why do the sun shine

The Sun shines because of nuclear fusion occurring deep within its core. This process releases enormous amounts of energy that eventually escape into space as light and heat. 
The Process of Nuclear Fusion
The Sun is essentially a massive, hot ball of gas, mainly hydrogen and helium. 
1. Extreme Conditions: In the Sun’s core, the pressure and temperature are incredibly high—about 15 million degrees Celsius (27 million degrees Fahrenheit). 
2. Hydrogen Fusion: Under these extreme conditions, hydrogen nuclei (protons) are forced to collide and fuse together in a process called the proton-proton chain reaction. 
3. Creating Helium: This chain reaction converts four hydrogen nuclei into a single helium nucleus. 
4. Mass-Energy Conversion: The resulting helium nucleus has slightly less mass than the four original hydrogen nuclei. This “missing” mass is converted into a burst of energy, as described by Einstein’s famous equation, E=mc^2. 
Energy’s Journey to Light
The immense energy released in the core is primarily in the form of high-energy gamma rays and fast-moving particles. This energy then travels outward through the Sun’s layers: 
• Radiative Zone: Gamma rays bounce around, being absorbed and re-emitted countless times, which can take over a hundred thousand years for a single photon to reach the next layer.
• Convective Zone: Hot plasma circulates, carrying the energy toward the surface. 
• Photosphere (The “Surface”): This is the outermost layer of the Sun that we see. By the time the energy reaches the relatively cooler photosphere (about 5,500 degrees Celsius or 10,000 degrees Fahrenheit), it is emitted as visible light, along with other forms of electromagnetic radiation like infrared and ultraviolet. This light travels across space to Earth, causing the Sun to shine. 
This process of nuclear fusion provides the constant, powerful source of energy that sustains the Sun and allows it to shine steadily.