The Universe in the Classroom

Keepin' an Eye on the Sun

Such a Magnetic Personality

Because the Earth is solid, all parts of it must spin around the axis at the same rate. Regions near the poles must spin around once a day, the same as regions near the equator, or else the planet would fly apart. The fact that the Sun is a gas, however, lets different regions pass by one another fluidly. In this way, the equator can spin around faster than the poles, and in fact it does. A point on the solar equator takes only 26 days to travel all the way around the Sun, whereas a point near the poles takes about 10 days longer.

This differential rotation has interesting ramifications for the Sun's magnetic field. Imagine the Sun's magnetic field lines as ropes that get stretched and kinked and tangled as different latitudes of the Sun move around at different rates. This bizarre distortion of the magnetic field results in active phenomena on the surface of the Sun. Sometimes the magnetic field restricts the energy flow to the surface, resulting in cooler, darker, roughly Earth-sized patches called sunspots. Sunspots look dark only compared to the surface of the Sun; if we could somehow float a sunspot in chilly outer space we'd see the spot glow red hot.
Giving up the ghost. The Sun occasionally throws off large amounts of hot, electrically charged gas in events referred to as coronal mass ejections. Thought to be due to instabilities in the solar magnetic field, CMEs can contain tens of trillions of kilograms of gas. This particular CME was observed in December 1996, as the Sun lay in the constellation Sagittarius, by one of the instruments on the SoHO spacecraft. The circular mask in the center of the image covers the Sun's bright photosphere to permit study of the tenuous corona. Image courtesy of SoHO/LASCO consortium. SoHO is a project of international cooperation between ESA and NASA.
More violent phenomena also result from the Sun's warped magnetic field. If field lines poke an elbow out the surface of the Sun, material can travel along them in streams, creating a prominence. These huge loops of gas are typically ten times the diameter of the Earth and can last for weeks. Magnetic instabilities in the solar field cause short, violent ejections of gas called solar flares-the most explosive events on the Sun.

The Sun's atmospheric activity peaks about every 11 years, a pattern known as the sunspot cycle. At the low point in the cycle, called solar minimum, there are very few sunspots, and those that do appear are located far from the Sun's equator. During the next solar maximum in the year 2001, however, there may be hundreds of sunspots on the Sun at once, and several prominences or flares.

Just when you think you've got our star figured out, out leaps another provoking magnetic characteristic: orientations of the magnetic poles reverse for every solar maximum. For example, if at one maximum the magnetic north pole is "up" and the south pole "down," at the next maximum the magnetic north pole will be "down" and the south "up." So the entire solar cycle lasts twice as long as the sunspot cycle, 22 years! This is the period of time needed for the Sun to return to the exact same configuration (the same point in the sunspot cycle with the same orientation of the poles).

(carefully) Caught in the Act

Note: NEVER look directly at the Sun, even for a short time. It can cause permanent damage to your vision without causing any physical pain.

There are two ways to observe the Sun, from Earth and from outer space. The SoHO spacecraft is returning an enormous quantity of exciting data, but Earth-based observers, despite the problem of a rising and setting Sun, can also obtain excellent information-especially if they combine their efforts!
Sun Diagram
There's no chewy middle. The Sun is an enormous ball of hydrogen, helium, and a tiny amount of heavier elements, that is shaped by the inward pull of gravity. Gas pressure in our star is directed outward and balances gravity's perpetual force at every point in the star. The gas pressure is due to energy liberated by thermonuclear fusion in the Sun's core. That energy heats the gas and ultimately leaks out of the Sun through its multi-layered atmosphere. Image courtesy of SOHO/MDI/SOI consortium. SOHO is a project of international cooperation between ESA and NASA.

A network of telescopes can be used for long observations in spite of the setting Sun. People used to say, "The Sun never sets on the British Empire," because the Sun was always shining on one of the British colonies scattered about the globe. Perhaps today people should say, "The Sun never sets on GONG" (the Global Oscillations Network Group). GONG incorporates six telescopes, at sites in Australia, Hawaii, California, Chile, Africa, and India, to piece together a continuous observation of the Sun that can last for days or weeks. Long observations by GONG provide much information on helioseismology, the branch of solar physics dealing with study of periodic motions on the Sun's surface. As earthquakes on Earth tell us about the inside of our planet, vibrations on the surface of the Sun communicate much about the internal solar structure. These experimental portraits of the Sun help test theories about the general workings of stars.

There are several safe ways for anyone to observe the Sun indirectly with little or no equipment. The easiest device to use is a pinhole camera. The opening can be any shape; the round image of the Sun will appear in the shadow of the paper or cardboard if the hole is small enough. You can even make a pinhole camera with a hand, curling up a finger to make a small hole. Look only at the shadow, not through the hole directly at the Sun! The projected image will show the surface of the Sun. See "Making a Pinhole Camera" in this issue for three variations on this simple device.

The Sun can also be viewed safely through special types of filters. Do not look at the Sun through any filter unless you know it to be safe. Safe filters include the following:

Many filters that are sometimes recommended for observing the Sun are not safe for direct-eye viewing, including a piece of glass blackened by soot from a candle, sunglasses, photographic neutral-density filters, and solar filters that operate at the eyepiece of telescopes or binoculars. Eyepiece filters are dangerous; they are placed at the point where the Sun's light is most concentrated and magnified. They can crack or even explode.

BONNIE D. SCHULKIN works at the Harvard-Smithsonian Center for Astrophysics on educational outreach materials for the upcoming AXAF mission. She also dabbles in freelance writing, private tutoring, quilting, and volunteering at the Boston Museum of Science. Her email address is

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