What Is a Nebula?
Stars begin life inside nebulae — vast clouds of gas and dust spread across space. These clouds are mostly made of hydrogen, with traces of helium and heavier elements.
Nebulae can span hundreds of light-years, yet are so thin that they would be considered a vacuum by Earth standards.
Gravity Starts the Process
Star formation begins when a region of a nebula becomes slightly denser than its surroundings. This can be triggered by:
Shockwaves from nearby supernovae
Collisions between gas clouds
Galactic gravitational forces
As gravity pulls material inward, the region collapses and heats up.
Protostars: A Star Is Born
As material collapses, it forms a protostar:
Dense and hot
Surrounded by a spinning disk of gas and dust
Not yet producing energy through fusion
This surrounding disk may later form planets, moons, and asteroids.
Ignition of Nuclear Fusion
When the core temperature reaches about 10 million °C, hydrogen nuclei begin to fuse into helium. At this moment, a true star is born.
Fusion creates outward pressure that balances gravity, stabilising the star.
Why Stars Are Still Forming
Star formation continues today because galaxies still contain large amounts of gas. The Milky Way forms several new stars each year.
Key Takeaways
Stars form in nebulae
Gravity triggers collapse
Fusion marks a star’s birth
Planets form from leftover material