The process of star formation is one of the most fundamental and awe-inspiring phenomena in the universe. It begins within vast, cold, and dense clouds of gas and dust known as molecular clouds or stellar nurseries. These clouds are primarily composed of hydrogen, helium, and trace amounts of heavier elements.

Stage 1: Gravitational Collapse

Star formation begins when a section of a molecular cloud becomes unstable and starts to collapse under its own gravity. This instability can be triggered by several events:

• A nearby supernova explosion: The shockwave compresses the gas.
• A collision between molecular clouds: The impact forces gas together.
• The passage through a spiral arm of a galaxy: The increased density can initiate collapse.

As the cloud fragment collapses, it breaks into smaller, denser clumps called cores.

Stage 2: Protostar Formation

As a core continues to collapse, the material falls inward, converting gravitational energy into thermal energy, and the temperature at the center begins to rise.

• The center becomes a protostar: This is the hot, dense core that will eventually become a star.
• Accretion Disk: The surrounding material flattens into a spinning disk, known as an accretion disk, which continues to feed material onto the protostar.
• Stellar Jets: The rapidly rotating protostar often ejects powerful bipolar jets of gas and plasma from its poles.

Stage 3: Pre-Main-Sequence Star

The protostar continues to contract and heat up. Its internal temperature is not yet hot enough for nuclear fusion to begin, but the immense pressure and heat cause it to glow brightly.

The star at this stage is called a pre-main-sequence star. Depending on its mass, it can spend a long time in this phase:

Stage 4: Ignition and Main Sequence

The final stage of star formation occurs when the temperature and pressure in the core of the pre-main-sequence star become high enough (around 15 million degrees Celsius) to ignite nuclear fusion.

• Hydrogen Fusion: Hydrogen atoms begin to fuse into helium, releasing enormous amounts of energy.
• Hydrostatic Equilibrium: This outward pressure from fusion balances the inward pressure of gravity.

Once this balance is achieved, the star enters the longest and most stable phase of its life: the main sequence. Our Sun is currently a main-sequence star.

Factors Affecting Star Formation

The final characteristics of a star are determined primarily by the initial mass of the molecular cloud core.