16-8 Sunspots are low-temperature regions in the photosphere

Granules, supergranules, spicules, and the solar wind occur continuously. These features are said to be aspects of the quiet Sun. But other, more dramatic features appear periodically, including massive eruptions and regions of concentrated magnetic fields. When these features are present, astronomers refer to the active Sun. The features of the active Sun that can most easily be seen with even a small telescope are sunspots (although only with a safety filter attached).

Observing Sunspots

Sunspots are irregularly shaped dark regions in the photosphere. Sometimes sunspots appear in isolation (Figure 16-17a), but frequently they are found in sunspot groups (Figure 16-17b; see also Figure 16-7). Although sunspots vary greatly in size, typical ones measure a few tens of thousands of kilometers across—comparable to the diameter of Earth. Sunspots are not permanent features of the photosphere but last between a few hours and a few months.

Figure 16-17: R I V U X G
Sunspots (a) This high-resolution photograph of the photosphere shows a mature sunspot. The dark center of the spot is called the umbra and has the lowest temperature. It is bordered by the penumbra, which is less dark, is hotter than the umbra, and has a featherlike appearance. (Even the umbra emits visible light, but not enough to capture in an image against the glare of the Sun.) (b) In this view of a typical sunspot group, several sunspots are close enough to overlap. In both images you can see granulation in the surrounding, undisturbed photosphere.
(a: Scharmer et al., Royal Swedish Academy of Sciences/Science Source; b: NOAO)

Each sunspot has a dark central core, called the umbra, and a brighter border called the penumbra. We used these same terms in Section 3-4 to refer to different parts of Earth’s shadow or the Moon’s shadow. But a sunspot is not a shadow: It is a region in the photosphere where the temperature is relatively low, which makes it appear darker than its surroundings. As we saw in Section 5-4, Wien’s law relates the color of a blackbody, such as the photosphere, to the blackbody’s temperature; the cooler the blackbody, the longer the wavelength at which it emits the most light. If the surrounding hot photosphere is blocked from view, a sunspot’s cooler umbra appears red and the warmer penumbra appears orange. The colors of a sunspot indicate that the temperature of the umbra is typically 4300 K and that of the penumbra is typically 5000 K. While high by earthly standards, these temperatures are quite a bit lower than the average photospheric temperature of 5800 K.

The Stefan-Boltzmann law (see Section 5-4) tells us that the energy flux from a blackbody is proportional to the fourth power of its temperature. This law lets us compare the amounts of light energy emitted by a square meter of a sunspot’s umbra and by a square meter of undisturbed photosphere. The ratio is

That is, the umbra emits only 30% as much light as an equally large patch of undisturbed photosphere, which is why sunspots appear so dark.

Sunspots and the Sun’s Rotation

Figure 16-18: R I V U X G
Tracking the Sun’s Rotation with Sunspots This series of photographs taken in 1999 shows the rotation of the Sun. By observing the same group of sunspots from one day to the next, Galileo found that the Sun rotates once in about four weeks. (The equatorial regions of the Sun actually rotate somewhat faster than the polar regions.) Notice how the sunspot group shown here changed its shape.
(The Carnegie Observatories)

Occasionally, a sunspot group is large enough to be seen without a telescope. Chinese astronomers recorded such sightings 2000 years ago, and huge sunspot groups visible to the naked eye (with an appropriate filter) were seen in 1989 and 2003. But it was not until Galileo introduced the telescope into astronomy (see Section 4-5) that anyone was able to examine sunspots in detail.

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Galileo discovered that he could determine the Sun’s rotation rate by tracking sunspots as they moved across the solar disk (Figure 16-18). He found that the Sun rotates once in about four weeks. A typical sunspot group lasts about two months, so a specific one can be followed for two solar rotations.

Further observations by the British astronomer Richard Carrington in 1859 demonstrated that the Sun does not rotate as a rigid body. Instead, the equatorial regions rotate more rapidly than the polar regions. This phenomenon is known as differential rotation. Thus, while a sunspot near the solar equator takes only 25 days to go once around the Sun, a sunspot at 30° north or south of the equator takes 27½ days. The rotation period at 75° north or south is about 33 days, while near the poles it may be as long as 35 days.

CONCEPT CHECK 16-15

If the center of a sunspot has a temperature of about 4300 K, why does it appear dark?

The Sunspot Cycle

The average number of sunspots on the Sun is not constant, but varies in a predictable sunspot cycle (Figure 16-19a). This phenomenon was first reported by the German astronomer Heinrich Schwabe in 1843 after many years of observing. As Figure 16-19a shows, the average number of sunspots varies with a period of about 11 years. A period of exceptionally many sunspots is a sunspot maximum (Figure 16-19b), as occurred in 1979, 1989, and 2000. Conversely, the Sun is almost devoid of sunspots at a sunspot minimum (Figure 16-19c), as occurred in 1976, 1986, 1996, and 2008. During the 2008 minimum in sunspot activity, there were fewer sunspots observed than in any other year since 1913.

Figure 16-19: R I V U X G
The Sunspot Cycle (a) The number of sunspots on the Sun varies with a period of about 11 years. The most recent sunspot maximum occurred in 2000. (b) This photograph, taken near sunspot maximum in 1989, shows a number of sunspots and large sunspot groups. The sunspot group visible near the bottom of the Sun’s disk has about the same diameter as the planet Jupiter. (c) Near sunspot minimum, as in this 1986 photograph, essentially no sunspots are visible.
(NOAO)

The number of sunspots increases and decreases on an 11-year cycle

The locations of sunspots also vary with the same 11-year sunspot cycle. At the beginning of a cycle, just after a sunspot minimum, sunspots first appear at latitudes around 30° north and south of the solar equator (Figure 16-20). Over the succeeding years, the sunspots occur closer and closer to the equator.

Figure 16-20: Variations in the Average Latitude of Sunspots The dots in this graph (sometimes called a “butterfly diagram”) record how far north or south of the Sun’s equator sunspots were observed. At the beginning of each sunspot cycle, most sunspots are found near latitudes 30° north or south. As the cycle goes on, sunspots typically form closer to the equator.
(NASA Marshall Space Flight Center)

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