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Physics LibreTexts

11.17 Sunspots

Aristotle, on philosophical grounds, declared the Sun a perfect, flawless sphere. Careful observers, however, noted that sometimes the Sun appears to be doted with imperfections. These dark spots are visible to the naked eye sometimes when the Sun is low on the horizon, or when it is dimmed by fog or a darkened piece of glass. (Warning: NEVER point any telescope or binoculars at the Sun because the concentrated light will burn your retina. Even staring at the Sun for a couple of seconds will damage your eyes. ) Others used a safer technique of projecting the Sun's light through a pinhole onto a white piece of paper and shielding it from stray light; this rudimentary form of "pinhole camera" magnifies the image and allows sunspots to be seen.

Astropedia Image
The Sun featuring several large sunspots. Click here for original source URL.


There is a rich history of pre-telescopic observations of sunspots. The Chinese astrononomer Gan De made a note of sunspots in a 384 B.C. in a star catalog he created. Astronomers of the Imperial Court were routinely observing sunspots in China by 28 B.C. Aristotle's student Theophrastus was the first to mention sunspots in the West, around 300 B.C. Through the Middle Ages, various observations were made and sunspots were noted, but they were often attributed to planetary transits.

In 1613, Galileo studied sunspots with his telescope and concluded that they are located on the solar surface and are carried around the Sun by solar rotation. You can replicate Galileo's observations for yourself with a small telescope or binoculars by projecting the sun's light onto a wall or stiff sheet of white paper. (Look at the shadow of the telescope or binoculars to align on the Sun; the shadow will be smallest when you are aligned.) Even then, Galileo had not been first to apply a telescope to sunspots. He was beaten to the punch by Thomas Harriot, the brothers Johannes and David Fabricus, and Christoph Scheiner. These early telescope observations convincingly refutred Aristotle — the Sun rotated, it changed, and it was not a perfect sphere. Ironically, soon after the telescope came along to make sunspot detection easier, there were less sunspots to detect! Edward Maunder noted the general lack of sunspots in the second half of the 17th Century. The period of low sunspot activity from 1645 to 1717 is known as the "Maunder minimum."

Today we know that sunspots are magnetically disturbed regions on the surface of the Sun that are cooler than their surroundings. A sunspot looks dark because its gases, at 4000 to 4500 Kelvin, radiate less than light than the surrounding gas at about 5770 Kelvin. The motion of solar gas near sunspots is not controlled by atmospheric forces, like terrestrial storms. Rather, magnetic fields near the Sun's surface drive the motion of solar gases. The gas that we see is composed primarily of Ions (charged atoms or molecules), which cannot move freely in a magnetic field but must spiral around the magnetic field lines and stream from one magnetic pole to the other. 

Sunspots are just part of a complex network of magnetic field lines, and they mark the places where field lines rise up through the surface of the Sun. Typically, spots are found in pairs, and they can be connected by prominences. These huge clouds of gas, larger than the whole Earth, can be seen silhouetted above the solar limb, or edge. When the field lines break, an event called a solar flare, the material previous caught up in the prominence gets blasted through space, and if it is blasted toward Earth, it can generate amazing atmospheric displays like aurora. These energetic particles can also interfere with telecommunications and cause problems for orbiting satellites.

The number of sunspots visible at any moment varies over time. The Sun's magnetic field varies over a roughly 22-year cycle. During a period called Solar Maximum there may be several hundred sunspots visible at one time, while during the Solar Minimum the number may dwindle to a handful and the Sun will start to resemble Aristotle's perfect, flawless sphere. The modern theory of sunspots dates to observations in the early 20th Century by George Ellery Hale, the pioneer who build the Mt. Wilson and Palomar observatories in California. Hale used a special spectrograph to observe splitting of hydrogen lines by the Zeeman effect, which indicates a magnetic field. He showed that the Sun reversed its magnetic poles twice each full sunspot cycle of 22 years.

The detailed physics of the sunspot cycle is complex and not yet full understood but the general features can be described. The theory involves thinking of a Sun as a dynamo, a machine that converts mechanical energy into electric and magnetic energy. The Sun has a dipole magnetic field, like the Earth, but unlike the Earth it rotates differentially, with the equation moving faster than the poles. As a result the magnetic field gets "wound up" over time and at some point that causes the magnetic field lines to cross and get tangled, with the effect of cancelling the field. Magnetic field lines are under tension, like rubber bands. The Sun plasma then reforms a small "seed" magnetic field which grows and then the process repeats. Why this chaotic process leads to such a regular reversal of polarity is not known. Our nearest star still presents us with some puzzles to solve.

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