AST-103L Spring 2001: The Sun Our Sun is a middle-aged, medium sized star, big enough to hold a million Earths. The ancient Greeks thought that the Sun was a perfect sphere of fire. Today we know that the Sun is a variable star that produces life giving light and heat as well as harmful radiation. It causes space weather that can harm astronauts working in space and can interfere with satellites orbiting our planet and communications. In this exercise, you will learn the basic features of the Sun’s surface, atmosphere, and interior, and begin to understand what causes some of these features. You will be introduced to sunspots and the solar cycle and calculate the rotation of the Sun. Objectives: Identify basic solar features and read about what causes some of them Understand what sunspots are, what the solar cycle is, and how the Sun moves Equipment: This write-up, calculator, lab notebook Data you downloaded from the SOHO website Figure 1. Artist’s rendition of solar structure. Page 1 of 8 AST-103L Spring 2001: The Sun I. FEATURES OF THE SUN’S SURFACE AND ATMOSPHERE Although the average distance from Earth to the Sun is a whopping 149,600,000 kilometers (93,000,000 miles), careful observation from Earth reveals a surprisingly large number of different visible features. Figure 1 is an artist’s schematic of the Sun. The most obvious and best known feature is the sunspot. Typically moving in groups, these dark, planet-sized features have been known to humankind for centuries. As sunspots form and disappear over periods of days or weeks, they also appear to move across the Sun’s surface because the Sun rotates. Caused by strong magnetic fields, sunspots are shaped much like a horseshoe magnet that rises from below the Sun’s surface. The rising hot gas is trapped by the sunspots’ intense magnetic field, which cools the sunspots from 6000°C to about 4200°C. The cool area appears dark compared to the area around it. Thus, from Earth, we see spots on the Sun (Figure 2). In some photographs, we can also see light colored areas around groups of sunspots that resemble tufts of cotton candy. We call these fluffy looking fringes, plages (Figure 3). Figure 2. Visible Sun. Visible or white-light images of Figure 3. H-alpha Sun. Light from hydrogen gas just the Sun show the photosphere and sunspots. Sunspots above the photosphere shows the bright plages that are regions of intense magnetic fields. They appear surround the active sunspot regions. Dar kstrong-like dark because they are somewhat cooler than the filaments are also seen that are in fact prominences surrounding gas. seen head-on. The bright structures on the limb are prominences. Sunspots are the sources of massive releases of energy (called coronal mass ejections) as well as solar flares (Figure 4), the most violent events in the solar system. In a matter of minutes to several hours, a solar flare releases about 10,000 times the annual energy consumption of the United States. Solar flares and coronal mass ejections give off radiation that includes X-rays and ultraviolet rays, and charged particles called protons and electrons. This sudden surge in radiation has dramatic effects on Earth. For example, in 1989, the Quebec province in Canada suffered a blackout because many transformers were destroyed by a large magnetic storm near Earth. The magnetic storm, caused by solar activity including solar flares, resulted in millions of dollars in damages. One of the most spectacular features of the Sun are solar prominences (Figure 5). They appear to stream, loop, and arch away from the Sun. The most recognizable prominences appear Page 2 of 8 AST-103L Spring 2001: The Sun as huge arching columns of gas above the limb (edge) of the Sun. However, when prominences are photo- graphed on the surface of the Sun, they appear as long, dark, threadlike objects and are called filaments. Like sunspots, prominences are cooler (about 10,000°C) in relation to the much hotter background of the Sun’s outer atmosphere (about 1,500,000°C). While solar flares may last hours, prominences can remain for days or even weeks. If you have seen photographs of a solar eclipse, then you have probably noticed a bright halo around the Sun, called the corona (Figure 6). The Figure 4. Solar Flare. A solar flare is a highly concentrated explosive release Sun’s corona changes with of energy within the solar atmosphere that occurs near sunspots. Flares emit sunspot activity. When there enormous amounts of energy. are more sunspots, the corona appears to be held closely to the Sun; when there are fewer sunspots, the corona streams out into space in a shape that resembles the spike on a warlike, peaked helmet. Sometimes parts of the corona appear to be missing. Logically, we call this a coronal hole. Scientists believe that the solar wind, a million mile per hour gale that blows away from the Sun, originates in coronal holes. Unlike wind on Earth, the solar wind is a stream of ionized (electrically charged) particles speeding away from the Sun (Figure 7). Figure 5. Extreme Ultraviolet. This extreme ultraviolet image of the Sun shows the chromosphere, which is a thin region of the Sun just above the photosphere. A giant prominence can also be seen. Page 3 of 8 AST-103L Spring 2001: The Sun Figure 6. Solar Eclipse. A solar eclipse shows the ethereal corona – the crown of light that surrounds the Sun. Prominences can also be seen as small white blotches in the photograph. Figure 7. Artist’s rendition of the solar wind bombarding Earth’s magnetic field with supercharged particles. Just as the Sun disappears behind the Moon during a total solar eclipse (Figure 6), a flash of bright red light appears. This colorful layer of the Sun, called the chromosphere, becomes visible for a brief instant. Although we know little about the chromosphere, there are curious, permanent features of the chromosphere, called spicules, that we can study in more detail. There are so many of these fine, bright, hairlike features, that they are always visible near the Sun’s edge, even though an individual spicule lasts only minutes. Like sunspots, spicules rise vertically above the Sun’s surface. Also visible for only minutes, are granulations in the Sun’s photosphere. Granulations are rising and falling columns of hot gases that look like fluffy marshmallows arranged in a honeycomb pattern. The tops of these granules form the Sun’s “surface.” Although we refer to the Sun’s “surface” as the photosphere, you probably know that the Sun has no solid surface, unlike Earth. It is an uneven sphere of hot, glowing gas. II. THE PHOTOSPHERE The photosphere is the visible surface of the Sun that we are most familiar with. Since the Sun is a ball of gas, this is not a solid surface but is actually a layer about 100 km thick (very, very, thin Page 4 of 8 AST-103L Spring 2001: The Sun compared to the 700,000 km radius of the Sun). When we look at the center of the disk of the Sun we look straight in and see somewhat hotter and brighter regions. When we look at the limb, or edge, of the solar disk we see light that has taken a slanting path through this layer and we only see through the upper, cooler and dimmer regions. This explains the limb darkening that appears as a darkening of the solar disk near the limb (Figure 8). A number of features can be observed in the photosphere with a simple telescope (along with a good filter to reduce the intensity of sunlight to safely observable levels). These features include the dark sunspots, the bright faculae, and granules. We can also measure the flow of material in the photosphere using the Doppler effect. These measurements reveal additional Figure 8. Example of limb darkening. features such as supergranules as well as large-scale flows and a pattern of waves and oscillations. The Sun rotates on its axis once in about 27 days. This rotation was first detected by observing the motion of sunspots in the photosphere. The Sun’s rotation axis is tilted by about 7.25 degrees from the axis of the Earth’s orbit so we see more of the Sun’s north pole in September of each year and more of its south pole in March. Since the Sun is a ball of gas it does not have to rotate rigidly like the solid planets and moons do. In fact, the Sun’s equatorial regions rotate faster (taking about 24 days) than the polar regions (which rotate once in more than 30 days). The source of this “differential rotation” is an area of current research in solar astronomy. III. FEATURES OF THE SUN’S INTERIOR We won’t study about the Sun’s interior here. We just want you to be familiar with the three basic components of the solar interior (see Figure 1). The core is the central part of the Sun where hydrogen fuses into helium to give off energy. This is effectively a nuclear power plant. Energy from the Sun’s core then travels outward through what we call the radiation zone. In the convection zone, the hotter gases rise and fall as they are heated from the radiation zone below much like a boiling pot of water. IV. WHAT IS THE “SOLAR CYCLE”? Every 11 years the Sun undergoes a period of activity called the “solar maximum,” followed by a period of quiet called the “solar minimum.” During the solar maximum there are many sunspots, solar flares, and coronal mass ejections, all of which can affect communications and weather here on Earth.