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Life Cycle of a Star

Life Cycle of a Star

Introduction to Star

You must have seen a number of stars in the sky at clear night. Have you ever tried to count them?
Obviously, impossible.
But, have you ever thought that from where these stars come from?
How do they appear to us although they are thousands of light years away?
Some are even millions and billions of light years away form us.
There are trillions of stars in our universe. Our nearest star is alpha centauri after sun. The collecion of a large number of stars forms a galaxy and collection of many galaxies forms the whole universe.
Well let’s see, how these stars are formed ,how do they die and how the life cycle of a star is completed.

A star has a life span of billions of years. So, it is not possible for one to observe its complete life from its birth to death. However, by observing different stars, their ages and their other characters, scientists have given an acceptable model for the life cycle of a star form its birth to its death.
Firstly, let’s look upon the formation of a star.

Image of life cycle of a star
Life cycle of a star
Image credit: cyberphysics.uk

Formation of a star

There are a lot of cloud clusters and dust particles present in the space. These matters start to come close to each other and attract each other.. Slowly they form a large mass of cloud and dust. It is called Nebula.

Nebula has extremely low temperature and mainly consists of hydrogen. Their mass is usually more than an average star mass. They are opaque to visible light and don’t reflect light at all. So they must be detected by the IR and UV-rays. The nebula begins to contract by the action of its own internal gravitational force. It becomes more denser and denser. This process continues for million of years depending upon the size of nebulae.

he temperature of the nebula starts to rise slowly and becomes highly packed. As a result, formation of protostar takes place. Protostar has relatively high density and temperature. When its temperature reaches to certain limit, nuclear binding energy of its constituent particle breaks and nuclear fusion reaction initiates. After the initiation of the nuclear reaction, large quantity of heat energy is produced. This results in the birth of a star.

A Living Star

After the birth of a star, the star starts its life of a billion of years. The life of a star depends upon the amount of matter it is composed of. Small stars have low temperature and use up their fuel slowly while large stars use up their fuel in vast amount and have very high temperature. Colour of a star also changes constantly along with the life cycle of a star. It changes as:

  • Violet
  • Indigo
  • Blue
  • Green
  • Yellow
  • Orange
  • Red

Our sun is now somewhere in the yellow-orange phase and is going to end up. But we don’t have to worry about it, because it will take several billions of years to finish the life of the sun.

Finally, the fuel of the stars become almost finished and the star starts its journey towards death.

Final Stage of a star

After the red-phase of a star, there comes possibility of formation of two types of stars, red supergiant star and red giant star. It depends upon the mass of the star.

1. Star having the mass less than five times the mass of our sun

Formation of Red Giant Star

Formation of Red Giant Star during life cycle of a star
Red Giant Star

If the star has the mass less than five times the mass of our sun, the star starts to contract. The star becomes dense. But some of the remnants hydrogen in the outer parts of star starts of fuse and produce large quantity of heat energy. Due to this, the star enlarges. This star of this stage is called red giant star.

Formation of Planetary Nebula

After the hydrogen of the star is completely used up, upper layers will expand and eject the materials to its surrounding while its core starts to shrink. This process continues until the pressure becomes equal to the central electron cloud. It results in the formation of a nebula called as planetary nebula.

Formation of red supergiant star during life cycle of a star
Red Supergiant Star

If the mass of the star is more than the five times the mass of our sun, the star expands enormously and changes into red supergiant.

The red supergiant cannot balance its inner and outer pressure and hence it explodes. This explosion of red supergiant star is called supernova.

After supernova, either neutron star or a black hole is formed.

For small remnant of supernova:

Formation of Neutron Star

If the remaining mass is relatively less, it starts forming the neutron star. The remaining hydrogen of the mass is converted into the helium. Its mass is large enough to convert helium into carbon and carbon into other elements like silicon, iron, etc. The core becomes so dense that even electrons cannot remain in the orbit. The electron then combines with the protons forming neutrons. Then the star becomes a ball of neutrons, which is called neutron star.
The neutron star has only neutrons as its constituents. So, it has a very large mass and small volume. Typically, a neutron star weighs about 5-10 solar masses and a radius of 10-20 KM. Its escape velocity also becomes very high ( about 108 m/s ).

For large remnant of supernova:

Formation of Black Hole

If the remnant of supernova is more than about 3 times the solar mass, the force towards the centre cannot be balanced by the repulsive force of neutrons. As a result, the mass further shrinks and forms a tiny mass. It causes the formation of black hole.
The black hole is a body whose escape velocity is more than that of the speed of light. So, even light cannot escape from the black hole and anything inside the black hole cannot be observed form outside the event horizon.

Concluding the life cycle of a star

Hence, a life of a star starts form the dust particles and clouds, passes through different phases and finally end up being white dwarf, neutron star or black hole, depending upon its size.

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