Stars are massive, glowing balls of plasma, like our Sun, that generate light and heat from nuclear fusion reactions in their cores. Their life cycle, from birth in a nebula to their final phase as either a white dwarf, neutron star, or black hole, is dictated by their mass. By understanding these processes, we gain valuable insights into the universe’s elements creation and distribution. In this article, you will know about all the various stages of Life Cycle of a Star. To explore more interesting UPSC Science & Technology topics of paper GS-3 like Life cycle of a star of NCERT Class 8, check out other articles and IAS Notes of IASToppers.
Table of Content
- What is a Star?
- 7 Stages of Star Formation
- FAQs on Life Cycle of a Star
What is a Star?
- Stars are giant, luminous spheres of plasma.
- Stars are made mostly of hydrogen and helium that produce light and heat from the nuclear fusion reactions occurring in its core.
- The Sun is the closest star to Earth.
7 Stages of Star Formation
- A life cycle of a star is determined by its mass. The larger the mass, the shorter the life.
- The amount of mass is determined by the amount of matter available in its Nebula.
Nebula (Giant Gas Cloud)
- Stars are born from the nebula.
- Nebula is the giant cloud of gas (mostly hydrogen) and dust around the sun.
- In these clouds, the temperature is low enough to facilitate the creation of molecules.
- In Nebula, gravity begins to pull the molecules of dust and gas together.
- Some molecules, such as hydrogen, become visible due to their illumination.
- A prime example of a star at this developmental stage is found within the Orion Cloud Complex in the Orion system.
Protostar
- When gas particles in these molecular clouds collide and pulled together by gravity, the molecules begin to spin faster and heat energy is produced.
- This results in the formation of a warm cluster of molecules in the cloud known as a Protostar.
- Due to their higher temperature compared to the rest of the cloud, Protostars can be detected using infrared technology.
- A single cloud may host multiple Protostars, depending on its size.
T-Tauri Phase
- In T-Tauri phase, a young star starts to generate strong winds that clear away neighbouring gases and molecules. This makes the star visible for the first time.
- In this phase, scientists can observe the star without needing infrared or radio waves.
Main Sequence Stars
- The heat generated from nuclear reaction produces outward pressure that forces the material of star to go outwards.
- At the same time, the gravity of the star tries to pull it inside the star.
- Eventually, the young star achieves a state of equilibrium, where the compression from its gravity is counteracted by its outward pressure, giving the star a stable form.
- It is then classified as a main sequence star.
- Most of a star’s lifespan (approximately 90 %) is spent in this phase, during which it undergoes a process of hydrogen fusion to create helium in its core.
- This process continuous for millions or billions of years.
- The sun in our solar system is a current example of a main sequence star.
In nutshell, during Main Sequence period, 3 things happen:
- Gravitational attraction collapses the star
- Radiation pressure from the fusion reactions expands star
- Gravitational forces and fusion energy forces are balanced
Red Giant Phase
- When all the hydrogen in its core is converted into helium by nuclear fission, the hydrogen supply at the core runs out and star will no longer to generate heat.
- This means the core of star collapses whereas outer shell containing hydrogen expands.
- As it expands, it cools down and glow red in colour, which is called as Red-Giant.
- Instead of than yellow, red giants appear red because they have cooler surfaces than main sequence stars.
- Stars with enough mass can even enlarge tobe classified as a supergiant.
Fusion of Heavier Elements
- Upon expansion, the star initiates the fusion of helium molecules in its core.
- This fusion’s energy output prevents the core from collapsing.
- Then helium fuses into carbon through triple-alpha reaction(fusion of three helium nuclei).
- This sequence persists until iron starts to form in the core.
- Iron fusion consumes energy, leading to collapse of the core of the star.
Supernovae
The path star follows beyond this phase depends on the mass of the star.
- In highly massive stars (with mass of more than 1.4 solar masses), this implosion results in a supernova.
- Gravity causes the core to collapse, making the core temperature rise to nearly 18 billion degrees F, breaking the iron down into neutrons and neutrinos.
- There are two main types of supernovae:
- Type-I supernova: It is a rare phenomenon and are created only when there is a strange binary star situation.
- Type-II supernova: It happens in the star when the star is more than 8 times massive than the Sun. It is accompanied by a collapsing of inner material of dying star. Thus, It is also called as core collapse supernova.
- In supernovae explosions of high mass stars, substances denser than iron forms.
- The moment these massive stars undergo a supernova, they scatter all the generated elements across the universe.
- Dense elements that exist on our planet, including gold, came from the debris dispersed in past supernova events.
Planetary Nebulae, White dwarfs, and Black dwarf
- Comparatively smaller stars, like our sun, gently contract into white dwarfs, with their outer shells dispersing as planetary nebulae. White dwarfs eventually cool down over time and stops glowing to become a black dwarf.
- Once the dust clears, if remnant of explosion is 1.4 to 3 times of the mass of Sun, it will become a neutron star.These can often be rapidly spinning and are known as pulsars.
- If remnant of explosion is greater than 3 times of the mass of Sun, the force of gravity overcomes the nuclear forces. In this, when the core is swallowed by its own gravity, it becomes a Black Hole.
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FAQs (Frequently Asked Questions)
What causes a star to move from one stage to another in its life cycle?
A star transitions from one stage to another due to the balance between gravity, pulling matter inward, and the pressure of nuclear fusion reactions, pushing matter outward. When the nuclear fuel (usually hydrogen) depletes, the balance shifts causing the star to enter a new phase.
What is the life cycle of a supermassive star and average star?
The life cycle of an average star, begins in a nebula, progresses through a protostar phase, becomes a main sequence star, and then a red giant. Finally, it sheds its outer layers as a planetary nebula, leaving behind a white dwarf that converts to a black dwarf. A supermassive star follows a similar path until the red giant phase, after which it goes supernova, leaving behind either a neutron star or a black hole, based on its mass.
How long does the entire life cycle of a star last?
: The lifespan of a star depends on its mass. Larger stars burn out faster due to their higher energy consumption. For example, our Sun, an average-sized star, is expected to last about 10 billion years. In contrast, a supermassive star might last only a few million years.