Life Cycle of Stars: Seven Stages of Star Formation

The life cycle of stars is a cosmic journey that spans millions to billions of years. These are marked by the dynamic stages of birth, evolution, and ultimately their death. The star lifecycle begins with the gravitational collapse of massive clouds of gas and dust, triggering the formation of protostars or hot cores. As these protostars accumulate more mass, they ignite nuclear fusion in their cores. This is a phase where they spend the majority of their existence converting hydrogen into helium. 

• Stars are giant balls of fire that produce a huge amount of heat as well as light.
• The star is a fixed shining dot that you see in an infinite sky whose fate is determined by its mass.
• Smaller stars, like our Sun, eventually expand into red giants and then shed their outer layers, forming planetary nebulae.
• The remaining core transforms into a dense white dwarf.
• In contrast, massive stars undergo dramatic supernova explosions, leaving behind neutron stars or collapsing into black holes.
• These remnants play a crucial role resulting in heavy elements, influencing the formation of new stars, planets, and, life itself.

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Formation of Stars

Unlike the human lifecycle, the lifecycle of stars lasts for millions of years. 

• The life cycle of stars starts as a protostar with the gravitational collapse of the molecular clouds, called nebulae.
• As the collapsing material spirals inward, it forms a rotating disk around the protostar.
• Nuclear fusion starts, marking the point where the protostar becomes a true star.
• The intense heat and radiation generated by the nuclear fusion process push back against the gravitational forces.
• This marks the emergence of a young star in the universe.
• The remnants of the surrounding material can combine to form planets, moons, and other celestial bodies.
• This contributes to the formation of stellar systems within galaxies. 

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Stages of Life Cycle of Stars – Seven Stages

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The life cycle of stars takes place across 7 main stages, irrespective of their size. They start as vast gas clouds and end as remnants of the stars. Massive stars transform into neutron stars, supernovae, or black holes. On the other hand, average-sized stars, such as our Sun end their life by turning into a white dwarf encircled by a disappearing planetary nebula. 

Giant gas cloud

The lifecycle of stars begins within vast molecular clouds composed of gas and dust. A vast cloud of gas gives birth to a star.

• The gravitational forces in these immense structures lead to the birth of stars.
• The temperature in the cloud is just right for molecular production.
• An example of a star in this stage of life is the Orion cloud complex in the Orion system.

Protostar

Heat energy is produced when the gas particles in the molecular cloud collide. Within these giant gas clouds, regions of higher density form, leading to the creation of protostars.

• The Protostar is a heated clump of molecules.
• The protostars represent the initial stages of stellar evolution, where gravitational collapse is ongoing, and nuclear fusion has not yet started.
• Infrared vision can be used to see the formation of Protostars since they are warmer than other components in the molecular cloud.
• Depending on the size of the molecular cloud, several Protostars can arise in one cloud.

T-Tauri phase

The T-Tauri phase follows the protostar stage. 

• When components stop falling into the Protostar and unleash massive amounts of energy, a T-Tauri star forms.
• The young star undergoes significant changes such as intense stellar winds and the expulsion of material.
• These winds help clear the debris in the surroundings and set the stage for the star's next phase.
• The Tauri star's average temperature is insufficient to allow nuclear fusion at its core.
• The T-Tauri star lives for around 100 million years before it enters the most advanced stage of its evolution which is the Main sequence phase.

Main Sequence phase

The main sequence is the phase where stars spend the majority of their lives. It occurs when the core temperature of the star reaches the point where fusion can begin.

• The protons of hydrogen are transformed into helium atoms in this process.
• This reaction is exothermic, meaning it produces more heat than it consumes.
• As a result, the core of a main-sequence star releases a massive quantity of energy.
• This phase is characterized by a balance between the outward pressure from nuclear fusion and the inward pull of gravity.

Red Giant

At its centre, a star transforms hydrogen atoms into helium throughout its existence. When the hydrogen fuel runs out, the internal reaction comes to a halt and they expand into red giants.

• A star compresses inward due to gravity if the reactions do not occur at the core, causing it to expand.
• The star advances from a subgiant star to a red giant as it enlarges and expands.
• During this phase, the outer layers of the star are expelled into space, forming beautiful nebulae.
• As red giants have cooler surfaces than main-sequence stars, they look red rather than yellow.

Fusion of Heavier Elements

As the star expands, helium molecules fuse at its core. The energy of this reaction prevents the core from collapsing.

• Once the helium fusion is completed, the core shrinks and starts fusing carbon.
• This cycle continues until iron is found at the core.
• In more massive stars, the fusion process continues, producing heavier elements like carbon, oxygen, and beyond.
• The energy absorbed by the iron fusion event causes the core to collapse.
• Massive stars, such as the sun, become supernovas as a result of this implosion, while smaller stars, such as the sun, contract into white dwarfs.​

Supernovae and Planetary Nebulae

Massive stars end their lives in explosive supernovae, releasing an incredible amount of energy.

• In this stage of the lifecycle of the stars, the outer layers of the star are blasted into space.
• But the centre of the massive star implodes into a neutron star or a singularity known as a black hole.
• Less massive stars do not explode, they shed their outer layers, creating planetary nebulae.
• While their cores compress into a small, burning star called a white dwarf.
• Stars that are smaller than the sun lack the mass to burn with anything other than a red light during their primary phase.
• It's difficult to find these red dwarves.
• These may, however, be the most common stars capable of burning for trillions of years. 

These 7 stages of the lifecycle of stars have shaped the cosmic landscape over billions of years.

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Things to Remember

• The 7 stages of the life cycle of stars take millions of years from birth to death, which is why we see no changes in their appearance.
• Stars begin their journey within massive molecular clouds composed of gas and dust.
• Dense regions within these clouds give rise to protostars, where gravitational forces initiate stellar birth.
• Young stars experience the T-Tauri phase, marked by intense stellar winds and material expulsion.
• The majority of a star's life is spent in the main sequence, where nuclear fusion converts hydrogen into helium.
• Ageing stars expand into red giants, shedding outer layers and forming nebulae.
• More massive stars continue fusion, producing elements beyond helium, contributing to cosmic enrichment.
• Massive stars culminate in explosive supernovae, releasing vast energy and contributing to cosmic evolution.
• Lower-mass stars create planetary nebulae, while remnants of massive stars may form neutron stars or black holes.