The spots on the Sun are an obvious indication of our star's activity. Galileo Galilei was the first to observe the spots on the Sun in 1610. The spots are cold (and, consequently, darker) regions of the Sun's photosphere, with a temperature of approximately 3500 K. It is the Sun's magnetic field that brings the spots into being. Smaller spots have the size of several thousand kilometers in diameter. Larger spots sometimes reach 100 000 km in diameter. The average lifespan of a spot on the Sun is approximately one month.
Sunspots have an inner structure: the darker central area known as the core has a penumbra around it. Sunspots often form groups that may occupy large areas of the solar disk. Thus, a group of spots with a total area equal to 6.5 billion km2 was registered on September 18, 2000. It was 13 times larger than surface of the Earth.
Light zones called the faculas sometimes surround the spots on the Sun. Faculas are hotter than the rest of the atmosphere by approximately 2 000 K. A sunspot is not a prerequisite for appearance of the faculas, as the latter sometimes appear on their own.
Faculas form as a result of convection that occurs in the deeper layers of the Sun. Their lifespan is equal to several weeks or months. A black spot sometimes appears between the granules in certain facula fields. It quickly grows, and turns into a sport with a clearly visible border on the next day. Penumbras form around such spots in the next 3-4 days. The area of these spots reaches its maximum on the tenth day, after which it slowly diminishes, and finally disappears altogether. The smallest spots are the first to vanish.
Sunspots may be observed through a small telescope. A telescope that is directed at the Sun will project a clear image onto a sheet of paper. Do not look directly into the ocular when observing the Sun -- that might cause damage to your eye.
The number of spots on the Sun is constantly changing. In some years, there may be no spots on the Sun at all. In other years, there could be dozens. Years of plentitude and years with no sunspots alternate approximately every 11 years (from 7 to 17 years). The most recent year with maximal solar activity was in observed in 2000.
Wolf number is a major characteristic of solar activity. It equals to the sum of total number of spots f and number of spot groups g multiplied by ten:
W = f + 10g.
Sunspots manifest in mid-latitudes during the minimums of solar activity, and near the equator during the periods of maximal activity. Spots are virtually never observed near the Earth's poles. Most spots are located at latitudes from 20? to 30? in the beginning of the eleven-year cycle of solar activity (Sporer's law). When dependence of latitude-sunspot visibility is plotted on a time-based diagram, the zones of activity form the so-called Maunder's Butterfly.
Activity cycle of sunspots is directly related to the Earth's climate. The thickness of tree rings has an eleven-year cyclical pattern. The weather in Europe was extremely cold in the end of XVII and the beginning of XVIII century, when virtually no sunspots existed.
The Sun is located in the center of this model. Observe Maunder's Butterfly in the bottom part of the screen. Latitude degrees are plotted along the vertical axis. Years are plotted along the horizontal axis. The color of each point on the diagram corresponds to the surface of the sun's area that is covered by sunspots in a given year and at given latitude.
Press "Run" button and observe the Sun. Note that the spots at different latitudes are moving with different angular speeds. The red line corresponds to the current year. Press "Stop" button to suspend the model, and press "Reset" to return it to its initial state.