The sun and Jupiter have very different compositions and internal pressures, which is why fusion occurs in the sun but not in Jupiter. Here's a breakdown:

Sun:

* Composition: Primarily hydrogen (70%) and helium (28%) with trace amounts of heavier elements.

* Mass: 333,000 times the mass of Earth. This immense mass creates immense gravity.

* Pressure: The gravity compresses the sun's core, creating enormous pressure (trillions of times Earth's atmospheric pressure).

* Temperature: The pressure and compression generate extreme temperatures (millions of degrees Celsius), reaching over 15 million degrees in the core.

These extreme conditions allow for nuclear fusion: The intense heat and pressure strip electrons from hydrogen atoms, creating a plasma of protons. These protons overcome their electrostatic repulsion and fuse together, forming helium nuclei and releasing immense energy in the process. This is the sun's energy source.

Jupiter:

* Composition: Primarily hydrogen (75%) and helium (24%), but with trace amounts of heavier elements.

* Mass: 318 times the mass of Earth (much smaller than the sun).

* Pressure and Temperature: Jupiter's gravity and internal pressure are much lower than the sun's. Although it has a core with a temperature estimated around 24,000° Celsius, this is not enough to sustain nuclear fusion.

Key difference: Jupiter simply lacks the mass and therefore the gravitational pressure and temperature necessary to initiate and sustain nuclear fusion. It's essentially a giant ball of gas, not a star. While Jupiter does emit some heat, this is generated by gravitational compression and not nuclear fusion.

In summary: The sun's immense mass and resulting gravity create the extreme pressure and temperature required for hydrogen fusion. Jupiter, despite being a gas giant, lacks the necessary mass and internal conditions for this process to occur.