We tend to think of our Sun as a paragon of stability. While other stars pulsate, go nova, collapse, or bubble and churn like overheated pots of oatmeal, Sol provides us with steady, dependable radiance. Of course there are variations, such as the 11 year sunspot cycle, but these are predictable and benign.
Recent evidence, however, suggests that we don't know all there is to know about the nearest star. Drs. David Gray (University of Western Ontario) and William Livingston (Kitt Peak) have been studying the Sun's temperature with a technique that compares the strength of absorption lines in the solar spectrum. Gray finds that in addition to a fluctuation of 1.5 degrees Kelvin over the 11-year cycle, the temperature of the sun is steadily increasing by 0.014 degrees/yr.
Of course, this doesn't mean that the Sun will just continue to heat up. The observed change may just be part of a much longer cycle. During the 17th to mid-18th centuries, astronomers noticed a complete absence of sunspots and geological records show that the Earth's temperature dropped by 1 to 2 degrees during that time. This may not seem like much, but it was enough to freeze the Thames river and shorten Europe's growing season, causing famine in many countries.
Gray notes that other stars also show this behaviour. In fact, two thirds of measured stars have variations an order of magnitude larger than those of the Sun - 15 degrees over cycle instead of 1.5 . Other stars, such as Tau Ceti and Eta Cephei, show no detectable temperature changes.
What is unique about our Sun is that temperature, luminosity, and magnetic activity are all linked: a drop in one means an immediate drop in the other two. In other stars, such as Beta Comae, the temperature begins to rise 3 years after we see the magnetic activity pick up. Cooler Epsilon Eradini has a gap of only 0.3 years. By this scheme, the Sun should have a lag of about 2 years, contrary to the simultaneous changes we observe.
Gray believes that fluctuations of this type are almost certainly due to the activity of dynamos which power the magnetic fields of cool stars. Researchers are currently trying to build a database to understand why stellar dynamos sputter and cough in this way.
What does this kind of variablity mean for the Earth's climate? It is possible that the current trend is partly responsible for the global warming - although this doesn't mean we can keep polluting the atmosphere. The greenhouse gases emitted by industry only serve to amplify any other warming effect. However, more research is necessary before we can say whether this small rise in the temperature of the Sun will continue, or will have any effect on us. Gray, for one, is interested in obtaining data over a longer time to see if this rise continues or flattens out in the long run.