The Suns Structure

The chromosphere, from the Greek "color ball," is a thin, transparent layer that extends about 10,000 km (6000 miles) above the photosphere. It is normally visible from Earth only during a total eclipse of the Sun, when it glows red due to its hydrogen gas. The temperature unexpectedly increases outward through the chromosphere, where the average temperature of matter is about 15,000 K.

The corona, from the Latin "crown," is the outermost atmosphere just above the chromosphere. It is a rarified, hot gas that extends many millions of kilometers into space. Because of its high temperature—up to 2 million K in the outer part—the corona shines bright at X-ray wavelengths. During a total eclipse of the Sun, it is strikingly visible as a jagged white halo around the briefly hidden photosphere (Figure 4.5).

Below the photosphere is the Sun's interior. Theorists figure that temperature and density increase inward from the surface. No known element can survive as a solid or liquid at the extremely high solar temperatures. So the Sun must be made of very hot gases throughout.

Deep inside, the temperature must rise to 15 million K, the pressure to 200 billion atmospheres, and the density to over 100 times that of water. The

Figure 4.5. The corona reaches outward for millions of kilometers in this V4 second exposure recorded during a total eclipse of the Sun.

core is the power plant where nuclear fusion reactions generate the Sun's energy (Section 5.5). There, hydrogen is fused into helium.

The intense energy released in the core provides heat inside the Sun and enough pressure to balance the inward pull of gravity. It is slowly transmitted outward. Photons are repeatedly absorbed and re-emitted at lower energies in the crowded radiation zone.

From there, circulating currents of gas in the convection zone transfer most of the energy as heat to the outer layers. It takes about 20 million years for energy produced in the core to surface and become sunshine.

Identify the regions of the Sun lettered on Figure 4.6. (a)_;

Answer: (a) Corona; (b) chromosphere; (c) photosphere; (d) convection zone; (e) radiation zone; (f) core.

The Sun Black And White
Figure 4.6. Regions of the Sun.


The Sun keeps turning around its axis in space, from west to east, as Earth does. But there is a difference. All of Earth makes a complete turn in a day. The whole Sun does not turn around together at the same rate.

The period of rotation, or the length of time for one complete turn, is fastest at the Sun's equator (about 25 days), slower at middle latitudes, and slowest at the poles (about 35 days). This strange rotation pattern probably contributes to the violent activity that takes place on the Sun, described in the sections that follow.

How is it possible for different parts of the Sun to rotate at different rates, in contrast to Earth, all of which makes a complete turn in a day?_

Answer: The Sun is a gaseous sphere and not a rigid solid as is Earth.

4.6 DATA

Summarize the data you have on the Sun's properties by filling in the convenient reference Table 4.1.

TABLE 4.1 Properties of the Sun


Method of Measurement


(a) Average distance from Earth Radar ranging of planets

Angular diameter in sky



Average density Solar constant

(solar energy incident on Earth) Luminosity

(h) Surface temperature

(i) Spectral type

(j) Apparent magnitude

(k) Absolute magnitude

(l) Rotation period

(m) Chemical composition of outer layers (n) Surface gravity

Solar telescope

Angular diameter and distance Planets' orbital motions Mass and volume High-altitude aircraft

Solar constant and distance from Earth Luminosity and radius Spectrograph Photometer

Apparent magnitude and distance from Earth Sunspots' motions; Doppler shift Sun's absorption spectrum

Mass and radius

Answer: (a) About 150 million km (93 million miles); (b) 32'; (c) 1,390,000 km (864,000 miles); (d) 2 x 1030 kg; (e) 1.4 g/cm3; (f) 1400 watts/m2 (126 watts/ft2); (g) 3.85 x 1026 watts; (h) about 5800 K; (i) G2; (j) -26.75; (k) 4.8; (l) equator: about 25 days; poles: about 35 days; (m) outer layers: about 71 percent hydrogen, 27 percent helium, 2 percent more than 70 other elements by weight; (n) 28 times Earth's or 294 m/s2.


Astronomers are using sophisticated tools and techniques to observe the Sun more closely and in more detail than ever before.

Optical solar telescopes at the U.S. National Solar Observatory ► ^ and worldwide continually image the Sun's visible surface

Figure 4.7a. Building the 2-ton, 25-foot wide (with solar panels) Solar and Heliospheric Observatory (SOHO). Lower Service Module gives power, thermal control, pointing, and telecommunications. Upper Payload Module holds scientific instruments.
Figure 4.7b. Sun in different wavelengths. Central image, ultraviolet. Clockwise from top: magnetic map, white light, 5 ultraviolet, and X-ray.

with its changing features. Arrays of giant radio telescopes receive and record radio waves. Infrared telescopes observe the solar limb and map sunspots.

In space, instruments monitor the Sun in all parts of the electromagnetic spectrum to detect solar features, radiations, particles, and fields normally blocked by Earth's atmosphere (Figure 4.7a). Ultraviolet, X-ray, and gamma ray telescopes on spacecraft record images of processes in the hottest and most active regions of the Sun (Figure 4.7b).

Spectroheliographs image the Sun in light of essentially a single wavelength belonging to one gas such as hydrogen or calcium. The monochromatic images are called spectroheliograms. Because hot gases produce light at specific wavelengths, the data reveal local surface temperatures and phenomena (Figure 4.8).

Formerly, the Sun's chromosphere and corona could be observed directly only during the few minutes of a total eclipse of the Sun when the much brighter photosphere was hidden. Now astronomers do not have to wait for one of these rare natural events to occur. They use a coronagraph, a telescope equipped with a disk that blocks light from the photosphere, to create an artificial eclipse at will.

The corona is very hot and dynamic, frequently discharging high-energy radiation and mass. European robot Solar and Heliospheric Observatory (SOHO) (1995- ) is first to record the Sun's atmosphere, wind, interior, and environment continuously. SOHO operates unimpeded in orbit around the Sun at Lagrangian point L1, 1.5 million km (1 million

Figure 4.8. The Sun with a big loop prominence, imaged in light of a spectral line at 30.4 nm, which shows the upper chromosphere at about 60,000 K.

miles) toward the Sun from Earth, where the gravitational forces of both are equal. ►^

While in orbit around the Sun at L1, U.S. robot Genesis (2001-2004) collected some 10 to 20 micrograms (comparable to a few grains of salt) of solar wind particles and returned its prize to Earth. Now scientists can measure precisely the composition of actual material from the Sun. ►http://genesis^

First to orbit nearly perpendicular to the ecliptic plane, European/U.S. robot Ulysses (1991-2009) imaged the Sun's magnetic fields, radiation and particles, and environment at all solar latitudes. Ulysses flew over the polar regions at minimum solar activity in 1994-1995, at maximum solar activity in 2000-2001, and a third time at the start of the latest solar cycle. ►http://ulysses^

Why do different features of the Sun appear in pictures taken in light of different wavelengths such as visible light, ultraviolet rays, or X-rays? Tip: Review Section 2.10 if necessary._

Answer: Different wavelengths are produced in regions of different temperatures where different conditions and activities prevail.


Optical telescopes reveal that the photosphere has a grainy appearance, called granulation. Bright spots that look like rice grains, called granules, dot the Sun's disk in high-resolution images (Figure 4.9).

Granules, cells up to 1000 km (625 miles) across, are the tops of rising currents of hot gases from the convection zone. Individual granules last an average of 5 minutes each. They look brighter than neighboring dark areas because they are about 300° hotter. The dark areas are descending currents of cooler gases.

Granules belong to supergranules, which are large, organized convection cells up to 30,000 km (19,000 miles) across, on the Sun's disk. Supergranules last several hours. They have a flow of gases from their centers to their edges, in addition to the vertical gas currents in the granules.

Spicules, jets of gas up 10,000 km (6000 miles) tall and 1000 km (600 miles) across, rise like fiery spikes into the chromosphere around the edges of supergranules. They change rapidly and last about 5 to 15 minutes.

Bright, white surface patches, called faculae, from the Latin "little

Figure 4.9. Solar granules. Bright points between granules represent locations of magnetic field concentrations.

torches," may be visible near the Sun's limb. Their appearance seems to signal coming solar activity.

What causes granulation?_

Answer: Gases rising from the Sun's hot interior.


Sunspots are temporary, dark, relatively cool blotches on the Sun's bright photosphere. They usually appear in groups of two or more. Individual sunspots last anywhere from a few hours to a few months.

The largest sunspots are visible at sunrise or sunset or through a haze. Observations of sunspots were first recorded in China before 800 b.c.

A typical sunspot is roughly twice as big as Earth. The largest sunspots may be bigger than ten Earths.

Sunspots really shine brighter than many cooler stars. They look dark only in comparison to the hotter, dazzling surrounding photosphere. The temperature is about 4200 K in the umbra, or core. The penumbra, or outer gray part of a large spot, is a few hundred degrees cooler than the photosphere.

Frequently sunspots appear in groups, or solar active regions, where the most violent solar activity occurs. The first telescopic observations of sunspots and their motions, reported by Galileo in 1610, had an historic impact (Section 8.7). Galileo correctly concluded that the Sun's rotation carries sunspots around.

Identify the umbra, penumbra, and photosphere, lettered on Figure 4.10, and indicate the approximate temperature of the umbra. (a)_;

Answer: (a) Photosphere; (b) penumbra; (c) umbra, 4200 K.

Figure 4.10. Umbra and penumbra of a sunspot, with granulation in the surrounding photosphere.


At any one time more than 300 sunspots—or none at all—may appear on the Sun's disk. The number of sunspots regularly rises to a maximum and falls to a minimum in an approximately 11-year cycle, called the sunspot cycle.

The sunspot cycle is watched carefully from Earth because it marks the solar activity cycle. The Sun is most active with greatest outbursts of energy and radiation for about 4.8 years during which sunspots are increasingly numerous. After sunspot maximum, the number of sunspots decreases for about 6.2 years to a sunspot minimum as solar activity lessens. The current cycle began in 2008.

Why is it important to keep track of the sunspot cycle?_

Solar Cycle White Light
Figure 4.11. Nearly an entire solar activity cycle imaged in ultraviolet light from the SOHO spacecraft.

Answer: The Sun is most active during the years of sunspot maximums, pouring the greatest amount of energy and radiation into Earth's environment.


Sunspots are like huge magnets. These regions of powerful magnetic fields are typically thousands of times stronger than Earth's magnetic field.

The magnetic field of a sunspot can be detected before the spot itself can be seen and after the spot is gone. Therefore, magnetic fields probably shape and control local conditions on the Sun. Astronomers analyze magnetic fields by measuring Zeeman spectral line-splitting (Section 3.10).

A weaker magnetic field spreads out over the whole Sun. It has a north magnetic pole and a south magnetic pole, with the magnetic axis tilted 15° to the rotation axis. It is split into two hemispheres. A display of magnetic field strength is called a magnetograph.

The Sun's magnetic field probably extends from its northern hemisphere through the solar system out beyond Pluto about 6 billion km (4 billion miles). Near the edge of the solar system, the magnetic field bends and returns to the Sun's southern hemisphere.

The complex solar magnetic field is generated by rotational and convec-tive motions of electrically charged particles that make up the Sun's hot gases. Apparently it energizes and controls violent outbursts of material and radiation on the Sun.

The polarity of the Sun's magnetic field is reversed about every 11 years shortly after the period of sunspot maximum. It takes two sunspot cycles of about 11 years each for the Sun's magnetic poles and sunspot magnetic polarities to repeat themselves. So the solar activity cycle is 22 years when counting the length of time required for the Sun to return to its original configuration.

What probably activates the violent outbursts of material that occur on the Sun?_

Answer: Very strong magnetic fields at the sites of sunspots.

You can observe a magnetic field by putting a magnet under a piece of paper. Lightly sprinkle iron filings on top of the paper. The filings will line up according to the strength of the magnetic force. By showing the regions of the magnetic force, they make the magnetic field visible to you.

What probably bends and controls the trajectory of the ejected gas in solar flares?_

Answer: Strong magnetic fields in the vicinity of sunspots.


A solar flare is a sudden, tremendous, explosive outburst of light, invisible radiation, and material from the Sun. One great solar flare may release as much energy as the whole world uses in 100,000 years (Figure 4.12).

Flares are short-lived, typically lasting a few minutes. The largest last a few hours. They occur near sunspots, especially in periods of sunspot maximums. Flares seem to be energized by strong local magnetic fields (Figure 4.13).

A flare usually follows the most energetic of all solar eruptions, a coronal

Figure 4.12. A widely spreading coronal mass ejection blasts more than a billion tons of matter out into space at millions of km an hour, from a SOHO spacecraft coronagraph. An enlarged ultraviolet image from a different day fills the occulting disk for effect.

Earth shown for size comparison

Figure 4.13. Ultraviolet image of an erupting prominence that extends over 35 Earths out from the Sun. Dot represents Earth for size comparison.

mass ejection, which blasts plasma out from the corona. A coronal mass ejection may include prominences, fiery arches of ionized gases on the Sun's limb that rise tens of thousands of kilometers up (Figure 4.13).

What probably bends and controls the trajectory of the ejected gases in solar flares and prominences?_

Answer: Strong magnetic fields in the vicinity of sunspots.


A huge flare can hurl fantastic amounts of high-energy radiation and electrically charged particles—as much energy as a billion exploding hydrogen bombs—into the solar system.

Gamma rays, X-rays, and ultraviolet rays reach Earth in just 8.3 minutes. Flare particles arrive a few hours or days later. These could destroy all life if Earth were not shielded by its magnetic field and atmosphere. Extra solar radiation is risky for airplane passengers, astronauts, and spacecraft electronics.

When electrically charged particles from the Sun strike Earth's atmosphere, they can stimulate the atmospheric atoms and ions to radiate light, producing aurora borealis (northern lights) and aurora australis (southern lights). Auroras are bands of light seen in the night sky in the Arctic and Antarctic regions but occasionally also down to middle latitudes about 2 days after a solar flare. They reach a peak about 2 years after sunspot maximum.

Strong blasts of electric solar particles interact with Earth's magnetic field and disturb it, causing geomagnetic storms. Compasses don't work normally then. The gusts can cause atmospheric storms, satellite damage, surges in electric power and telephone lines, and blackouts.

High-energy radiation heats the upper atmosphere, making it expand. Then friction and the drag on spacecraft in low Earth orbits increase. The

Figure 4.14. Effects of solar flares on Earth's environment.

drag is greatest during times of maximum solar activity when satellites may plunge from orbit and be destroyed on reentry. The first U.S. space station, Skylab (1973-1979), was a casualty of a solar maximum. By increasing ioniza-tion, solar outbursts can disrupt radio transmission and navigation signals.

Because solar flares affect modern life so much, the U.S. National Oceanic and Atmospheric Association (NOAA) with partners worldwide monitors the Sun's magnetic field and activity daily (Figure 4.14). Its Space Environment Center ► www.sec.noaa.govM issues space weather alerts, warnings, and forecasts.

List two effects that large solar flares have on modern technology on Earth. (1)_; (2)_

Answer: (1) Disruption in power transmission; (2) disruption in radio communications.


The solar wind is a plasma, or stream of energetic, electrically charged particles that flows out from the Sun at all times. It is much faster, thinner, and hotter than any wind on Earth.

The solar wind is observed by instruments carried on spacecraft above Earth's atmosphere. Near Earth, the average solar wind speed is about 450 km/second (1 million miles/hour). Earth's atmosphere and magnetic field ordinarily protect us from harmful effects of the solar wind.

Big blasts of solar wind occur at coronal mass ejections. The wind is strongest during periods when many sunspots are visible and solar activity is great. Strong blasts of solar wind produce especially brilliant auroras. ►www.spaceweather.comM

The solar wind comes mainly from coronal holes, regions in the Sun's corona where gases are much less dense than elsewhere. Magnetic fields are relatively weak there, allowing high-speed solar wind streams to escape.

Voyager spacecraft instruments (Section 8.12) continue to measure the solar wind. They could detect the heliopause, the boundary where the solar wind merges with the diffuse material between stars.

What is the solar wind?_

Answer: A stream of energetic, electrically charged particles that flows out from the Sun.


Scientists believed that they understood what makes the Sun shine until solar neutrino experiments raised some doubts.

Theoretically, the Sun's energy is produced by the conversion of hydrogen into helium in nuclear fusion reactions. Solar neutrinos, neutral elementary particles with almost no mass, are a by-product. Neutrinos interact very weakly with matter and travel at virtually the speed of light.

Scientists cannot look directly deep inside the Sun's core to test their theory. But the theory predicts that the neutrinos produced in the core should escape. So they look for the solar neutrinos instead.

Neutrinos produced in the center of the Sun have been detected. They show that the theory is basically correct.

Scientists built several neutrino traps deep inside the Earth. The number of neutrinos detected over 30 years was lower than the number theory predicts. New experiments and analysis explain that this solar neutrino problem was due to neutrino oscillations. Neutrinos change from one type to another, and some were not counted before.

Helioseismology is the study of the Sun's internal structure and condition by measuring global oscillations on its surface. Pressure waves of different frequencies penetrate to different depths. They reveal the density, temperature, and rotation rate inside the Sun, just as seismic waves expose Earth's interior. Solar oscillations are observed spectroscopically through Doppler shifts in spectral lines (Section 3.9). Six stations of Global Oscillation Network Group (GONG) are located around the world to obtain nearly continuous observations. ►www.gong.noao.eduM

Explain the small number of solar neutrinos detected in early experiments.

Answer: Neutrino oscillations: Neutrinos change from one type to another, and some were not counted before. (Astronomers rely on the results of the experiments.)


Astroseismology extends this study to other stars. Apparently other stars have regions of violent activity like those of our Sun, including starspots and starspot cycles, although stars are so far away that these must be deduced from their spectral lines and brightness variations rather than observed directly. Recent X-ray observations indicate that nearly all types of stars also have similar coronas with temperatures of at least a million degrees.

Write a short summary describing three phenomena that indicate violent activity on the Sun (and by extension to other stars), and name their probable cause.

Answer: Your answer should briefly describe (1) sunspots, or dark, relatively cool, temporary spots on the Sun's photosphere; (2) flares, or sudden, short-lived outbursts of light and material near a sunspot; (3) coronal mass ejections, or blasts of plasma from the corona.

Probable cause: the Sun's powerful magnetic fields.

Most violent activity on the Sun seems to be caused and controlled by very strong local magnetic fields.


The Sun, like all other stars, is racing through space.

With respect to nearby stars, the Sun is speeding toward the constellation Hercules at 20 km/second (45,000 miles/hour), carrying its planets along with it.

The Sun with its planets is inside the Milky Way Galaxy. It goes around our Galaxy's center as the whole Galaxy turns around in space. The Sun travels at about 250 km/second (563,000 miles/hour) (Section 6.2).

This self-test is designed to show you whether or not you have mastered the material in Chapter 4. Answer each question to the best of your ability. Correct answers and review instructions are given at the end of the test.

1. List three reasons why modern astronomers study the Sun.

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