Convection And Granulation

Radiation streaming out from the Sun's energy-generating core is stopped at the bottom of the convective zone, which occupies the outer 28.7 percent of the solar radius. In this region, the relatively cool, opaque solar gas absorbs great quantities of radiation without re-emitting it. ttis causes the material to become hotter than it would otherwise be, and the Sun must find another way to release the pent-up energy.

tte heated material expands, becomes less dense than the gas in overlying layers, and rises to the visible disk of the Sun in roughly ten days, tte hot material then cools by radiating sunlight into space, and sinks back down to become reheated and rise again, tte roiling currents of hot and cool gas create a churning, wheeling motion that carries heat from the bottom to the top, like a boiling pot of water or other liquid that is similarly heated from below (Fig. 2.4).

In 1801 the English astronomer William Herschel (1738-1822) noticed that the Sun has a granular appearance. More than a century later, high-resolution photographs revealed that this granulation is composed of closely packed cells having bright centers surrounded by dark lanes, ttey mark the top of the convective zone.

Recent images of the Sun's white light, or all the colors combined, resolve the granules when taken under conditions of excellent seeing (Fig. 2.5). tte mean angular distance between the bright centers of adjacent granules is about 2.0 seconds of arc, corresponding to about 1,500 kilometers at the Sun. ttat seems very large, but an individual granule is about the smallest thing you can see on the Sun when peering through the Earth's turbulent atmosphere.

ttere are at least a million granules on the visible solar disk at any moment, exhibiting a non-stationary, overturning motion, tte bright center of each granule, or convection cell, is the highest point of a rising column of hot gas. tte dark edges

FIG. 2.4 Benard Convection When agasor liquid is heated from below, convection takes place in vertical cells, known as Benard cells. Warmer material rises at the centers of the cells and cooler material falls around their boundaries. This figure shows the hexagonal convection pattern in a layer of silicone oil that is heated uniformly from below and exposed to ambient air above; light reflected from aluminum flakes shows fluid rising at the center of each cell and descending at the edges. The regular motion, long duration, and polygonal shapes of these cells are somewhat distorted in the Sun's turbulent convective zone. (Courtesy of Manuel G. Velarde and M. Yuste, Universidad Nacional de EducaciĆ³n a Distancia, Madrid, Spain.)

Hexagonal Convection Cells

FIG. 2.5 Double, Double

Toil and Trouble When optical telescopes zoom in and take a detailed look at the visible Sun, they resolve a strongly-textured granular pattern. Hot granules, each about 1,500 kilometers across, rise at speeds of 500 meters per second, like supersonic bubbles in an immense, boiling cauldron. The rising granules burst apart, liberating their energy, and cool material then sinks downwards along the dark, intergranular lanes. This photograph was taken at 430.8 nanometers with a 1-nanome-ter interference filter. It has an exceptional angular resolution of 0.2 seconds of arc, or 150 kilometers at the Sun. (Courtesy of Richard Muller and Thierry Roudier, Observatoire du Pic-du-Midi et de Toulouse.)

of each granule are the cooled gas, which sinks because it is denser than the hotter gas. And each individual granule lasts only about 15 minutes before it is replaced by another one, never reappearing in precisely the same location.

tte granules are superimposed on a larger cellular pattern, called the supergranulation. Unlike the granules, which move up and down, the supergranulation is detected as a pattern of horizontal flow across the solar disk, and it remains invisible in white-light images that show the granules. By subtracting long- and short-wavelength images of the Sun in 1960, the American physicist Robert Leighton (1919-1997) discovered the supergranulation, a pattern of convection cells that are about 30,000 kilometers across, or about twice the size of the Earth. Roughly 2,500 supergranules are seen on the visible solar disk, each persisting for one or two days (Fig. 2.6).

Material in each supergranule cell rises in the center, moves away from the center with a typical velocity of about 400 meters per second, and sinks down again at the cell boundary, tte Sun's magnetic field is carried along with this flow, piling up at the supergranular cell edges and creating a network of concentrated magnetic field.

Modern instruments aboard the SOlar and Heliospheric Observatory, abbreviated SOHO, have shown that the supergranulation outflow is relatively shallow, disappearing at depths of about 5,000 kilometers or about one sixth of a cell diameter. Moreover, the supergranulation undergoes oscillations and supports waves that move across the Sun, resembling the "wave" of spectators at a baseball, football or soccer game. Since the supergranulation wave moves in the same direction as the Sun rotates, the supergranulation appears to rotate faster around the Sun than the other visible solar gas and magnetic features like sunspots.

tte visible disk of the Sun, with its granulation and supergranulation, caps the con-vective zone and completes our current model of the solar interior. As we shall next see, its ingredients and dimensions change dramatically over the eons, on cosmic time scales.

2.6 THE SUN'S REMOTE PAST AND DISTANT FUTURE

So how long has the Sun been shining, and how long will it keep on doing so? Nothing in the Cosmos is fixed and unchanging, and nothing escapes the ravages of time. Everything moves and evolves, and that includes the seemingly constant and unchanging Sun. It formed together with the planets about 4.6 billion years ago, when a spinning interstellar cloud of dust and gas fell in on itself, tte center got denser and denser, until it became so packed, so tight and so hot that protons came together and fused into helium, making the Sun glow. Ever since then, the Sun has slowly grown in luminous intensity with age, a steady, inexorable brightening that is a consequence of the increasing amounts ofhelium accumulating in the Sun's core.

Although the Sun is consuming itself at a prodigal rate, the loss of material is insignificant in comparison with its total mass, tte mass of the Sun is two thousand trillion trillion tons or 2 X 1030 kilograms, about a third of a million times the mass of the Earth. Over the past 4.6 billion years the Sun has consumed only a few hundredths

FIG. 2.6 Supergranulation Thousands of large convection cells, the supergranules, are detected in this Dopplergram which shows motion toward and away from the observer, along the line of sight, as light and dark patches, respectively. It was obtained using the Doppler effect of a single spectral line with the Michelson Doppler Imager, abbreviated MDI, instrument on the SOlar and Helio-spheric Observatory, or SOHO for short. The image contains about 2,500 supergranules, each about 30,000 kilometers across. Near the disk center, where the Doppler effect detects radial motion, the supergranulation is hardly visible at all, thus indicating that the velocities are predominantly horizontal. Supergranules flow horizontally outward from their centers with a typical velocity of 400 meters per second. (Courtesy of the SOHO MDI/SOI consortium. SOHO is a project of international cooperation between ESA andNASA.)

of one percent of its original mass, tte reason is essentially because very little mass (just 0.007 or 0.7 percent) is annihilated in forming a helium nucleus.

A more significant concern is the depletion of the Sun's hydrogen fuel within its nuclear furnace. Since thermonuclear reactions are limited to the hot dense core, the Sun will eventually run out of hydrogen - in about 7 billion years, tte Sun will then expand to engulf the the closest planet, Mercury.

As the hydrogen in the Sun's center is slowly depleted over time, and steadily replaced by heavier helium, the core must keep on producing enough pressure to keep the Sun from collapsing in on itself. And the only way to maintain the pressure and keep on supporting the weight of overlying material is to increase the central temperature. As a result of the slow rise in temperature, the rate of nuclear fusion gradually increases and the Sun inexorably brightens.

ttis means that the Sun was significantly dimmer in the remote past, and the Earth should have been noticeably colder then, but this does not agree with geological evidence. Assuming an unchanging terrestrial atmosphere, with the same composition and reflecting properties as today, the decreased solar luminosity would have caused the Earth's global surface temperature to drop below the freezing point of water about 2 billion years ago. tte oceans would have been frozen solid, there would be no liquid water, and the entire planet would have been locked into a global ice age something like Mars is now.

Yet, sedimentary rocks, which must have been deposited in liquid water, date from 3.8 billion years ago. ttere is fossil evidence in those rocks for living things at about that time, ttus, for billions of years the Earth's surface temperature was not very different from today, and conditions have remained hospitable for life on Earth throughout most of the planet's history.

tte discrepancy between the Earth's warm climatic record and an initially dimmer Sun has come to be known as the faint-young-Sun paradox. It can be resolved if the Earth's primitive atmosphere contained about a thousand times more carbon dioxide than it does now. Greater amounts of carbon dioxide would enable the early atmosphere to trap greater amounts of heat near the Earth's surface, warming it by the greenhouse effect, ttat would keep the oceans from freezing.

Over time the Sun grew brighter and hotter, tte Earth could only maintain a temperate climate by turning down its greenhouse effect as the Sun turned up the heat. Our planet's atmosphere, rocks, oceans, and perhaps life itself, apparently combined to decrease the amount of carbon dioxide over time, keeping the Earth's temperature steady as the Sun slowly brightened.

But the Sun cannot shine forever, for it will eventually use up the hydrogen fuel in its core. Although it has converted only a trivial part of its original mass into energy, the Sun has processed a substantial 37 percent of its core hydrogen into helium during the past 4.6 billion years. And the Sun will have used up all its available core hydrogen in another 7 billion years. It will then balloon into a red giant star with a dramatic increase in size and a powerful rise in luminosity (Fig. 2.7).

Although the outer solar atmosphere will cool and redden, the Sun will also expand and move much closer to the Earth. Mercury will become little more than a memory, being pulled in and swallowed by the swollen Sun. tte giant Sun will be 2,300 times

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