Cycles in the Sun
In a classic Far Side cartoon by Gary Larson, we see a man sitting at a desk surrounded by fans, reading a newspaper. On the wall beside him is a large lever with two positions labeled "rise" and "set", and the caption reads: "Inside the Sun". Our nearest star is actually much more dynamic than it might first appear, and a short paper published this week has revealed fresh evidence of something very interesting beneath the surface.
To anyone who has ever seen the Sun through a telescope (equipped with a special filter to protect your eyes!), the most obvious features are sunspots -- small dark patches about the size of the Earth that are strongly magnetic and cooler than their surroundings. If you recorded the position of the spots every day for weeks, you would see them come and go tracing the fluid rotation of the Sun -- moving faster near the equator and more slowly towards the poles. If you kept careful records of the sunspots over decades, you would notice a regular rise and fall in the number of spots every 11 years. This is the most visible manifestation of an underlying magnetic cycle in the Sun, where the magnetic bubbles that appear as sunspots are periodically stretched out, reorganized and recycled by the rotation and other motions deeper in the interior.
Although we cannot see spots on other stars directly, long-term studies of stars like the Sun show similar magnetic cycles. All other things being equal, stars that rotate faster generally have shorter cycles, since they are more efficient at recycling their magnetic bubbles. However, when astronomers examine in detail the relationship between rotation and the length of the magnetic cycle, there seem to be two different types of stars -- "active" stars that spin faster than the Sun and have magnetic cycles every 400 rotations, and "inactive" stars that spin more slowly and exhibit magnetic cycles every 90 rotations. Some of the "active" stars show both types of magnetic cycles simultaneously, suggesting that the two types of cycles might actually just be operating in different regions of the star.
The paper published this week by a team of mostly British astronomers used an innovative tool to study the magnetic cycle of the Sun -- they peered beneath the surface with the help of sound waves that bounce around inside and set up standing waves with specific frequencies, a technique known as helio-seismology. As the Sun moves through its magnetic cycle, the frequencies of these standing waves change slightly. Looking at the changes in these frequencies over 25 years the team noticed not only the expected 11-year variation, but also a regular 2-year variation that appeared to be operating independently. Although the Sun itself seems to be peculiar, other stars that show an "active" magnetic cycle every 11 years also show a secondary "inactive" magnetic cycle -- every 2 years. This is the most direct evidence to date that the Sun might actually have two different magnetic cycles operating simultaneously on the inside.
Although the Sun does not seem to be a typical star, astronomers can study it in much more detail to understand how stars work in general. As this new observation suggests, it is equally important to study a variety of other stars to identify what is peculiar --- and what is normal -- about our nearest star.
