Stars shine because they are hot. Since stars are hot and dense, they emit energy like a black body, with a luminosity proportional to the fourth power of their temperature.
If a star had no power source to replace the energy radiated away in the form of photons, it would cool down. It would take some time to cool down, however, like an iron that's been unplugged. If the Sun, for instance, had no internal power source, it would cool to invisibility in 20 million years.
Chemical reactions, such as burning, are not powerful enough to keep the Sun shining for billions of years.
For instance, consider burning hydrogen:
2H2 + O2 --> 2H2O + energy
Burning one kilogram of hydrogen releases 140,000,000 Joules of energy. (In other units, this is equal to 39 kilowatt-hours, or 34,000 Calories -- enough energy to keep a 100-watt bulb lit up for over two weeks.)
If the Sun were initially made entirely of hydrogen, burning that hydrogen (never mind the problem of where to get the oxygen) would only provide enough energy to power the Sun for 20,000 years. After that the Sun would go into its 20 million year long cooling-off period.
We need a much more powerful energy source.
Atoms within a molecule are glued together relatively weakly by
electric forces. The protons and neutrons within an atomic
nucleus are glued together much more strongly, by a force which
is called (appropriately) the strong nuclear force.
Thus, chemical reactions, which involve breaking
and merging molecules, release much smaller amounts of energy
than nuclear reactions, which involve breaking
and merging atomic nuclei.
(Parenthetical note on vocabulary: Reactions which involve
breaking up nuclei are called fission reactions.
Reactions which involve merging nuclei are called fusion
reactions.)
For an example, consider the fusion of hydrogen:
4 1H --> 4He + energy
(The little superscript in front of each chemical symbol tells you the total number of protons and neutrons in each nucleus.)
Fusing one kilogram of hydrogen into helium releases 650 trillion Joules of energy. This is over 4 million times the energy released by burning the same amount of hydrogen.
If the Sun were initially made entirely of hydrogen, fusing that hydrogen into helium would provide enough energy to power the Sun for 100 billion years (a time comfortably long compared to the Sun's current age of approximately 5 billion years).
So, if fusion is such a potent energy source, why don't we all have a portable fusion reactor in our basement? After all, fusing just half a ton of hydrogen into helium would supply the entire energy needs of the United States for one day. Well, the problem with fusion is that atomic nuclei are positively charged, since they are packed with protons. Since like charges repel, in order to bring two nuclei close enough together for them to fuse, you must overcome the electric repulsion between them.
Overcoming the electic repulsion between hydrogen nuclei requires high random velocities (T > 10,000,000 Kelvin). Even at these high temperatures, fusion is inefficient; only a minuscule fraction of collisions between nuclei result in fusion. To compensate for the low efficiency of fusion, you need high densities (> 10 grams/cubic cm) of hydrogen. These hot, dense conditions occur in the centers of stars.
In the very hot, dense conditions of a star's center, all matter will be ionized. Hydrogen nuclei (which consist of nothing but a proton) will be streaking about at high speed, entirely naked, with no electrons bound to them.
Four protons (or hydrogen nuclei) will rarely collide simultaneously. Instead, the fusion of hydrogen nuclei into helium nuclei will occur by a series of two-particle collisions. In low-mass stars (M < 1.1sun), the dominant process for producing fusion is the proton-proton chain.
It is called the proton-proton chain because the first step consists of fusing two protons together:
1H + 1H --> 2H + e+ + neutrino
In the above equation,
The next step in the proton-proton chain involves fusing a proton and a heavy hydrogen nucleus:
2H + 1H --> 3He + gamma-ray photon
In the above equation,
Repeat the two reactions above, so that you have a total of two light helium nuclei. The last step involves fusion the two light helium nuclei together:
3He + 3He --> 4He + 1H + 1H
THE BOTTOM LINE: four hydrogen nuclei are converted into one helium nucleus. The energy released emerges in the form of positrons, neutrinos, and gamma-ray photons. The neutrinos zap straight out of the Sun and are lost, but the positrons and photons are trapped inside, and act to heat up the Sun. (Mind-boggling fact for the day: Every second, day and night, over one trillion neutrinos go zapping straight through your body from the Sun.)
A graphic depiction of the proton-proton cycle (The red dots are protons; the green dots are neutrons):
More massive stars (those with M > 1.1 Msun) have hotter central temperatures (T > 16,000,000 Kelvin). At these high temperatures, fusion of hydrogen into helium occurs by a different process: the CNO cycle. (CNO stands for Carbon, Nitrogen, Oxygen.) In the CNO cycle, a carbon nucleus acts as a catalyst for the conversion of hydrogen into helium. During the course of the CNO cycle, the carbon nucleus is converted into a nitrogen nucleus and an oxygen nucleus, but in the end, emerges unscathed. For completeness, I am showing the complete CNO cycle below (but it won't be on the quiz!)
THE BOTTOM LINE is the same for both the proton-proton chain and the CNO cycle: four hydrogen nuclei are fused together into a single helium nucleus. In the process, energy is released which keeps the Sun hot.
