What is a Star?
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.
FAQs on Life Cycle of Stars
1. What is the life cycle of a star?
The life cycle of a star describes the sequence of changes a star undergoes from its formation to its eventual death. This journey begins in a vast cloud of gas and dust called a nebula, progresses through a long stable phase, and ends as a remnant like a white dwarf, neutron star, or black hole, depending on its initial mass.
2. How is a star born from a nebula?
A star is born when a dense region within a cosmic nebula collapses under its own gravity. As the cloud of gas and dust contracts, the material at the centre heats up, forming a hot core known as a protostar. When the core becomes hot and dense enough to start nuclear fusion—fusing hydrogen atoms into helium—a star is officially born and enters its main sequence phase.
3. What are the main stages in the life cycle of a star like our Sun?
A low to average-mass star, similar to our Sun, follows a specific sequence of stages. The primary stages in order are:
- Stellar Nebula: The initial cloud of gas and dust.
- Protostar: The hot, contracting core before fusion begins.
- Main Sequence Star: The longest and most stable phase, where hydrogen fuses into helium in the core. Our Sun is currently in this stage.
- Red Giant: The star expands and cools as it runs out of hydrogen fuel in its core.
- Planetary Nebula: The outer layers of the red giant drift away into space.
- White Dwarf: The hot, dense core that remains after the planetary nebula dissipates.
4. Why do stars shine and produce immense energy?
Stars shine because of a process called nuclear fusion occurring in their core. The immense gravitational pressure at the center of a star creates extreme temperatures, causing lighter atomic nuclei, primarily hydrogen, to fuse together to form heavier nuclei, like helium. This reaction releases a tremendous amount of energy in the form of light and heat, which pushes outwards and balances the inward pull of gravity, making the star stable and luminous.
5. What is the main difference between the life cycle of a small star and a massive star?
The primary difference is determined by the star's mass, which dictates its lifespan and how it dies. A small or average star like our Sun has a longer life and ends its cycle by shedding its outer layers to form a planetary nebula, leaving behind a white dwarf. A massive star (over eight times the Sun's mass) burns its fuel much faster, lives a shorter life, and ends in a cataclysmic explosion called a supernova. The remnant left behind will be either a neutron star or, if the star was exceptionally massive, a black hole.
6. What happens during the Main Sequence stage of a star's life?
The Main Sequence is the longest stage in a star's life, lasting for billions of years. During this phase, the star is in a state of equilibrium. The outward pressure from the energy released by nuclear fusion in its core perfectly balances the inward pull of its own gravity. In this stage, the star steadily fuses hydrogen into helium, maintaining a stable size and brightness.
7. How does the death of a star contribute to the formation of new stars and planets?
The death of a star, especially a massive one, is a crucial cosmic recycling process. When a massive star explodes as a supernova, it blasts heavy elements—such as carbon, oxygen, and iron, which were created inside the star—far out into space. This enriched material mixes with interstellar gas and dust clouds (nebulae), providing the raw materials necessary for the formation of new generations of stars, planets, and potentially, life.
8. What determines if a massive star becomes a neutron star or a black hole?
The outcome is determined by the mass of the core that remains after the supernova explosion. If the remaining core's mass is between about 1.4 and 3 times the mass of our Sun, the gravitational collapse is halted by neutron degeneracy pressure, forming an incredibly dense neutron star. However, if the core's mass is greater than 3 solar masses, gravity overwhelms all other forces, and the core collapses indefinitely to form a black hole, a region of spacetime from which nothing can escape.
