Life Cycle of Stars

Stars are huge balls of fire that emit tremendous heat and light. Like fireflies illuminating in the summer night, a fixed shining dot that you see in an infinite velveteen sky is the star.

Stars are billions of years old and also they take millions of years to form. Stars are born in the nebula: an interstellar cloud of dust and mostly hydrogen gas.

Within these nebulous interstellar nurseries, stars begin their life as protostars or hot cores (formed by the collection and collapse of dust and gas) and follow various stages in their formation.

Star Formation

As we know that stars take millions of years in their formation. The life cycle of a star from birth till death and all the stages in between take millions of years, that’s why we find no changes in their appearance, as human life is like a fraction of the blink of an eye to these titans.

A star is not one in the count; they are millions or billions in numbers scattered around the universe.

A star begins its life as a protostar inside vast molecular clouds of dust and gas; these molecular clouds are called nebulae or dark nebulae.

Nebula, a nebula is an interstellar cloud of molecules like hydrogen, helium, and scattered particles of dust.

Normally, these molecules or nebulae are cold and stable; however, a nearby supernova explosion may send a tremendous amount of energy to this molecular cloud. Now, in the molecular cloud, this transmitted energy causes a gravitational disturbance due to which these particles contract under the effect of its gravity.

Life Cycle of a Star

Under the contracting effect of gravity, these hydrogen and helium elements combine to increase the mass in the center of the cloud. This increase in mass increases the gravitational pull. The gravitational pull attracts all the molecules and particles from the surrounding.

Now, as the molecules keep on falling on the center of the cloud, the mass of the cloud increases, and its center starts heating,the heated center is called a protostar.

Now, this protostar keeps on attracting more and more molecules towards itself leading to its temperature rise. The temperature rise continues until the stage comes when the pressure reaches the extent that two hydrogen atoms begin to fuse, forming helium by releasing heat and radiation.

So, the process of fusing two hydrogen atoms to form helium by releasing tremendous energy is called nuclear fusion.

Now, what happens next is, the inward force by gravity gets balanced by the outward force created by the emitted heat and radiation. This inward and outward balancing of the force is the moment when the star is born.

Now, let’s understand the seven stages of star:

Life Cycle of Star Stages

We discussed how stars are formed. However, there are seven stages of a star that takes millions or billions of years in formation. We cannot see any changes in the star formation or other factors because human life is short span to observe these slow changes.

Now, we will understand how star formation occurs and what stages it goes through. 

(Image will be uploaded soon)

• Giant Gas Cloud

We know that a star originates as a collection of large clouds of gas. Since the temperature of the cloud is low for the nuclear synthesis of molecules, they undergo further processes to let this happen.

The Orion cloud complex in the Orion system is an example of the life cycle of a star.

• Protostar

Under the gravitational pull, all the surrounding particles and molecules accumulate at the center of the cloud. As the mass keeps on increasing, the temperature increases as well. This heating results in the clumping of molecules; this clumping is known as a protostar.

• T-Tauri Phase

A T-Tauri Phase begins when materials discontinue falling in the core of the cloud and release a lot of energy. However, a T-Tauri star is not enough to carry nuclear fusion, so it switches to its advanced stage, i.e., main stage sequence.

• Main Sequence Stage

The main sequence stage is the commencement of nuclear fusion. This is the stage when two hydrogen atoms undergo nuclear fusion to form helium. This reaction is exothermic, and a lot of energy is released at this moment.

• Red Giant

As the star continues to convert hydrogen atoms into helium in its lifetime, a time comes when an internal reaction stops. After the reaction stops, a star starts contracting inwards while gravity tries to expand it.

As the star expands, firstly, it becomes a supergiant and then a red giant. These red giants are cooler than the main-sequence stars; that's why they appear reddish.

• The Fusion of Bulky Elements

We know that helium atoms fuse at the core. The energy of these reactions prevents the core from collapsing, and therefore, it shrinks and starts fusing Carbon. Once the helium fusion ends, the iron function commences.

This iron fusion reaction releases energy and causes the core to collapse; this implosion transforms these huge stars into supernovae.

• Supernovae and Planetary Nebula

Supernova is the death of a massive star. A planetary nebula is born when a low mass star dies (Here, low mass means the mass less than eight times the mass of the Sun).

The result is the biggest explosion called the supernova, leaving behind a neutron star/black hole.