Recall, the center of the Sun, a typical star is
T = 15 Million Kelvin Degree, and the Density = 150 x Water!
Pressure ~ (is proportional to) Temperature x Density
Why is the Sun HOT and DENSE at its center?
This internal PRESSURE pushes outward against the Gravitational PRESSURE
THERMONUCLEAR FUSION
THERMO = heat (in this case EXTREME temperatures)
NUCLEAR = having to do with the nuclei of atoms
FUSION = the fusing of two things to become one.
At the center of OUR SUN, 4 Hydrogen nuclei are fused to become 1 Helium nucleus.
The single Helium nucleus weighs less (smaller mass) than the 4 Hydrogen nuclei.