* Gravity: The Sun's immense mass exerts a strong gravitational pull inwards, trying to collapse the star.
* Internal Pressure: The Sun's core undergoes nuclear fusion, releasing enormous amounts of energy. This energy creates an outward pressure that counteracts gravity.
This delicate balance between gravity and internal pressure is called hydrostatic equilibrium. Think of it like a balloon. The air inside pushes the balloon outwards, while the rubber skin pulls inwards. As long as these forces are balanced, the balloon maintains its shape.
Here's how this applies to the Sun:
* Nuclear fusion: The Sun's core fuses hydrogen into helium, releasing energy in the form of light and heat. This energy travels outwards, pushing against the star's mass.
* Stable Equilibrium: The outward pressure from nuclear fusion perfectly balances the inward pull of gravity. This means the Sun is neither collapsing nor expanding.
Key takeaway: The Sun's size and shape are a result of a continuous battle between gravity trying to crush it and the immense internal pressure from nuclear fusion trying to expand it. This balance is what keeps the Sun stable and shining.
