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A Brief History Of Solar Observations, The Solar Wind, A Small Blue PlanetA journey through the 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.

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 Nuclear fusion and the production of gamma rays, takes place in the core of the Sun. The radiative zone is so dense that it can take a million years for a photon to pass through. Illustration by Argosy. The Gale Group. 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.

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