The Sun has the reputation of being the most stable burning star, astronomers observe. Its extreme stability allows humans to exist on Earth. But this stability won’t last forever.
Over billions of years, the Sun has grown increasingly brighter and larger as the nuclear fusion of hydrogen into helium progresses outward from its center. Today the Sun burns about 35% brighter than it did when God created the first primitive life on Earth 3.86 billion years ago.1 In another 30 million years, the intensified brightness will be too great for any Earth-life to survive.2
Even short-term variations in the Sun’s burning, depending on the intensity of the fluctuation, can render advanced life impossible. Astronomers designing detailed models of stellar interiors recognize that all stars, including the Sun, must exhibit some short-term fluctuations. The size of the short-term fluctuations, then, proves crucial to life’s existence.
In principle, the Sun’s fluctuations should be easy to measure. Gravity operates to shrink the Sun while radiation works to expand it. The effect of gravity depends on the Sun’s total mass. While the solar wind causes a loss of mass to outer space, and absorption of comets and dust causes a small gain, these effects influence only long-term variations. Astronomers calculated that energy gates and dams operating in the Sun’s interior must produce short-term fluctuations in its radiation output. The level of such fluctuations, however, would cause only minute changes in the Sun’s diameter, measuring an arcsecond or less (one arcsecond = 0.00055 of the Sun’s apparent angular diameter).
However, in practice, measuring the diameter to the required degree of accuracy proves difficult. Over the past 300 years, astronomers made several attempts to measure variations in the Sun’s diameter with ground-based telescopes.3 Early space-based measurements provided results 10 times more precise.4 But, these results were either inconclusive or marginal.
New results from the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) satellite delivered the long-sought measurements. This amazing instrument supersedes the precision of ground-based telescopes by several hundred times. The MDI measures the solar diameter to an accuracy of better than one milliarcsecond (one part in 1,800,000).
The MDI detected a correlation between the solar diameter and the eleven-year sunspot cycle.5 Solar diameter fluctuations measured only 20 milliarcseconds (much smaller than predicted by some researchers). Independent helioseismic (sunquake) measurements made about the same time confirmed this tiny amplitude.6
Astronomers anticipated the relationship between the solar diameter and sunspot activity, since the Sun’s magnetic field drives sunspots. The rate of nuclear fusion determines total flow of energy from the Sun. Therefore, when magnetic energy rises, heat energy falls. Less heat energy means less force to drive solar expansion. Thus, astronomers had expected to find that the Sun would shrink slightly when the magnetic field grows stronger and expand slightly when the magnetic field weakens.
However, the MDI instrument revealed the opposite correlation: the stronger the magnetic field on the surface, the larger the solar diameter. The solution to this mystery comes from recognition that photons produced near the core take nearly a million years to reach the surface. A deep-seated cause for variations in the Sun’s diameter and surface magnetic field strength is consistent with the apparent out-of-sync correlation seen on the surface. Since magnetic field disturbances travel faster through the interior than the photons arising from nuclear fusion, the magnetic field and radiation fluctuations arising from deep-seated disturbances arrive on the surface at different times.
These new findings validate predictions arising from astronomers’ most detailed solar models. The confirmation of a deep-seated source for the observed solar oscillations and the low level of those oscillations strengthened the certainty of astronomers’ understanding of solar physics.7 This confirmation maintains consistency with the recent resolution of the solar neutrino problem. (See “Missing Solar Neutrinos Found,” pages 10-11.)
These findings further establish that careful solar design enables life’s existence. The right structure, composition, interstellar environment, size, and age produce the radiation flow just right for the long-term maintenance of primitive life and the short-term sustenance of human life on Earth.