What's it all mean?

What are the conditions for nuclear reactions to occur? Why can stars do it and we can't (easily)? For hydrogen, the temperature has to be in excess of 10 million degrees Kelvin ( K). We never encounter temperatures like this on Earth, or I should say never did until 1947. In 1947 the United States produced the first hydrogen bomb, or fusion bomb. This used the tremendous heat generated by a fission bomb to cause the proton-proton cycle to ``go''.

In the Sun (and other stars), these temperatures occur naturally and more benevolently.

The Sun can and should be viewed as a magnificent equilibrium. The enormous gravitational force of such a large mass produces huge pressure and temperature in the center. The huge temperatures are sufficient to cause fusion reactions to occur. The fusion reactions release energy which keeps the central temperature high and allows the star to support itself against its own weight. When you look at the Sun you are looking at a controlled H bomb which has been ``going off'' for billions of years.

Let's collect these ideas.

• The energy for the prodigious energy requirements of the Sun come from its mass itself. The energy given off by nuclear fusion reactions is the only process that can satisfactorily account for these energy requirements.
• By extension, the other stars in the sky are also thermonuclear furnaces. We will see additional evidence that this is the case.
• Since a star obtains its energy from its own mass, the bigger the mass the longer it will last, BUT the higher the power output (Luminosity is the correct technical term), the less long it will last.