Sun

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Sun

The Sun is the star at the center of our solar system . It has a diameter of about 420,000 mi (700,000 km) and a surface temperature of 9,981°F (5,527°C). Its visible "surface" is actually a thin gas, as is the rest of its atmosphere and interior.

The Sun shines as a result of thermonuclear fusion reactions in its core, and the energy produced by these reactions heats the gas in the Sun's interior sufficiently to prevent the weight of its own matter from crushing it. This energy also is the source of heat and life on Earth , and small variations in the Sun's energy output, or even in the features present in its atmosphere, may be sufficient to profoundly affect terrestrial climate. Although the Sun is by far the nearest star, the processes causing solar variability are still poorly understood and continue to challenge astronomers.

A brief history of solar observations

The Sun is about 90 million mi (150 million km) away from Earth. It is modest by stellar standards, although it is over 100 times larger than the earth and over 300,000 times more massive. Although it consumes more than half a billion tons of its nuclear fuel each second, it has been shining continuously for five billion years, and it will continue to shine for another five billion.

To the ancient Egyptians the Sun was a god, for anyone who felt its warmth and watched the renewal of crops it brought in the spring realized it was a bringer of life. Greek mythology recounts the tale of Icarus, who died in a brazen attempt to fly too close to the Sun.

Ancient dogma held that the Sun orbited Earth. It was not until 1543 that Nikolaus Copernicus (1473-1543) published the heliocentric theory of the Universe, in which the Sun lay at the center and Earth was relegated to a circular orbit around it. Copernicus refused to have this magnificent work published until near his death. This was a wise decision, since earlier publication would have resulted in his equally earlier death at the hand of Church authorities who regarded his work as heresy.

Around 1610, the newly invented telescope was trained on the Sun. The early solar observers, including Galileo (1564-1642), noticed that the surface of the Sun had dark spots on it. Not only that, the spots moved across the solar surface, leading Galileo to conclude that the Sun rotated. This sent further shock waves through the religious world. The poor cardinals had hardly recovered from Copernicus's heresy, and now along came Galileo telling them that the backup embodiment of celestial perfection, the Sun, had ugly blemishes on its surface. Galileo got into serious trouble with the Church for this and other statements he made, and it would be 380 years before Pope John Paul II exonerated him.

In the nineteenth century an amateur German astronomer named Heinrich Schwabe (1789-1875) did some of the first scientific analysis of solar data. Observers had kept records over the years of the number of sunspots visible, and Schwabe noticed that the sunspot number rose and fell in a cyclic fashion, with a period of about 11 years. Later workers confirmed this activity cycle, and the sunspot number remains today an important piece of information about our star.

The nature of sunspots was first investigated by George Ellery Hale (1868-1938), who devoted his life to studying the Sun from the observatory he founded on Mt. Wilson near Los Angeles. Hale discovered that sunspots were cooler than the surrounding solar surface—which is why they appear dark—and that sunspots are associated with strong magnetic fields. Hale also discovered that the sun's magnetic field reverses itself with each 11-year cycle, so that there is a more fundamental 22-year magnetic activity cycle behind the sunspot cycle.

Since Hale's time, solar research has developed along two great lines of investigation. First, a huge observational database has been created. The national solar observing facilities of the United States are located at Sacramento Peak just east of Alamogordo, New Mexico, and at Kitt Peak, 30 mi (48 km) west of Tucson, Arizona. In addition to sunspots, these observatories have monitored prominences and flares, which are spectacular, eruptive events in the solar atmosphere, the solar magnetic field itself, the solar granulation, which is a result of the turbulent movement of the gas beneath the solar surface, and the total energy output of the Sun. Space-based satellites have studied the Sun in wavelength regimes, such as the ultraviolet and the x ray, not accessible from the ground. The result of these investigations is a detailed set of observations about the phenomena that occur on our star.

The other line of investigation is the theoretical analysis of processes within the Sun and in its atmosphere. Scientists have developed models to explain what is observed; if a model fails to fit the observations, it must be discarded. The goal of this work is to explain why the various features seen on the Sun appear. Why is there an 11-year-long solar activity cycle ? What do the forces producing granulation have to do with the nature of the solar magnetic field? Where does the magnetic field originate? These questions, and many others, are only partially answered.

A fruitful line of research toward answering these questions involves observing not just the sun, but many other stars like the Sun. You would have a difficult time understanding what the human race was like by observing only one person; similarly, it is hopeless to try to understand the complex nature of the stars by observing only one of them. Long-term observations of solar-like stars have been carried out at the Mount Wilson Observatory near Los Angeles and at Lowell Observatory in Flagstaff, Arizona. Understanding the so-called "solar-stellar connection," derived from observations of other stars, may yield important clues about the processes at work in our own star.

Two million years after our earliest ancestors looked up and shielded their eyes from the brilliant, life-giving object in the sky, we still have only a rudimentary understanding of it.

A journey through the Sun

The solar furnace

At the sun's core, the temperature is 26,999,541°F (14,999,727°C). The matter here has a density of roughly 2.2 lb (1 kg) per cubic centimeter—about 150 times the density of water . It is compressed to this degree by the crushing weight of all the matter between it and the surface—it is about 210,000 mi (350,000 km)—to the surface.

There are no atoms in the core. No atom (a nucleus of protons and neutrons, orbited by electrons) could exist in this inferno. There is nothing but a swirling sea of particles. We know from physics that the hotter a medium is, the faster its particles move. In the Sun's core the protons race around at blinding speeds, and because they are so tightly packed, they are constantly crashing into one another.

What is a proton ? It is a hydrogen ion-a hydrogen atom that has had its sole electron stripped away. The sun is made mostly of hydrogen. In the cool regions of its atmosphere the hydrogen exists as atoms, with a single electron bound to the proton; in the core there are only the ions.

Four hydrogen nuclei smash together in a quick series of collisions, with catastrophic force . So violent are these collisions that the protons' natural tendency to repel one another—they all have the same positive charge—is overcome. When the various interactions are over, a new particle has emerged: a helium nucleus containing two protons and two neutrons. The helium nucleus gets its share of battering by the other particles, but it is larger and tightly bound together, and even the maelstrom cannot disrupt it.

There is one more product of this fusion reaction. In the series of collisions leading to the formation of the helium nucleus, two particles called photons are produced. A photon is a bundle of electromagnetic radiation , also known as a ray of light . The photons race away from the Sun's core at the incredible speed of 180,000 mi (300,000 km) per second, the speed of light.

Toward the surface

Photons do not travel to the surface in a straght line. They constantly hit other particles, bouncing off them in a random direction. Sometimes an atom absorbs its energy, only to re-emit it a fraction of a second later in a different direction. This is the so-called random walk, and it describes how photons work out from the Sun's core.

Then toward the surface, the temperature, pressure, and density of the gas drop. There is not as much weight compressing the gas, so it does not need to be at as high a pressure to support the material above it. Lower pressure means lower temperature and density.

Halfway from the Sun's core to its surface, we are in the zone of radiative energy transport, where uncountable trillions of photons flow away from the Sun's core where they were produced. As they flow past, new photons, freshly created in the core, flow into it from below.

Into the convection zone

A little more than two-thirds of the way to the surface, gas cools to 179,492°F; (99,700°C). Instead of individual particles, atoms exist, which are capable of absorbing the photons rather than simply scattering them in a different direction. Photons have difficulty flowing through this cool gas. As they get absorbed, new photons flow into the gas from below, heating it even more. The gas begins to overheat. As a result, energy transport is now more efficient if a huge bubble of hot gas forms and begins to rise toward the surface. This is called convection, and we are now in the Sun's zone of convective energy transport.

A hot gas bubble rises into progressively cooler gas, releasing heat into smaller bubbles reaching the very cool region just below the Sun's surface. The bubbles release their pent-up heat. With their heat gone, they are now cooler than their surroundings, so they sink back into the Sun's interior, to pick up more heat and begin the convective cycle anew.

In the atmosphere

It takes about 30,000 years for a photon to reach the Sun's surface. Had the photon gone in a straight line, it would have reached the surface in just over one second, but 10 billion trillion interactions with matter in the Sun's interior slowed it considerably.

At the surface is a thin (300 mi/500 km) layer of matter called the photosphere. The temperature here is 9,981°F; (5,527°C), and for the first time, a photon of visual light—that is, with a wavelength that places it in the visual portion of the spectrum—has a chance of escaping directly to outer space. The density of the gas is now so low that it is nearly a vacuum , thousands of times less dense than air, and so little matter is left that photons escape with no further interactions.

The photosphere is a seething region of hot, rising granules and cooler, sinking ones. In places there are great, dark spots, perhaps 6,200 mi (10,000 km) across, where the temperature is only 6,692°F; (3,700°C) and where matter is constrained to flow along the intense and tangled lines of the strong magnetic fields that permeate the spots. (One phenomenon thought to contribute to the tangling of the solar magnetic field is the Sun's rotation . The Sun's equator rotates once every 26 days, its poles once every 36 days. This differential rotation contributes to twisting the magnetic fields and producing active features like sunspots.) The magnetic fields are invisible, but observations have revealed that they can arch high into the Sun's atmosphere, forming loops. Hot gas becomes confined in these loops, forming spectacular prominences. Violent rearrangements or eruptions in twisted magnetic fields result in flares, which spew matter and intense radiation into space. Some of this radiation may interrupt radio communications on Earth, while the particles will soon stream into Earth's atmosphere, causing aurorae.

Just above the photosphere, the temperature starts to climb, reaching 17,492°F; (9,700°C) a few thousand miles above the photosphere. This is the chromosphere. Most of it is ten million times less dense than air. The causes for the temperature rise are still not fully understood. One possibility is that mechanical energy from the convection zone—the energy associated with the motion of the gas—is deposited into the Sun's upper atmosphere, heating it. Because it is so thin and tenuous, the chromosphere is very faint, and under normal circumstances is invisible with the brilliant photosphere behind it. We can see the chromosphere by photographing the Sun with special filters sensitive to light that originates in the chromosphere, or during an eclipse, when the Moon blocks the photosphere and the chromosphere appears as a glowing ring girdling the solar limb.

Now 1,800 mi (3,000 km) above the photosphere, the temperature rises sharply—1,935,541°F; (19,727°C), then 179,541°F; (99,727°C), then 899,541°F; (499,727°C). A narrow transition region opens to the corona, an incredibly tenuous and hot—3,599,541°F; (1,999,727°C)—region extending 3,000,000 mi (5,000,000 km) above the photosphere. The corona is also very faint, and can only be observed in visible light with the photosphere blocked, as it is during an eclipse. Because the corona is so hot, it is also spectacular in x ray photographs, which can be obtained only from space-based observatories.

The solar wind

The end of the corona marks the last of the Sun's strong magnetic field regions. photons race into empty space.

There are also swarms of particles. There are only a few per cubic centimeter—an almost perfect vacuum. They are all part of the solar wind , a continuous stream of matter flowing away from the sun. Slowly, the Sun is losing material to space. The rate of this loss is very small, so it will not seriously affect the Sun's evolution . (Some stars, however, have powerful winds that can carry off a substantial fraction of their mass.) The solar wind permeates the entire solar system, and beyond.

A small blue planet

Eight minutes after a photon leaves the Sun's photosphere, it reaches Earth. Along with countless billions of other photons, it streams through Earth's atmosphere. The photon, a product of a hydrogen fusion reaction 30,000 years ago, has finished its trip. Obviously the Sun has a profound impact on doings here on Earth, but recent research suggests the connections may run deeper than initially thought.

The link may lie in the solar activity cycle, which is the periodic variation in active features such as sunspots, prominences, and flares, in the Sun's atmosphere and on its visible surface. The cause of the activity cycle is not well understood, but astronomers generally agree that the Sun's differential rotation, combined with the turbulent motions in its convection zone, create a magnetic dynamo that results in a perpetual tangling and rearrangement of the Sun's magnetic field. When magnetic field lines, which normally lie below the photosphere, become tangled and burst into the Sun's atmosphere, active features such as sunspots and prominences invariably form. When the magnetic field becomes tangled to a critical level, it rearranges and simplifies its configuration, and the amount of solar activity decreases correspondingly. The sunspot cycle typically has a length of about 11 years, but there is compelling circumstantial evidence that variations in the length of the solar activity cycle are closely related to changes in the global temperature, with shorter solar cycles corresponding to warmer temperatures on Earth.

And in the end, the sun will have its final and greatest impact on Earth. What of the core, which our photon left a million years ago? Five billion years from now, the seemingly countless hydrogen nuclei will all have been converted into helium "ash"—the Sun's fuel will be gone. To stave off destruction by the inexorable force of gravity, the sun's core will contract and heat to the point that the helium will ignite. In the process, the sun will expand into a red giant star , swallowing the innermost planet, Mercury, and turning Earth into a charred wasteland. But the helium is the last fuel reserve the sun will be able to use, and it will eject its outer layers, leaving behind only its collapsed core, a small, dying white dwarf.

Resources

books

Seeds, M.A. Horizons, Discovering the Universe. New York. Wiley, 1991.

periodicals

Giampapa, Mark S. "The Solar-Stellar Connection." Sky &Telescope (August 1987): 142.

Pasachoff, J. "The Sun: A Star Close Up," Mercury (May/June 1991): 66.

Jeffrey C. Hall

KEY TERMS

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Chromosphere: —The narrow middle layer of the Sun's atmosphere. It is about 17,492°F; (9,700°C) and is very faint relative to the photosphere.
Convection zone: —The outermost third of the solar interior. Here heat is transported to the surface in giant convective bubbles of gas, which rise to the surface, release their heat, and then sink back into the interior to pick up more heat.
Core: —The central region of the Sun, where thermonuclear fusion reactions take place.
Corona: —The highest and hottest layer of the solar atmosphere. Matter in the corona may have a temperature of 3,599,541°F (1,999,727°C) and may be several million miles above the photosphere.
Photosphere: —The lowest layer of the solar atmosphere, where most of the visible light is emitted. Because this is the layer we see in white light photographs, it is often called the solar "surface," even though it is a very thin gas.
Prominence: —A large region of glowing gas suspended in magnetic fields, often arching far above the photosphere. Some prominences are quiescent, remaining for days, while others are eruptive, and dissipate violently.
Radiative zone: —The central two-thirds of the solar interior. Here energy is transported by the flow of photons, or light waves, through the matter.