In a previous answer I described how SMM had confirmed that the output of the Sun seems to be modulated by the presence of absence of sunspots and other solar active regions. At a level of 1 part in 100 of its output, the Sun is indeed a variable star.
In astronomy, it used to be that a star could be variable only for structural reasons such as it is an evolved star undergoing pulsation, a supernova or nova; and then there were eclipsing variables whose light variations are caused by periodic eclipses with a companion star. There are now any number of phenomena that can change a star's brightness and it is becoming clear that ALL star may be variable at some level and for some span of time.
The SMM satellite carried a special photometer to measure the Sun's brightness, and as the figure above shows, this brightness follows the solar cycle/sunspot cycle pretty nearly. Further research is planned to track the Sun's brightness for several solar cycles. The total change is a fraction of a percent in the power we receive at the Earth's distance which in the units of the above figure are about 1368.
For more information on solar irradiance changes, visit the SMM Page at the NASA Marshall Space Flight Center.
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.