|MadSci Network: Astronomy|
A very a good question and particularly relevant these days when scientists are trying to understand the things which might affect global warming, ice ages, and general climate change.
The sun is a very stable star and is not prone to sudden changes in its energy output. There are of course minor fluctuations like solar flares and solar prominences (see Solar Flare Research). While these may involve very large quantities of energy on a human scale, they are fairly small fluctuations on the solar scale. They do have some effect on the earth because they are associated with the solar wind and a large number of charged particles may be captured by the earth's magnetic field. The main effect of this is apparent in the aurora (Northern or Southern Lights). A major solar disturbance may cause radio interference, damage satellites outside the earth's protective atmosphere, and possibly trigger other atmospheric disturbances. (Some interesting accounts can be found at: http://image.gsfc.nas a.gov/poetry/storm/storms.html.)
The presence of sun spots (cool spots) on the sun's surface can alter its brightness by as much as 0.1%. (See http://science.nasa.go v/ssl/pad/solar/whysolar.htm ) without too much effect on the earth. A bigger change in the energy output of the sun of say a few percent would have a long term affect on earth's climate. Oddly the effect might not be as obvious as one might think because other atmospheric effects may come into play in unexpected ways. For example would an increase in solar heating cause an increase or decrease in cloud cover? Would warming cause clearer skys or maybe evapourate more of the sea and cause more cloud? The effect of increasing or decreasing cloud cover might also have unexpected effects. Would increased cloud cover keep us warmer by insulation, or cooler by reflecting more sunlight? There are many contributing factors which interact in complex ways, and I'm afraid their interaction is very poorly understood. However the sun itself is very stable and will remain so for many billions of years - until it uses up most of its hydrogen fuel and undergoes a radical re-configuration to keep going. Not all stars are so stable. Some pulse regularly (or intermittently) some blowing-off clouds of hot ionised gas (plasma) from time to time. When the sun uses up its hydrogen fuel, it will first collapse, this will start up the helium burning process causing to swell up to become a red giant which will give out a lot more energy than at present (although the red giant phase will not last for so long in astronomical terms). The sun is so stable there is even an astronomical parameter called the solar constant which is the average quantity of radiation per unit area at the average distance of the earth from the sun. (It's about 1.37kW/m2 - and about 60% makes it to the surface of the earth but that depends on where you are). This is used by scientists designing satellites to determine whether solar power or nuclear power should be used to power the satellite. Solar power is adequate for anything closer to the sun than Mars, nuclear is necessary for probes to Jupiter and beyond. My company built the rechargeable batteries (and some other parts) which Beagle 2 will land on Mars this week (Christmas day). We calculate there is enough sunlight on Mars to charge them up each day with solar power - so we're hoping the sun's output remains pretty stable for the next few weeks! :-). Our friends at NASA will be hoping the same for the rovers Spirit and Opportunity arriving in January. Regards, Gavin Ward
Try the links in the MadSci Library for more information on Astronomy.