The Sun produces a lot of light every second and it has been doing that for billions of years. How does it or any other star produce so much energy for so long? This section will cover how stars produce their energy. Astronomers have known for a long time that the Sun produces a tremendous amount of energy. The first part of

Solar Luminosity---huge energy output!

• The first basic question about the Sun is how bright is it? It puts out A LOT of energy every second. How much? The answer from our measurements is 4 × 10²⁶ watts. Such a large number is beyond most of our comprehension, so let's put the Sun's total energy output (ie., its luminosity) in more familiar units. It is equal to 8 × 10¹⁶ of the largest power plants (nuclear or hydroelectric) on the Earth. Our largest power plants now can produce around 5,000 Megawatts of power. Another way to look at this is that the sun puts out every second the same amount of energy as 2.5 × 10⁹ of those large power plants would put out every year---that's over two billion!

Possible Sources of Energy

• What could produce that much energy every second? Let's first rule out other likely candidates. How about chemical reactions? The most efficient chemical reaction is combining two hydrogen atoms and one oxygen atom to make a water molecule plus some energy. Such a reaction has a very small ``efficiency'' (something like 1/66000000 of one percent). The efficiency = amount of energy released/(mass× c<sup>2</sup>), where ``mass'' is the total mass of all of the atoms involved and c is the speed of light. The amount of energy the Sun has stored = the efficiency × (the mass of the fuel source) × c².
• To find out how long the Sun would last, you need to find out how much energy the Sun has stored in its account and know how fast it makes withdrawals on its account. The amount of time it would last is the amount of energy stored divided by the rate of withdrawal: lifetime = energy stored/consumption rate = E stored/Luminosity. Makes sense, yes? If the Sun could use all of its hydrogen to make water, the chemical reactions would only power the Sun for about 18,000 years. However, the amount of oxygen is much less than the hydrogen, so the chemical reactions can power the Sun for only 30 years.
• We need a reaction with a higher efficiency. How about the ultimate in efficiency---a complete matter to energy conversion with 100% efficiency. Such a reaction could power the Sun for 10¹³ years. Unfortunately, there are problems with this because the number of heavy particles (protons + neutrons) in the Sun must stay the same and protons are extremely stable---they do not spontaneously change into energy (photons).

Gravitational Contraction Doesn't Power the Sun Long Enough

• How about gravitational settling? This is a fancy way of referring to the converting of the potential energy of the falling layers to kinetic energy. When you hold a rock above the ground it has stored energy (``potential energy''--it has the potential to do some work). The stored energy is released as you let it fall. The rock gets kinetic energy because it is moving. Kinetic energy can heat things up. This is what would happen to the layers of the Sun if they were to fall inward toward the center of the Sun. The gas would be compressed and, therefore, would heat up. In addition to the expected heating, the gas would also radiate light.
• Until the beginning of this century, this was the idea physicists strongly argued for. This gravitational energy (with an efficiency of 1/10000 of one percent) could power the sun for 30 million years---a nice long time except for the nagging but ever louder criticism of the biologists who needed more time for evolution to occur and the geologists who preferred the idea of an unlimited age for the Earth but would stomach something like a few billion years for the age of the Earth. A good article on the age-of-the-Earth debate is in Scientific American August 1989 pages 90 to 96. Eventually, physicists had to change their minds about the age of the Sun (and Earth) as radioactive dating indicated a 4.6 billion year age for the solar system and, therefore, the Sun. It was the fact that the Sun could not last long enough being powered by gravitational contraction that motivated the search for nuclear power sources.

Nuclear Fusion Needs Extreme Temperatures and Densities

• Nuclear power is the only thing left to power the Sun for as long as it has been shining. There are two types possible: fusion and fission. They both transform the nucleus of an atom into another type of nucleus. Fission produces energy by breaking up massive nuclei like uranium into less massive nuclei like helium and lead. Fusion produces energy by fusing together light nuclei like hydrogen to make more massive muclei like helium. Atomic power plants and the Atom Bomb use fission to get the energy. Stars and Hydrogen bombs use fusion.