The most important energy machines in the universe are stars. They can produce the right blend of heat and light to sustain planets and in at least one case, life. They also produce the elements needed to make planets and us.

What powers these campfires in the cosmic dark?

Our remote ancestors saw the sun, moon and stars as being part of the heavens, along with the immortal gods and remote from all of us living down here on the ground. They were exempt from our rules. However, in the Renaissance we came increasingly to accept the idea that we are all part of the same universe, and that the laws of physics applying to the stars and objects in the sky are the same as the ones applying to us. The sun was a physical object just like the earth, and since it has been shining more or less steadily on our world since the beginning it had to be at least as old. For a long time the best estimate of the age of the Earth was that made by Bishop Ussher, who estimated it is about 6,000 years old. Having a sun shine for that long should not be a problem.

However, geologists were coming up with more and more evidence that the Earth has to be very much older. The rocks revealed story after story of repeated erosion, deposition, heating, compression, uplifting and tilting. This meant the Earth has to be tens or hundreds of millions of years old. In fact we now know the Earth is about 4.6 billion years old, as must be the sun.

The sun has a diameter of about 1.5 million kilometres. A ball of fossil fuel that size, with a supply of oxygen to help it burn, could sustain the sun鈥檚 current energy output for a few million years. Another idea was that the sun produces its energy by collapsing under its own gravity. This process does better; it should work for some tens of millions of years, which is still not enough.

In the early 20th Century Albert Einstein came up with his famous relationship between energy and mass, and physicists were making significant progress in understanding atomic processes and the fundamental nature of matter. They realized such processes have the potential to release colossal amounts of energy, and that stars might be powered by nuclear reactions. The first suggestion was that in the extremely hot and highly compressed cores of stars, matter is totally converted into energy. This 100 per cent efficient process raises a different problem. The consumption of fuel would be so low that all the stars around us would be still very young. There would be no ageing stars, burning the last dregs their fuel, at least not for tens of billions of years. In fact there are lots of old stars in the sky, we see stars dying and many remains of stars that died in the past. We need something a bit less efficient.

We now know that stars are nuclear powered. They get energy by converting small atoms into larger ones, and in the process turn a very small amount of the material into energy. For the Sun, its energy production process turns about 4 million tonnes of material into energy every second. Fortunately the sun contains a lot of material. There are stars that are producing energy at rates of over a million times that of the sun, and converting billions of tonnes of matter a second.

When we drop matter into a black hole, as it vanishes inside we get a flash of energy very close to 100 per cent conversion of mass into energy. However, the energy production rate varies hugely depending on what the black hole is eating at the time, and the radiation is so intense, rich in X-rays and dangerous radiation, that any planets unfortunate enough to be close by will be sterilized and eventually the cinders will be swallowed.

Top of FormJupiter is high in the southern sky overnight and Saturn rises before midnight. Venus lies low in the dawn glow, and Mercury even lower, the moon will reach Last Quarter on May 18.

Ken Tapping is an astronomer with the NRC鈥檚 Dominion Radio Astrophysical Observatory, Penticton.