Ken Tapping, January 20, 2010

For millions of years, every square metre of the top of the Earth's atmosphere has been receiving solar energy at a rate close to 1366 Watts. There have been slight variations that have been reflected in changes in climate, such as the "Little Ice Age", in the 17th Century. However, on the whole, the solar energy supply to the Earth has remained remarkably stable. It is this energy supply that drives our environment and most ecosystems. Even a small change, of say a few percent would be catastrophic. Since the rate at which we receive energy from the Sun changes with the distance between the Earth and Sun - like getting closer to or further away from a fire - there is a small range of distance we have to stay in for life to remain possible. Venus is too close to the Sun, and Mars is close to the outer edge of the habitable range.

Over the centuries we have been observing the sky, astronomers have discovered lots of stars that are not as well-behaved as the Sun; some vary by a few percent, others by a factor of ten or more. Some cycle in a well-behaved pattern while others vary irregularly. Actually, considering what stars are, at first sight it is remarkable that stars like the Sun are so stable, rather than that others vary.

Stars form from the collapse of huge clouds of cosmic hydrogen. The result is a ball of gas, where the weight of the overlying layers compresses and heats up the core so that nuclear fusion starts. Hydrogen is converted into other elements, and a lot of energy is released. This further heats the core and provides the energy that makes the star shine. Increases in temperature in the core increase the pressure. This pushes the overlying layers upwards, which relieves the additional pressure and allows the core to cool off a bit. This process acts like the governor on a steam engine, or the safety valve on a boiler, stabilizing the star's energy output. In lots of cases, the stabilizing process gets out of kilter and allows the temperature in the star's core to oscillate. We've all heard the engines in power mowers and lawn tractors. Sometimes they tick over steadily, and sometimes they go "rhum, rhum, rhum".

Some stars oscillate in brightness in a manner that enables us to calculate their energy output. By measuring how bright they look in the sky, we can deduce how far away they are. These stars, called cepheid variables, are invaluable rulers for measuring the distances of distant galaxies. We just look for cepheids and then measure them.

During a star's life, its core gets loaded with waste products from energy production. This causes the star to brighten as it ages, and then swell into a red giant star, and to vary slowly and irregularly in brightness. This will happen to the Sun in about three billion years' time.

When enjoying the next sunny day, don't just take our star for granted, remember the processes we all have to thank for our being here to appreciate it, and hope it remains stable, and doesn't decide to go "rhum, rhum, rhum" for a bit.