The enormous pressure at the core of the Sun, caused by the weight of the Sun's mass under its own gravity, is great enough to crush the nuclei of hydrogen together. Normally the positive electrical charges of the hydrogen nuclei repel one another, preventing close contact between them.
When the nuclei are crushed closely enough together, they are attracted to each other by the strong nuclear force, which overpowers the repulsive electrical force. The two nuclei are then "fused" together into a single helium nucleus, releasing a considerable amount of energy in the process. This released energy is the Sun's source of heat and light.
For five billion years the Sun has been consuming its hydrogen fuel at a huge rate, each second releasing the energy of 100 billion-megaton hydrogen bomb detonations! Fortunately, the Sun is very large and contains enough fuel to continue burning at this rate for another five billion years.
Sun has a diameter of about 875,000 miles, about 110 times that of Earth. The volume of space that the Sun occupies is over 1.3 million times that of Earth. Volume-wise, 1.3 million Earths could fit inside the Sun. This image compares the sizes of a limb of the Sun next to the Earth. Don't blink. The Sun contains almost all of the matter in the Solar System: 1,989,100,000,000,000,000,000,000,000,000 kilograms--almost 2000 billion billion billion kilograms. That is roughly 330,000 times the mass of Earth.
The distance at which the Earth orbits the Sun is about 93 million miles (148 million kilometers), 400 times farther than the Earth-Moon distance. Coronal Mass Ejections, traveling at up to a million miles per hour, still take at least 4 days to travel from the Sun to the Earth.
Image Credit: SOHO/EIT Consortium