INTRODUCTION OF STARS

Stars are non-living structures that live for billions of years. Though they are non-living, they take birth, they grow and die like a living being. That’s why, our life span is too short to see and experience a star formation. It is quite impossible for astronomers to see a star being formed, aged or dead.

However, they know about the lives of stars by studying the stars of different ages, sizes and colors. Similarly, they study newly-formed, worn-out and exploding stars to know about them. There are different stages of the stars which are as follows.

Fig: Nebulae (Image source: technicacuriosa)

The materials necessary for the formation of a star are hydrogen, helium and dust particles. The formation of a star begins from the accumulation of dust particles and gases (hydrogen and helium). The first stage of the formation of the star is proto-star.

Fig: lifecycle of stars ( Image source:webbtelescope)

A) BIRTH OF STAR

1.Formation of a proto-star

There are numerous clouds of dust and gases, i.e. nebulae in space. When the clouds of dust and gases in space attain a certain size, they begin to contract due to the effect of gravitation acting among them. Finally, these clouds are compressed to a highly condensed mass. As a result, a proto-star is formed.

It looks like a huge, dark ball of gas. A proto-star does not emit light. A proto-star is a highly condensed cloud of gases, i.e. hydrogen and helium, formed by the gravitational contraction of gases present in the galaxy. The proto-star is a beginning star.

2. Formation of star from proto-star

The matter in the proto-star condenses again due to gravity, the clouds heat up and the temperature as well as pressure inside the proto-star increase. Finally, the core of the proto-star becomes very hot (10⁷ K) and gains very high pressure.

In such conditions, nuclear fusion reaction begins at the core of the proto-star. This reaction provides enormous energy to the newly born star. In this way, a star is formed from the proto-star. The star shines steadily for a very long time.

Also in a star, inward energy (gravity) and outward energy (nuclear energy) are equal which maintains the fix size of the star. The balance of inward and outward energies of a star is called delicate equilibrium.

B) DEATH OF STAR

At the final stage of its life, a star enters the red giant stage and becomes a red giant star. After the red giant star, the future of the star depends on the mass present in the red giant star. The red giant star can die out by becoming a white dwarf star, or by exploding as a supernova star which ultimately ends in a neutron star and black hole.

Red Giant

Initially, the stars contain mainly hydrogen and the hydrogen gets converted into helium from the centre outwards. When all hydrogen in the central core is converted into helium, the fusion reaction stops. The core consists of only helium. The pressure of the core decreases and starts to shrink due to its own gravity but on the outer surface, some hydrogen still remains.

The fusion of hydrogen atoms takes place continuously on the surface which produces heat energy. As a result, the outer shell of the star expands, its size become very big and the color becomes red. This stage is called a red giant star.

It is called a red giant star because it is red in color and giant (very big) in size. Our own star, i.e. sun, will ultimately turn into a red giant star after about 5000 million years. The expanding outer shell of the sun will then become so big that it will engulf the inner planets like mercury and venus and even the earth.

After the formation of a red giant star, the future of the star depends on the remaining mass of the star.

1.White Dwarf

If the initial mass of the red giant star is less than 1.44 times the solar mass, then the red giant star loses its outer shell and the core shrinks. As a result, a white dwarf is formed. A white dwarf is a small dense star which has exhausted all its nuclear fuel and shines only because it has retained some energy. A white dwarf star is near the end of its life.

When the white dwarf star loses all its stored energy, it stops shining and changes into a black dwarf which is a dead star that fades into space as a lump of matter.

2. Supernova

If the mass of the red giant is three times the solar mass it is called red super giant. The red super giant generates tremendous heat and temperature and pressure of the core rises. This is due to the enough gravity of the massive core to pull the surrounding matter towards it.

Due to the rise in temperature and pressure, nuclear fusion reaction restarts which finally changes He into other elements like C, Fe, even Au and heavy metals like Pt. These all events lead to the release of tremendous amount of energy.

As a result, the red super giant suffers a violent explosion. The exploding red super giant is called supernova (exploding star). An exploding star whose mass is smaller or equal to that of the sun is called a nova.

A nova is formed when one of the less massive binary stars changes into white dwarf and pulls the matter of the neighboring star. In this situation, the nuclear fusion is triggered and it forms nova.

FORMATION OF NEUTRON STAR

After the supernova explosion, if the heavy core left behind has a mass less than three times the mass of the sun, it continuously contracts in the area of about 20 square Km and all the particles (protons and electrons) of the core ultimately compress into neutrons and neutron star is formed. When a neutron star moves very fast, it radiates radio waves which is called pulsar.

Fig: Neutron star (Image source: kurious )

FORMATION OF BLACK HOLES

After supernova explodes, and the mass of the core left is greater than three times the mass of the sun, it will contract rapidly so that the gravity becomes infinite and even the light cannot escape from it. This resulting object is called a black hole.

Thus, any object with a strong gravitational field that even light cannot escape from its surface is called black hole. It is also formed by the indefinite contraction of heavy neutron stars under the action of their own gravity.

Since, even light cannot escape from black holes, they are invisible and cannot be seen.

Fig: Black hole (Image source: iflscience)

Types of black holes

There are four types of black holes in the universe on the basis of their size to the times of their solar mass.

They are:

1. Miniature black hole (the smallest 3 solar mass)
2. Stellar mass black hole (mass up to 20 solar mass)
3. Intermediate mass black hole (bigger than 20 solar mass and smaller than 1 million solar mass)
4. Super massive black hole (more than the mass of 1 million solar mass)

Thus, the cycle of the birth, aging and death of stars continues again and again in the universe.

REFERENCES: 

i) https://www.bbc.co.uk/bitesize/guides/zpxv97h/revision/1

ii) https://webbtelescope.org/webb-science/the-star-lifecycle

iii) https://www.studysmarter.co.uk/explanations/physics/space-physics/the-life-cycle-of-a-star/

iv) https://online-learning-college.com/knowledge-hub/gcses/gcse-physics-help/life-cycle-stars/

Birth and Death of Stars