The Sun is a mass of incandescent gas because of the incredible pressure and temperature at its core, which triggers nuclear fusion. Here's a breakdown:

* Gravity's Role: The Sun's immense gravity pulls all its material inward, creating incredible pressure at its core.

* Nuclear Fusion: This pressure and the resulting high temperature (millions of degrees Celsius) force hydrogen atoms to fuse together, forming helium. This process releases a tremendous amount of energy, which is what makes the Sun shine.

* Incandescent Gas: This released energy heats up the Sun's material, causing it to glow brightly. The Sun is essentially a giant ball of plasma, which is a superheated state of matter where electrons are stripped from atoms. This is why we call the Sun a mass of incandescent gas.

In summary:

1. Gravity causes immense pressure at the Sun's core.

2. Pressure and temperature trigger nuclear fusion.

3. Nuclear fusion releases energy, making the Sun hot and bright.

4. High temperature creates plasma, which is why the Sun is a mass of incandescent gas.