Our Quiet Star

An Internet guide to exploring the mysteries of sunspots

sun
NASA/JPL-Caltech/NRL/GSFC

On a visit to the newly renovated and expanded Griffith Observatory in Los Angeles I heard Jay M. Pasachoff, an astronomer from Williams College in Massachusetts, tell about his plans to travel to China to observe the total solar eclipse of July 22. Having witnessed twenty-eight total eclipses before that one, he had already seen more than anyone else. But what really got my attention was his remark that lately the Sun has been unusually devoid of sunspots. Those relatively cool blemishes on the Sun’s surface normally wax and wane in an eleven-year cycle, but this is the deepest “solar minimum” since 1913, according to NASA.

New discoveries about the nature of sunspots abound, but first, what are they? The Exploratorium in San Francisco has a short primer on the enigmatic spots. According to George Fischer, a solar astronomer at the University of California who has a video interview on the site, “A sunspot is a dark part of the sun’s surface that is cooler than the surrounding area. It turns out it is cooler because of a strong magnetic field there that inhibits the transport of heat via convective motion in the sun. The magnetic field is formed below the sun’s surface, and extends out into the sun’s corona.” The site includes the fascinating history of sunspot science, which notes that Greek philosophers made references to sunspots in their writings as early as the fourth century B.C., and astronomers in China made systematic observations as early as 28 B.C. The fourth page of the history section shows you how you can safely use binoculars to project an image of the Sun on a piece of paper for viewing.

Looking directly at the Sun to check for spots can easily damage your eyes. In ancient times, astronomers could sneak a glance at the solar disk just before sunset, or when it was covered by a thin veil of clouds, but looking directly at the Sun is never a good idea. Galileo, one of the first astronomers to train a telescope on the Sun, used a method devised by his protégé Benedetto Castelli to project the Sun’s image from a telescope onto a surface to safely study sunspots. Visit The Galileo Project, hosted by Rice University in Houston, Texas, to learn more about early sunspot observations. At this site you can view a remarkable animation made from Galileo’s original drawings, showing the sunspots as they traveled across the solar disk during the summer of 1612.

A Website at Goddard Space Flight Center in Maryland has more on the “Magnetic Sun,” of which sunspots are just one manifestation. Here the Mount Wilson astronomer who discovered the true nature of sunspots is given his due: “The nature of sunspots remained unclear until 1908, when George Ellery Hale, using an instrument that observed the Sun in narrow ranges of color emitted by selected substances, reported that the light from sunspots was modified in ways that indicated it was produced in intense magnetic fields.”

The Internet is the safest (and cheapest) way to observe our star’s spots. One of my favorite sites, Astronomy Picture of the Day has three images to get you started. Zoom in over a gaping, planet-sized hole at The Sharpest View of the Sun. Go to An Active Sunspot Viewed Sideways to see a beautiful image formed by glowing gas as it traces the magnetic sweeping out from the sunspot’s interior. Another image, Sunspot Stack, shows how the magnetic disturbances look as seen from the photosphere, the chromosphere, and the corona.

For current images go to the homepage of the Solar and Heliospheric Observatory (SOHO), a joint mission of the European Space Agency (ESA) and NASA. The SOHO spacecraft is locked in an orbit between the Sun and the Earth at about four times the distance to the Moon from us. Launched in 1995, it is the first solar observatory to have a continuous view of our star. Click on the “sunspot” image on the top right to see the latest image of the solar disk. One of the best features of this site is that it allows you to generate time-lapse movies (much like the one using Galileo’s drawings) of the sunspot activity over a period of months. Go to the menu bar at the top and click on “data/archive” and then select “SOHO Movie Theatre.” Next, click on “MDI Continuum” for the image type and enter the dates for your sunspot movie to start and stop. Using it to watch the Sun for the last few months, I saw barely a blemish. In contrast, a movie from a few years ago showed spots galore on the rotating star.

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