The birth of a new star is marked by a process called nuclear fusion, which begins when a cloud of gas and dust in space collapses under its own gravity. This event is generally triggered by external forces such as shock waves from nearby supernovae or the collision of molecular clouds.
As the gas and dust accumulate, they form a dense core that heats up due to gravitational pressure. When the core reaches a sufficient temperature (around 10 million degrees Celsius), hydrogen atoms start to fuse into helium, releasing massive amounts of energy in the process. This energy creates a balance between gravity pulling inwards and thermal pressure pushing outwards, allowing the star to stabilize.
Thus, the transition from a collapsing cloud to a steady star that actively fuses hydrogen signifies the birth of a new star in the universe.