It is said that the Sun emits more energy in one second than mankind has consumed in the whole of its history! The inherent source of the solar output, in a word, is fusion! In the core of the Sun, nuclear reactions take place and convert the matter into energy. Within the core, density is about 12 times the density of solid lead or 160 times the density of water. The temperature reaches a peak of some 15 million degrees Celsius. under such extreme conditions, nucleus of one hydrogen atom fuses with that of another. The net result is the conversion of four hydrogen protons into one helium-4 nucleus, but some mass is lost during the process and transformed into energy. (See diagram below).
With many fusion reactions taking place simultaneously, the Sun loses 4.3 million tons of mass every second, by converting 600 million tons of hydrogen into just under 596 million tons of helium. Thus, only a small fraction (0.7%) of the mass of hydrogen going into nuclear fusion reaction does not show up as the mass of helium. Yet this lost mass is converted into huge amount of energy because Einstein’s equation E=mc2 is at work here. (e denotes energy, m is mass lost and c is speed of light, squared). To take specific example, consider the conversion of 1 kilogram of hydrogen into helium. Despite the fact that a kilogram of hydrogen goes into the nuclear reaction, only 0.993 kilogram of helium is produced. Using Einstein’s formula, it is easy to ascertain the energy output:
E=mc2 = (0.007 kilogram) (3,00,000)2
= 6.3 x 1014 joules
This is equivalent to energy released by burning 20,000 tons of coal. And remember, we are talking about of only 0.007 kilogram of the Sun’s lost mass, whereas the total mass it loses every second is nearly 40 million tons. Although the Sun has been releasing prodigious amount of energy since last 4.5 billion years, it has so far lost only a few hundredth of 1% of its original mass of hydrogen.