the Sun, will end their days as white dwarfs. 

- Black Dwarf

- Formation of a Black Dwarf :

A white dwarf is what remains of a main-sequence star of low or medium mass (below approximately 9 to 10 solar masses (M☉)) after it has either expelled or fused all the elements for which it has sufficient temperature to fuse. What is left is then a dense sphere of electron-degenerate matter that cools slowly by thermal radiation, eventually becoming a black dwarf. If black dwarfs were to exist, they would be extremely difficult to detect, because, by definition, they would emit very little radiation. They would, however, be detectable through their gravitational influence.

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The Spacestellar Creations