Which conditions are required for nuclear fusion to begin? The gas cloud must be very dense. Gravitational attraction must stop. Atoms must be stripped of their electrons. Helium atoms must break down into hydrogen. Temperatures must exceed 14 million Kelvin.

Fusion requires temperatures about 100 million Kelvin (approximately six times hotter than the sun's core).

At these temperatures, hydrogen is a plasma, not a gas ...

The sun achieves these temperatures by its large mass and the force of gravity compressing this mass in the core.