The Sun is the brightest and most familiar object in the sky. Life on Earth would not be possible without it:

• The food we eat exists because of sunlight falling on green plants, and the fuel we burn comes either from such plants, or was accumulated by them (in the forms of coal, oil and natural gas) long ago.

• The Earth would probably not be fit for life. Life as we know it needs liquid water, and Earth is the only planet to have it: without the Sun, Earth would be an icy rock in space. Even now, Earth is probably the only place in our solar system fit for life: any water on Venus and Mercury would become steam, any on Mars or on more distant planets would freeze.

All hot substances radiate light, either the visible kind or beyond the rainbow spectrum, in the "infra red" (IR; "below red") and "ultra violet" (UV; "above violet") ranges. This glow [called "black body radiation" by physicists--the glow of a body with no color of its own] is the way a red-hot piece of iron or the filament in an electric light bulb produce light. The hotter the object, the brighter it shines, and the further away from red is its color. Conversely, the color of a hot object (if it is dense) tells us how hot it is. In the case of the Sun, the color of the photosphere suggests a temperature of 5780 degrees Kelvin (degrees Celsius measured from the absolute zero, about 5500° C.)

The Heating of the Earth

Think it over! If all of Earth's heat comes from the outside (neglecting internal heat), and if it maintains a steady temperature, no other way exists. Of course, only the average temperature is steady. Actually the ground is heated only in the daytime, but radiates back day and night, so nights, when energy only goes out and hardly any comes in, are cooler than days.

The "Greenhouse Effect"

• Clouds in the atmosphere reflect some of the sunlight before it reaches the ground, reducing the heating of the ground.

• The atmosphere absorbs the infra-red (IR) light radiated from the ground and thus delayes the escape of heat to outer space, keeping the ground warmer than it would otherwise be.

The second process is stronger, so the net effect is that like a blanket, the atmosphere helps keep Earth warmer than it would be otherwise. This is called the "greenhouse effect," because the same process operates in greenhouses used for growing vegetables in cold climates. A greenhouse is enclosed and roofed by glass panes, which let sunlight enter, but absorb the IR emitted back by the ground, and thus keep the greenhouse warm.

The chief absorbers of IR in the atmosphere are not nitrogen and oxygen, the main constituents of air, but a relatively minor percentage of "greenhouse gases" such as water vapor (H₂O), carbon dioxide (CO₂) and methane (CH₄), which are strong absorbers of IR.

Another molecule, responsible for an important effect even though only a very small amount of it is present, is ozone, a variant of the oxygen molecule--O₃ rather than the usual O₂ produced at high altitudes, with its peak around 25 kilometers. It is also a greenhouse effect, but more important, it absorbs the Sun's ultra-violet (UV) light, which on can cause skin burns and hurt eyes. The ozone found near the ground and forming part of the urban air pollution comes from a completely different process.

. High altitude ozone is destroyed by the presence of chlorine, and recently attention has been drawn to ozone removal by chlorine produced by escaping refrigerant gases, of the types preferred until recently for use air conditioners, refrigerators, aerosol cans and also some industrial applications. These gases are very, very stable, and can persist in the atmosphere for many years. Unfortunately, sooner or later their molecules wander into the stratosphere, where the ultra-violet sunlight is capable of breaking them up and releasing chlorine. Because of the damage from these gases to the ozone layer, their use is being phased out.

The greenhouse effect helps keep Earth at temperatures comfortable for life, but that is a finely balanced situation. In the last half century, the burning of fossil fuels--coal and oil-- has steadily increased the atmospheric content of CO₂. The average temperature of the Earth has also risen, and this rise is believed to be due to the added CO₂.

Further Exploring:

Site on the Earth's raditation balance, with several subfiles.
Site on the global solar heat budget.
Similar to the above, from a university course.

A hurricane viewed from space.

Rising air expands, and expansion of a gas cools it down, which is why mountaintops are cooler. Ultimately, a height is reached where not enough air remains on top to stop the IR radiation from escaping to space. The air then cools by radiation and stops rising, producing a relatively stable layer of the atmosphere known as the stratosphere.

Just below the boundary of the stratosphere ("tropopause"), air which has cooled is forced down again by warmer air rising from below. The result is a circulation of air, rising hot and returning cold, going around again and again, a motion known as convection. On a cold winter day such convection also occurs in homes: near poorly insulated windows the air cools and descends (as the flame of a candle will show--but careful with that fire!), while further inside the room it rises again. The region between the ground and the stratosphere where convection and weather take place is known as the troposphere.

Sunlight also evaporates water--from the oceans, from lakes and rivers and from green plants. Energy is invested in turning liquid water into vapor, and therefore humid air has more energy stored in it than dry air.

The capacity of air to hold water vapor depends strongly on temperature, and is smaller in cold air (just as less sugar can dissolve in cold water). As warm humid air rises, it expands and cools, and since it then cannot hold as much water as before, the excess is forced out: Initially into the tiny droplets of clouds, then if the cooling is more drastic, into raindrops.

The remaining air is drier and warmer--warmed by water vapor turning back to liquid and returning energy to its surroundings--and warmer air is better able to radiate its heat into space. That is how water, clouds and rain play a major role in the transport of solar heat from the ground back to space and help create the complex patterns of weather and climate.

Additional Exploring

 Two recent books discuss the history of the greenhouse effect:
Greenhouse: The 200-Year Story of Global Warming,
    by Gale E. Christianson, Constable/Walker 1999
Historical Perspectives on Climate Change,
    by James Rodger Fleming, Oxford Univ. Press, 1998.
Both books are reviewed by Robert J. Charlson in Nature, vol 401, p. 741, 21 October 1999.

Around and around goes the wind, and from its circuits returns the wind.
        Ecclesiastes, ch. 1, v.6

Pressure and Convection

Let us start with the flow of air. Suppose a "parcel of air" is heated near the ground (by conduction of heat, the flow of heat due to direct contact). Heat makes it expand, it becomes less dense than the surrounding air and buoyant, and it rises like a hot-air balloon (or like a drop of oil in a bottle of water).

At the higher levels of the atmosphere, this warm bubble again gives up its heat (to other flows or perhaps to cold space), cools down, and other bubbles coming from below push it to the side, where it descends again. (diagram on the board). Such a circulating flow is called convection.

More generally, convection is any flow which

1. picks up heat at one place,
2. drops it at another, and
3. is driven by this transport of heat.

The important thing to remember when dealing with convective flows, is that the higher one is in the atmosphere, the lower are the pressure and density of the air. What compresses it is the weight of air above it which it must support. On top of Mt. Everest, less air is piled up on top and therefore the pressure is lower.

At ground level, the compressing weight of the atmosphere amounts to about 1 kilogram on each square centimeter. That pressure does not bother our bodies, because the air inside us is at the same pressure, and the fluids of the body (like blood) do not compress easily. For the same reason, fish have no problems with depth--even at a depth of 100 meters, with a pressure 11 times larger (10 kilogram water above each square centimeter, plus the weight of the atmosphere) they feel no discomfort.

(Divers too can stand such pressure, provided the air they breathe is similarly compressed--except that the mixture must be changed, otherwise they may take in too much oxygen, and too much nitrogen is dissolved in their bloodstream.)

At an altitude of about 5 kilometers, only half the atmosphere is above us, the other half is below, so only the weight of half the atmosphere must be supported, and the pressure is reduced to one half.

By "Boyle's law" (named for Robert Boyle, 1627-91), the density is also reduced to one half (ignoring any variation in temperature). Rising an additional 5 kilometers, the pressure again falls by half, to 1/4 of what it was on the ground, and at 15 kilometers, it is halved again to about 1/8. All this is approximate and depends on temperature, but the trend should be clear.

The cabin of a jetliner flying at 10 km must be sealed and pressurized, because passengers breathing air at 1/4 the sea-level density would be starved for oxygen and might lose consciousness. On the very rare occasions when a jetliner loses its pressure, masks connected to oxygen canisters drop down automatically, allowing the passengers to breathe normally while the pilot quickly descends to a lower altitude.

Weather

First, a word of caution: what follows below is a very crude and simplified picture of a much more complicated process. While the circulation in the drawing below resembles the one proposed by Hadley in 1735, the actual situation is much more involved.

(a) Local Weather

When the atmosphere is stable, the higher we go, the cooler the air is.

Air is warmest near the ground, which absorbs receives heat from sunlight. It is coldest above the level where jetliners fly, at 10-15 kilometers, the region from where it radiates most of its heat into space. That is why mountaintops are cold and the highest mountains have snow on their tops.

(Still higher layers may get quite hot again, by absorbing UV and "extreme UV," but they have little effect on what goes on below them).

How exactly does this happen?

Suppose some "parcel of air" (dry air, for now--humidity is an additional factor, considered later) is heated by the ground and rises. Higher up the pressure is lower, so the air expands: but expansion cools it.

Similarly, if for some reason the parcel is blown down, is is compressed again and heated by the compression. Such up-and-down motions happen all the time, and the net result is that when conditions are stable, the temperature drops at a steady rate as we go higher.

The motion of the rising parcel of air depends on its surroundings. It always cools by expansion--but is it still warmer than the still air around it? If it is, it continues to rise; if not, it stops. As will be seen, this is where the humidity of air has an important effect.

[ On an ordinary day, direct heating by the ground only carries the air a few hundred meters, perhaps a kilometer, creating above the ground a "boundary layer" with many small convective flows. Large scale motions like thunderstorms usually occur higher up (see below).]

(b) Global Weather.

Convection also operates on a global scale. The greatest heating occurs near the equator, and air heated there rises and flows poleward, to cooler regions of the Earth.

The rotation of the Earth greatly modifies this flow, by the Coriolis effect, as explained below.

• At the equator the east-west motion of the air matches that of the equator.

• Away from the equator, however, the Earth's surface is closer to the rotation axis, the distance it covers in 24 hours is smaller, and so its west-to-east speed is slower. If the air moving away from the equator (right side of the drawing above) persists at its original west-to-east speed, it will overtake the local surface, making the predominant winds blow from west to east ("westerlies").

• The cooler air returns equatorward (middle part of the drawing above) at lower altitudes, completing the loop. If it still kept its original west-to-east speed, it would again match the local rotation of the equator.

• Actually, the air loses velocity to friction with the ground. Therefore, the time it returns to the equator, it lags behind the rotating ground, and the average wind is easterly.

In the age of sailing ships, sea-captains took advantage of this system. Sailing from Spain to America, they would go closer to the equator, a more southern route that took advantage of the easterly "trade winds. " Sailing back home they would go further north and use the westerlies. The many Spanish wrecks off Florida, some containing quite rich cargo, were lost on this home voyage back to Spain, loaded with gold and silver from Mexico and South America.

Jetliners flying across the US cannot swing quite as far. However, when flying west to east, their pilots often exploit the fast core of the westerlies, known as the jet stream and flowing at high altitudes. Flying westward they try to avoid the jet stream.

Water vapor

We look at two examples of humidity in action.

1. In a thunderstorm, hot humid air rises, as in ordinary convection. As it rises to regions of lower pressure, it cools by expanding. However, cold air cannot hold as much humidity as warm air, and the extra water is therefore squeezed out. In moderate convection it forms clouds (as in the second example below), but in a vigorous thunderstorm, there is too much of it and it turns into rain.

   Giving up water heats the air, or rather, slows down its cooling, because the heat invested by the Sun in evaporating the water is now passed back to the air. As a result, the rising air is still warmer than the air layers around it, and it continues to rise vigorously. It squeezes out still more rain and forms the tall thunderstorm clouds, which pilots know well to avoid.

   (In very vigorous thunderstorms, the "updrafts" of rising air may rise so quickly that they blow raindrops into the higher and colder sections of the cloud, where they freeze, producing hail. Some hailstones are picked up again and again, adding layers of ice on each upwards journey. That is how unusually big hailstones can form.)

2. On a hot clear day, many fluffy small clouds may form. A light plane flies across the land, and every time it comes under a cloud, the pilot feels that it is bodily lifted. What is happening?

   --Here is the reason. The heating of the ground by the Sun has created many small convection currents rising upwards. Their air contains humidity, not enough for a serious thunderstorm, but enough to produce small clouds when droplets of water condense as the rising air cools.

   The small clouds mark the top of the "boundary layer" near the ground, with many small circulating flows. Each cloud sits on top of a rising convection current, which lifts the airplane as it flies across it. Since "what goes up must come down" the pilot can also expect downdrafts between the clouds, where the air goes down again, as part of the circulating convection. Such up-and-down motions may make passengers on low-flying airplanes quite airsick!

Further Exploring

   Who first wrote "Everybody talks about the weather, but nobody does anything about it"? Most would claim it was Mark Twain (just type the first 5 words into a search engine and see!), but it ain't necessarily so. It looks like Twain's style, but actually the words first appeared in an editorial in the Hartford (Conn.) Courant on 24 August 1897, written by Charles Dudley Warner.

    Warner was a good friend of Twain, who himself had lived in Hartford for many years (he left before 1897). He was a newspaperman in Hartford and the two had collaborated on an 1873 book "The Gilded Age." Warner is also remembered for other quotes, e.g. "Politics make strange bedfellows."

Concerning the quote, see on the web
        http://www.m-w.com/wftw/99aug/082499.htm.
and more about Charles Dudley Warner (and Twain) at
        http://courant.ctnow.com/projects/twain/warner.htm.

How to Observe the Sun

The safe method for observing the Sun was apparently introduced by Christopher Scheiner, one of the three observers who claimed to have discovered sunspots (Galileo and Fabricius were the others). He projected the Sun's magnified image from his telescope upon a flat white surface, and observed it there. That method is still the choice of observers. It is used by the world's greatest solar telescope, at Sacramento Peak ("Sac Peak") in New Mexico, which can throw a big image of the Sun onto a table in a cool underground room. The tube is fixed, built into the ground, while the Sun is tracked by a movable mirror at its top.

If you want to view the Sun--during an eclipse, for instance--do not look at it directly, but project its image onto a flat surface using a telescope (which may have a special attachment) or even a piece of cardboard with a pinhole. Alternatively, sun-filters exist for telescopes, and you may also look at the Sun through a totally blackened black-and-white film (old x-ray films may have suitable portions) or a welder's shield. Whatever you use, the Sun must appear comfortably dim.

On some rare occasion, the Sun near the horizon shines through a thick haze which dims it but does not make it appear less sharply. At such times large sunspots can be seen by the eye, and have in fact been reported long before Galileo by Chinese observers and others. But the Sun must appear dim, like an orange on the horizon.

How Far Away is the Sun?

By observing the motions of various planets, astronomers can readily obtain the value of T for each, from which a can then be calculated, giving planet's mean distance from the Sun in AU. We thus get a pretty good idea of the relative size of planetary orbits. But to know the actual distance in km or in miles, one of these distances, at the very least, must also be measured.

Given a map drawn on an unknown scale, we need only know the true value of any of its distances to calibrate all others. The same holds here: the distance from Earth to any planet is sufficient. When, for instance, radio signals were first bounced off the planet Venus from the giant radio-telescope at Arecibo, Puerto Rico (an immovable dish supported by a bowl-shaped valley), the time delay provided the value of the astronomical unit with greater accuracy than ever before. It is possible that even greater accuracy was obtained by tracking the radio signals of Voyager 2 as it passed Uranus and Neptune (giving a longer baseline), or of the Viking landers on Mars (giving a more sharply defined position).

The Layers of the Sun

What appears to us as the "surface" of the Sun is actually a relatively thin layer (about 100 km or so, which is 1/7000 of the Sun's radius) called the photosphere. Layers below it cannot be seen, but as in the Earth's atmosphere, one expects them to become progressively denser with increasing depth, compressed by the weight of the layers on top of them. They are also presumed to get increasingly hotter, since the Sun's heat is generated in its core and flows outwards.

Similarly, as one proceeds upwards from the photosphere, layers of the solar atmosphere become increasingly rarefied, and become transparent to the light coming from below them. Until the 20th century, these layers could only be observed during a total solar eclipse, when the disk of the Sun was blocked by the Moon.

Observations of the eclipsed Sun show the next layer above the photosphere--the reddish chromosphere ("chromos"=color), about 5000 km thick. Above that came the solar corona ("crown"), glowing tufts that faded into the distance, though time exposures with sensitive cameras managed to trace them to several solar radii.

Above the middle latitudes of the Sun the streaks of the corona sometimes displayed arches, suggesting that magnetic fields there determined its structure. That impression was reinforced by "polar plumes" above the north and south poles, spread out like the iron filings at the ends of a magnet, suggesting that the Sun, like the Earth, had two magnetic poles.

The heat of the Corona

    The high temperature is inferred from the light emitted by their atoms. In a rarefied hot gas, emitted light no longer follows a simple pattern dictated by the temperature, like red-hot iron or a lightbulb filament (both of which are solid matter). Instead (see section S-4), it is concentrated in narrow ranges of color ("spectral lines") which depend on the atoms from which they came. Observations of emissions from atoms (or rather, ions) such as iron with 13 of its electrons missing suggest a coronal temperature of around 1,000,000°C (degrees Celsius), while emissions from chromospheric ions similarly suggest about 30,000°C.

    The high temperature of the corona can also be seen from the x-rays and the extreme ultra-violet light which it emits, and indeed, images in these emissions (which must be taken outside the atmosphere) are now the preferred way of observing the corona (see section S-6). Such an image, color-coded to indicate regions of different temperature, is linked here. It shows that the heat is not evenly distributed:

    The way the corona is heated is a mystery. It cannot be ordinary heat flow from the photosphere below, because ordinary heat radiation can never create a temperature higher than that of its source. Suppose you concentrate sunlight with magnifying glasses and mirrors, and suppose you somehow keep your sample from evaporating. You can never get it hotter than the Sun. As the sample heats up, it too radiates, and by the time its temperature reaches that of the Sun, it loses as much heat as it gains.

    Some early theories proposed that the energy was coming from the outside, in the form of a steady rain of meteorites, greatly accelerated by the Sun's gravity, but that was soon disproved. Today it is believed that some plasma process is responsible, fed by local magnetic fields, but we really do not know much about that process. One idea is that it creates a sub-population of very fast ions, which do not share their energy because of the lack of collisions. At levels above the photosphere, solar gravity holds back the slower component, so that only very hot ions populate the higher layers, giving the corona a very high effective temperature.

    Other theories suggest that plasmas waves coming from the photosphere arrive in the corona and, unable to proceed into the rarefied gas, dissipate their energy. Because the gas receiving that energy is so rarefied, it heats up to a high temperature.

(For more about the corona, see here.)

The Solar Wind

    The Sun's gravity is much stronger, but a million-degree atmosphere is too much for it. As a result, a steady stream of hot plasma known as the solar wind flows out of the corona into space. The corona however is not likely to disappear any time soon, since it is constantly replenished from below. The solar wind was predicted by Eugene Parker in 1958 and its existence received increasingly stronger confirmation from scientific instruments aboard the 2nd Soviet moon rocket in 1959, NASA's Explorer 10 in 1961 and the interplanetary probe Mariner 2 in 1962. Streamers in the corona outline the start of the solar wind.

    At the orbit of the Earth, the solar wind has an average density of about 6 ions/cm³, compared to 2.5 10¹⁹ molecules/cm³ in the Earth's sea-level atmosphere; it is more rarefied than the best laboratory vacuum. Earth receives solar wind from low and middle latitudes of the Sun, with an average outwards speed of 400 km/sec in the direction of Earth; above the Sun's poles the velocity nearly doubles, as reported by the Ulysses space probe. Past the Earth's orbit, the solar wind continues with undiminished speed (but with decreasing density, as its particles spread out), well past the orbit of Pluto. Scientists hope that Voyager 2, now racing away from the Sun, will still be transmitting when it crosses its outer boundary ("heliopause") or at least the "terminal shock, " a discontinuity preceding it, which should happen some time in the 21st century.

Exploring Further

Sunspots

Galileo and Christopher Scheiner observed dark spots on the face of the Sun, and from their motion they deduced that the Sun rotated, with a period of 27 days close to the equator, relative to the moving Earth (25 days, relative to the stars). The period increased to about 29.5 days at higher latitudes, showing the Sun's surface was not solid.

The sunspots, they guessed, were clouds floating above the surface, blocking some of the sunlight. We now know that sunspots are darker than their surroundings because they are moderately cooler, since their intense magnetic fields somehow slow down the local flow of heat from the Sun's interior. The process which causes this is still unclear.

            What is a "magnetic field," anyway?

What follows below is a brief summary of magnetism; more details can be found on the files linked below, all of them parts of the web site

Magnetism

The magnetism of rare natural "lodestones" was known in ancient Greece--supposedly first noted in the town of Magnesia, from which comes the name. The magnetic compass (a Chinese discovery) was used by Columbus and other early navigators, but it was not until 1820 that a Danish professor, Hans Christian Oersted (pictured on the left), found by accident that an electric current in a wire could deflect a nearby compass needle (click here for the full story). A Frenchman, André-Marie Ampere, showed soon afterwards that the basic magnetic phenomenon was the force between two electric currents in parallel wires; they attracted each other when they flowed in the same direction, and repelled when they were opposed (click here for a more detailed discussion).

Just as lines of latitude and longitude help us visualize positions on the Earth's globe, so magnetic field lines (originally named by Michael Faraday lines of force) help visualize the distribution of magnetic forces in 3-dimensional space. Imagine a compass needle which can freely turn in space to wherever the magnetic force tries to point it (such needles exist--see bottom of this web page). Magnetic field lines are then imaginary lines which mark the direction in which such a needle would point.

A compass needle, for instance, has two magnetic poles at its ends, of equal strength, the north-seeking (N) pole and the south-seeking (S) pole, named for the directions on Earth to which they tend to point. Suppose the needle is free to point anywhere in 3 dimensions. If placed near the north pole, it would everywhere point towards the pole, and field lines therefore converge there (see drawing). If placed near the south pole, it would point away from it in all directions, and therefore field lines would diverge there, coming out of the Earth in a pattern that is a mirror-image of the pattern at the north pole. In between the lines form big arches above the Earth's equator, with their ends anchored in opposite hemispheres.

Any bar magnet has a pattern of field lines like that of the Earth, suggesting that the Earth acted as if a short but very powerful bar magnet was inside it. Actually such a magnet does not exist, and the pattern comes from electric currents in the Earth core, and slowly changes, year by year; still, the "terrestrial bar magnet" remains a useful visualization aid.

When two bar magnets are brought together, their (N,S) poles attract each other, their (S,S) and (N,N) poles repel: thus if a bar magnet were hidden inside the Earth, its S pole would be the one that pointed northwards, attracting the N pole of the compass needle. This strange mix-up of terminologies often confuses students: it is best to recognize the mix-up exists and then to ignore it.

  Michael Faraday who in the early 1800s introduced the concept of magnetic field lines, believed that space in which magnetic forces could be observed was somehow modified. His was a somewhat mystical view, but later mathematical developments found it quite useful, and today we refer to such a region of space as a magnetic field.

The Sunspot Cycle

Ever since then solar observers have carefully followed sunspot cycles, and have also reconstructed earlier cycles from available observations, defining a suitable "sunspot number" index to gauge the level of sunspot activity. The nature of sunspots remained unclear until 1908, when George Ellery Hale, using an instrument that observed the Sun in narrow ranges of color emitted by selected substances, reported that the light from sunspots was modified in ways that indicated it was produced in intense magnetic fields.

Sunspot fields turned out to be as intense as the ones we find near the poles of iron magnets--but extending across regions many thousands of kilometers wide. In conventional units, the magnetic intensity intensity in them reached about 1500 gauss (0.15 Tesla), while the field near the surface of Earth is typically 0.3-0.5 gauss, depending on location. In interplanetary space at the orbit of Earth, the magnetic field (carried from the Sun by the solar wind) is much weaker, typically 0.00006 gauss, while at the orbits of the outer planets, it is 20 times weaker still. Yet even there the instruments of spacecraft such as Voyager 2 still measure it reliably.

Sunspots display many interesting features. Generally (though not always) they appear in pairs, with opposite magnetic polarities. In half the solar cycles, the "leading" spot (in the direction of the Sun's rotation) will always have an N polarity, and the "following" spot an S polarity; then in the following cycle, polarities are always reversed. The general magnetic field, producing the Sun's north and south magnetic poles, also reverses polarity at each cycle, the reversal typically occuring 3 years after sunspot minimum. All these suggest that the 11-year cycle is a magnetic phenomenon. Astronomers believe that the electric currents which flow in the solar plasma and create those fields get their energy from the unequal rotation of the Sun--faster at the equator--which in its turn is driven by large-scale flows of the solar gas.

Solar Activity

The fastest and most significant among these was the solar flare--a brightening of the chromosphere near a sunspot group, rising within minutes and typically lasting 10-30 minutes.

Flares are usually observed in the red light emitted by hot hydrogen (Ha or "H-alpha line"), but it so happened that the first observation in 1859 was of a rare "white light flare" observable with an ordinary telescope (see here and here for the full story). This was followed 17 hours later by a huge magnetic storm, a world-wide disturbance of the Earth's magnetic field: something apparently was ejected from the Sun, and took that long to reach Earth.

We now know that "something" was probably a fast-moving plasma cloud, plowing through the ordinary solar wind, which takes 4-5 days to cover the same distance. The arrival at Earth of such clouds, with the shock wave that forms ahead of them, can be quite dramatic (see here for one story).

The most remarkable aspect of such activity is the speed with which it takes place. If a typical big flare spreads over 10,000 km in 10 minutes, it must propagate quite rapidly. Some of its features begin much more abruptly, e.g. the associated x-rays (observable from space) can rise in just a few seconds. All this suggests that the energy source is not the heat of the Sun, which spreads and changes rather gradually, but the intense magnetic fields of sunspots. (For more on these matters, see here.)

Exploring further

A small bar magnet, on gimbals that allow it to point in any direction in space, can be procured from its manufacturer, Cochranes of Oxford, Ltd., Leafield, Oxford OX8 5NT, England. Two types are available, Mark 1 with jewelled bearings for $36.60, Mark 2 with simple bearings for $12.65. For details see their web site:
    http://www.cochranes.co.uk/BNRVP30/edu5.htm

The Law of Field Line Preservation

When a spacraft breaks away from the influence of the Earth's magnetic field into interplanetary space, it finds there a weak magnetic field. The field may be weak, but it extends over huge distances, and can have important effects. From the observed direction of interplanetary magnetic field lines, we believe this field comes from the Sun, carried by magnetic field lines dragged out by the solar wind.

Here this "dragging" process will be explained, and it will be used to obtain the expected shape of those field lines.

But if the field is weak, then the plasma rules and pushes the field lines around. A rule which is fairly well obeyed states then that if two or more ions start out located on the same field line, they will always share the same field line. If they then manage to move, the field line gets deformed.

(For any space physicist reading this: the deformation process also involves electric fields.)

Using this "law of field line preservation", we will now derive the shape of interplanetary magnetic field lines.

1. In the middle of the bottom (short side) of a sheet of paper, draw a small circle, about one inch across: that will represent the Sun, viewed from far above its north pole. If the Sun rotates once in 27 days, then each day it rotates by
   360°/27 = 13.3°

   Draw from the center of the Sun a line perpendicular to the bottom of the page, extending most of the way to the top. Using a protractor and a ruler, draw on each side of that line 3 additional radial "spokes" from the center of the Sun, each making angles of 13.3° with its neighbors.

(Alternative method: Let center of the Sun be the origin of a system of cartesian coordinates, with the x-axis parallel to the bottom of the page. Draw the two axes, with the y-axis extending to near the top of the page. With a pencil, draw faintly the line y=4, parallel to the x axis but 4" (4 inches) above it.
    On that line mark points at distances 15/16", 2" and 3 3/8" on both sides of the y-axis, then draw radial lines from the center of the Sun through those points, extending them until they are 1/2" from the sides of the sheet or 1" from the top.
    For those used to metric units, let the radius of the Sun be 1 cm (diameter=2cm), the pencil line follows y=10 cm and the marks on it are at distances of approximately 23.7, 50.2 and 83.9 millimeters from the y-axis. Extend the lines until they reach within 1 cm of the sides or 3 cm of the top.)

2. On each of the spokes, mark the point where it emerges from the Sun, and mark from there, along each spoke, additional points at intervals of 1.5 inches. Each interval marks the distance the solar wind covers in one day.

   (Yes, the Sun is drawn much too big on this scale, but we will ignore the difference this makes. Besides, the solar wind does not start moving from the Sun's surface, but from some greater distance.)

   The magnetic field at all these points is already so weak that the solar wind overpowers it and shifts its field lines, while its own motion--radially outward--remains unchanged. We will now derive the shape of those lines.

   (As a shortcut, you can download the drawing here. A version with higher resolution is linked here; you will not see any difference on your screen, whose resolution is limited to 72 dpi, but you can copy the file and use it with some graphic program that has higher resolution.)

3. Since the Sun is viewed here from north, and it rotates in the same sense as the Earth, on this scale drawing, does it rotate clockwise... or counterclockwise?
       Decide for yourself and write it down--no peeking!

Marking the Spokes

4. Mark with the number 1 the point where the line furthest to your right emerges from the Sun. You are told that 7 ions are located at that point, close to each other and on the same magnetic field line. You are also told that in the coming week, all 7 are destined to join the solar wind, one day apart. On day one, however, they are all still at the starting point, although one ion has just started moving outwards

5. Go to the next "spoke" on the left of the first one. The Sun rotates counterclockwise (yes!), so on day 2 all of them are at the base of that line--except for the one which started the previous day, which has moved radially and is now at "first base," the next point on the first line. Also, still another ion has just started moving outwards from the starting point. Mark both points with the number 2.

   On day 3, the ion which started out first is at "second base," and the one which started on day 2 is at the first point out on its spoke. All others are at the base of the 3rd spoke, to which the Sun has now rotated, and one more ion has just begun to move. Mark all three point with the number 3.

   On day 4, the Sun has rotated to the 4th spoke and 4 ions remain at the base point of that spoke, including one which is just starting to move. The other three, in the order they were released, are at 3rd, 2nd and 1st "base." Mark all four points with the number 4.

   And so on, day after day. The points marked 5, for instance, are where the particles are on the 5th day. Obviously, you must give up on marking any ions which have gone past the limits of the paper.

   Each of the radial lines is now marked with the day on which "its" particle reaches each marked point. If any unmarked points are left, you may, if you with, extend the marking further (8, 9, ...), to days for which we have no starting particle.

Spiral field lines

6. Now connect--preferably with a red pen, or in some color different from that of the rest of the drawing--all points with the number 2, also the ones with 3, 4, 5, 6 or 7, and perhaps also those with 8, 9 and 10. Take those marked 6: they show where the ions are after 5 days have passed--about the time the first of them reaches Earth's orbit. Since at the beginning they were all on the same magnetic field line, after 5 days they still are. The line you have drawn therefore gives the expected shape of an interplanetary magnetic field line.

   You may use a straight ruler for the connection: the actual lines curve smoothly, but even with lines composed of straight sections it becomes clear that the shape is a spiral. This agrees with observations at the Earth's orbit, where the average interplanetary magnetic field is found to make an an angle of 45° with the flow of the solar wind, similar to what the drawing shows. In other words--after being 5 days on their way, and reaching 150,000,000 km from the Sun, the magnetic field lines still "remember" the Sun's rotation.

7. They "remember" it for months afterwards, even as the solar wind speeds past the orbits of Saturn and Uranus. If you continue this graphic excercise to such large distances, you will find that the spiral gets wound tighter and tighter, until the field line direction is very close to that of circles around the Sun. The space probe Voyager 2 has shown that this indeed does happen.

It was also noted that with increasing distance from the Sun, the spiral shape of interplanetary magnetic field lines becomes more and more tightly wound, until their shape differs little from circles.

Both points were well illustrated by the phenomena that followed intense solar activity in April-May 1998, reported by Robert Decker of the Applied Physics Lab of the Johns Hopkins University in Maryland. That activity created a disturbance in the solar wind, as well as a flow of protons with energies about 1000 times that of the solar wind, and these were observed by a number of spacecraft--ACE at the L1 Lagrangian point (near Earth, distance from the Sun about 1 AU), by Ulysses (5 AU), and by Voyagers 1-2--Voyager 2 at 56 AU and Voyager 1 at 72 AU.

The solar wind disturbance arrived at Voyager 1 about 7.5 months later, propagating radially at the velocity of the solar wind flow in which it was embedded. The protons, on the other hand, although they moved much faster, were relatively few in number, which forced them to spiral along field lines. They were observed by Voyager 1 after 6 months--1.5 months before the disturbance in the solar wind reached that distance--and Dr. Decker calculated that their spiral path took them 10 times around the Sun, a total distance of about 2000 AU.

Color: What is it?

[For the same reason, a simple glass lens brings different colors to a focus at different distances. In Newton's time, if an astronomer focused a telescope to give (say) a sharp yellow image of a star, that image would be surrounded by unfocused patches of red and green. Newton thought the problem was insoluble, and proceeded to invent a new kind of telescope, based not on lenses but on concave mirrors, which reflect all colors equally. In later times optics were created which focused all colors together, using a combination of several lenses made of different kinds of glass, and these are nowadays found in cameras, projectors and small telescopes. However, all big modern telescopes follow Newton's idea and use mirrors.]

Perceived color

Even with the rainbow explained, the puzzle of color still baffled scientists. The riddle was solved around 1860 by James Clerk Maxwell (pictured on the left), the brilliant Scottish physicist who also gave us the basic equations of electricity, the ones that predicted electromagnetic waves (see Section S-5). Maxwell showed, while still a student, that two kinds of color existed, depending on whether it was perceived by an instrument or by the human eye:

1. "Spectral color, " i.e. the colors of the rainbow and their combinations. The amount which each part of the rainbow spectrum contributes to a beam of light can be determined by splitting the beam with a prism.

2. "Perceived color" reported by the human eye to the brain.

An instrument using prisms ("spectrograph") will reveal that the human eye can be fooled: different combinations of rainbow colors may look the same to the eye.

Our eye contains three kinds of light-sensitive cells, each sensing a different band of colors--one band centered in the red, one in the green and one in the blue. Any color which we see--including brown, olive-green and others absent in the rainbow--is an impression our brain conveys as it combines signals from these 3 color bands. Color-blind persons lack some types of eye cells, and their world lacks certain colors, or even (for those having only one kind of cell) any color at all. Color blindness is much more prevalent in men; in women, on the other hand, a rare mutation exists whose eyes have four different kinds of receptor cells. The rest of us can only guess what colors those ladies must be able to see! (For more about it, see here.)

That is why color TV and color printers can be based on the three "primary colors" red, green and blue. These devices do not in any way reproduce the true spectral color of the objects they show, but they are still capable of representing any color our eyes can see. Here and at the bottom of this page you can link to a program letting you experiment with 3-color combinations, using the color monitor of your computer.

The Spectrum

(1) In light emitted from solids, liquids or extensive bodies of dense gas such as the Sun, the colors are distributed continuously. Their exact distribution ("black body spectrum") depends on the temperature at which it is produced--a warm hand radiates mostly in the infra-red, a glowing bar of iron is cherry-red, a lightbulb filament is bright yellow, and sunlight is white-hot.

[Also of this type is the distribution of microwave radiation left over from the "big bang" when the universe apparently began, a radiation observed by NASA's COBE satellite, the Cosmic Background Explorer. When the observed COBE spectrum was first shown before a meeting of astronomers, it caused a great stir. Observed values generally show some experimental error, but here they were so close to the predicted theoretical curve that the first impression of the viewers was that the presenters had drawn the curve first and then placed their points on top of it.]

The Wave Nature of Light

A variety of instruments allow physicists to actually measure the wavelength of light. The one most likely to be used by students is a diffraction grating, a plate ruled with fine parallel grooves, with a constant distance between each one and the next. Inexpensive plastic gratings are available, pressed from a metal grating and mounted on cardboard frames like photographic slides. The incoming wave resonates with the spacing between the grooves and some of it is deflected, by an angle which depends on the wavelength, and knowing the angle and the spacing allows the wavelength to be calculated. Thus gratings can separate a beam of light into its colors the way prisms do, and they are often used in spectroscopes.

[Lit from the side with a reflecting surface behind them, gratings will shimmer in many colors, making them a popular item of costume jewelry. The same process is responsible for the shimmering of laser disks used in recording music and computer data, which also contain many narrow parallel grooves.]

Spectra

In addition, however, sunlight also contains many bright emission lines, characteristic of hydrogen, calcium and other elements. One yellow line, discovered in 1868, was first identified as the yellow line of sodium, but it did not have the proper frequency and did not fit the spectrum of any other known substance. The British astronomer Norman Lockyer finally proposed that here was a new substance, unknown on Earth, and he was right: "helium" (from "helios", the Sun) was identified in terrestrial material by William Ramsay in 1895 and was later isolated by him.

Astronomers studying the Sun enjoy a big advantage: the object which they observe is very, very bright. It is therefore possible to extract from its light just one narrowly defined color--a single spectral line--and still have enough brightness left to give a detailed picture.

Ever since George Ellery Hale in 1892 found a way to observe the Sun in this manner, astronomers have used it to look at the Sun in the light of hydrogen, calcium or helium. The detailed pictures of the Sun presented (for instance) on the world-wide web, the ones that show clouds, streaks, tufts and other structures, are all created in this manner. Other monochromatic images are obtained in extensions of the visible spectrum, e.g. UV, EUV (extreme UV) and X-rays.

One feature shown in such pictures are prominences, large clouds of denser and cooler gas, rising high above the photosphere. Some of them stand out against the dark background sky on the visible edge ("limb") of the Sun, and if one watches them for a while, one can see material falling back towards the Sun. Others are seen in the middle of the solar disk, where they appear as dark filaments, because being cool, they absorb the spectral line in which the observation is made. Prominences turned out to be important for understanding coronal mass ejections, discussed here in a later section.

Electromagnetic Waves

With that term added (the "displacement current"), the equations of electricity and magnetism allowed a wave to exist, propagating at the speed of light. The drawing below illustrates such a wave--green is the magnetic part, blue the electric part--the term Maxwell added. The wave is drawn propagating just along one line: actually it fills space, but it would be hard to draw that.

Electromagnetic Wave (see text above)

Maxwell proposed that it indeed was light. There had been earlier hints--the velocity of light had appeared unexpectedly in the equations of electricity and magnetism--and further studies confirmed it. For instance, if a beam of light hits the side of a glass prism, only part of it enters--another part gets reflected. Maxwell's theory correctly predicted properties of the reflected beam. The next obvious question was: if this was an electromagnetic wave with wavelength around 0.5 microns, what about other wavelengths?

Heinrich Hertz in Germany calculated that an electric current swinging very rapidly back and forth in a conducting wire would radiate electromagnetic waves into the surrounding space (today we would call such a wire an "antenna"). With such a wire he created (in 1886) and detected such oscillations in his lab, using an electric spark, in which the current oscillates rapidly (that is how lightning creates its characteristic crackling noise on the radio!). Today we call such waves "radio waves". At first however they were "Hertzian waves, " and even today we honor the memory of their discoverer by measuring frequencies in Hertz (Hz), oscillations per second--and at radio frequencies, in megahertz (MHz).

Light and radio waves belong to the electromagnetic spectrum, the range containing all different electromagnetic waves. Over the years scientists and engineers have created EM waves of other frequencies--microwaves and various IR bands whose waves are longer than those of visible light (between radio and the visible), and UV, EUV, X-rays and g-rays (gamma rays) with shorter wavelengths. The electromagnetic nature of x-rays became evident when it was found that crystals bent their path in the same way as gratings bent visible light: the orderly rows of atoms in the crystal acted like the grooves of a grating.

[It should be stressed that such vaves are very different from sound waves, which have no connection to electricity but are pressure waves, associated with the elasticity of solids, liquids and gases. The wavelength ranges of radio waves and sound overlap. However, radio is stopped by a metal screen because it conducts electricity, while penetrating non-conducting solid walls (you can listen to your radio indoors!). Sound on the other hand is stopped by walls but penetrates the mesh of the screen.]

Photons

To find how a light wave passes through a telescope, one calculates its motion as if it filled the entire focusing mirror. Yet when that same wave gives up its energy to one individual atom, it turns out that it acts like a particle. Regardless of whether a light beam is bright or dim, its energy is always transmitted in atom-sized amounts, "photons" whose energy depends only on wavelength.

Observations have shown that such duality also existed in the opposite direction. An electron should in principle have at any time a well-defined location and velocity, yet experiments that measure them give a blurred result. Quantum physics tells us that arbitrary precision in such observations cannot be attained, but that the motion may be described by a wave.

This may be a good place for introducing new quantities and notations. An electromagnetic wave of wavelength l (lambda, small Greek L) covers a distance of c meters each second, where c is the velocity of light in space, close to 300,000,000 meters/second. Its frequency n (nu, small Greek N)--the number of up-and-down oscillations per second--is also the number of wave crests in that distance, and is therefore obtained by dividing c with the wavelength:

A basic quantum law then states that the energy E in joules of a photon of light of frequency n is

Exploring further: A web page on electromagnetic waves, part of an extensive and detailed site on "The Amazing World of Electrons and Photons". Click here for a map of that site.

Wavelength and Energy

The process also works the other way around: when "excited" atoms give up their excess energy to an electromagnetic wave (energy they might have received, say, through a collision with some fast atom in a glowing gas) they can only do so in photon-sized amounts. The fact that atomic emissions appear in narrowly defined "spectral lines" suggests that "excited" atoms cannot contain extra energy in arbitrary amounts, but must be in one of their "energy levels" which resonate with their structure, each associated with a precisely defined amount of energy.

Each atom also has a "ground state, " its lowest energy level and the one in which it prefers to stay. When it descends from some excited state to the ground state, the starting and final energies of the atom are precisely specified energy levels. The energy emitted, equal to the difference between the two, is thus narrowly defined, producing a photon with a precise wavelength. The great success of quantum mechanics has been its ability to calculate and predict the energy levels of various atoms and combinations of atoms.

The formula E = hn = hc/l means that the shorter the wavelength l, the more energetic the photon. A photon of UV contains more energy than one of visible light, and photons of X-rays and g-rays (gamma rays) are more energetic still. One therefore expects that hotter regions of the Sun, where individual particles have more energy, will emit electromagnetic radiation of shorter wavelength, and that is indeed observed.

The temperature of a gas is proportional to the average energy of each of its particles (the formula, by the way, is E = 3/2 kT, where T is the absolute temperature in degrees Kelvin--like Celsius, just different zero point--and k is a fixed number, "Boltzmann's constant."). Thus while the photosphere emits mainly visible light, the hot corona is better observed in EUV (extreme UV) or in long-wavelength X-rays. Flares give even higher energies to ions and electrons, and to trace locations where those particles are produced and absorbed, shorter X-rays and g-rays are needed. All these ranges have been observed by instruments aboard spacecraft. They cannot be studied from the ground, because all short-wavelength photons are easily absorbed by the atmosphere and do not reach ground level.

Coronal Holes

The best-known early observations of the Sun from space were the ones made in 1973 from the

On right: Yohkoh X-ray image of the Sun. For bigger image, click here.

As might have been expected, the brightest coronal emissions came from above active sunspot regions. Separating such bright patches were large dark areas, named "coronal holes"; they were appparently dark because of lower density and less heat dissipation in the lower corona.

The discovery of coronal holes helped solve a long-outstanding puzzle. Long before Skylab, space probes such as Mariner 2 in 1962 detected fast streams in the solar wind, flowing not at 400 km/s but perhaps at 600 km/s or more. They tended to recur at intervals of 27 days--the rotation period of the low-latitude Sun--suggesting that whatever their source was, it rotated with the Sun. Even earlier, around the turn of the century, series of moderate magnetic storms were observed which tended to recur at 27-day intervals, prevalent not near the peak of the sunspot cycle but, perversely, near its minimum.

Skylab showed that both phenomena were associated not with sunspots but with the dark areas of coronal holes, which also seemed to contribute much of the solar wind. Apparently loops of magnetic field lines above sunspots help trap plasma (a bit like they do in the Earth's radiation belt) and hold it back.

Field lines of coronal holes, on the other hand, seem to extend far outwards, their ends dragged by the solar wind to the Earth's orbit and far beyond it. Since plasma motion tends to be guided by magnetic field lines, such lines provide an easy exit to solar wind plasma. Above the poles of the Sun, as already noted, field lines stick nearly straight out, creating two large, permanent "coronal holes. " As expected, the space probe Ulysses found those regions filled with fast-moving solar wind, similar to the kind observed in solar wind streams.

Coronal Mass Ejections

In the future NASA plans to conduct such observations from twin sun-observing spacecraft of the "Solar Stereo" mission. Located on the Earth's orbit but 60° ahead and behind it (at the L4 and L5 Lagrangian points of the Earth-Sun system), the cameras on these spacecraft will be able to observe CMEs heading for Earth and even (because of their different vantage points) obtain some information about their 3-dimensional structure.

More recently the Sun has been monitored in EUV and X-rays by Yohkoh, a highly successful Japanese satellite. Its images give a clear and detailed view of coronal holes, coronal bright spots and CMEs.

Another notable observer of CMEs has been the LASCO instrument aboard the SOHO spacecraft, stationed at the Lagrangian L1 point, sunward of Earth. For some SOHO images, see here and here. Sophisticated image-enhancement procedures on LASCO images made it possible for SOHO investigators to see CMEs even when headed towards Earth; an example can be seen here.

With all these modes of observation, a great deal was learned about CME since 1973, and it is now believed that much of the magnetic storm activity at Earth formerly credited to flares is actually associated with CMEs. Their energy apparently comes from the coronal magnetic field, and their material from prominences which are blown away by the process. They need not arise in sunspot regions.

Exploring Further: Click here to reach a web site which covers CMEs in greater details than is done here.

High-Energy Particles

Physicists on Earth use electromagnetic devices--high-energy accelerators--to accelerate electrons, protons and other electrically charged particles to great velocities, in order to study their collisions with matter and so learn about their make-up and about the forces that bind them. Some pretty sophisticated accelerator tools is needed for this, but it appears that Nature, too, can do so. This is shown when flare events, once or twice per year during active parts of the solar cycle, emit streams of high-energy ions and electrons, which can flood interplanetary space for some hours, even at the Earth's orbit and beyond it. CME shocks may also do so, and the relative roles of CME and flares is still being debated. For more about such events, see here.

NASA is rightly worried about such particles. They do not threaten life on Earth, which is shielded by a thick atmosphere. Even astronauts in space stations near the Earth's equator are shielded, by the Earth's magnetic field. However, any humans beyond the Earth's inner magnetosphere--for instance, in transit between Earth and Mars--would need to be sheltered in some way.

The way those accelerations occur is still unclear, but it is widely held that they are associated with small regions in space in which magnetic fields from adjoining sources (e.g. sunspot groups) cancel each other, creating "neutral points" of zero field intensity. Such points--unfortunately for experimenters--are usually well above the photosphere, in region whose magnetic fields are difficult to measure. Acceleration may also occur at shocks associated with CMEs.

Some information about accelerated particles may however be deduced from the radiation they emit: fast electrons, in particular, excel in producing x-rays. Medical x-rays are produced when beams of fast electrons, created inside a tube from which all air has been evacuated, come to a sudden stop against a metal target; on the Sun, when fast electrons are stopped by the surrounding gas, a similar process takes place. Such x-rays can rise much faster than other flare emissions--a minute in some cases, but only a second or two in others.

In one such event (picture on right), Yohkoh actually observed the position of an x-ray burst, localized at the top of a magnetic arch, well above the limb (edge) of the visible Sun. Note that in the picture, two places are particularly bright--the top of the arch, where the acceleration (presumably) took place, and one "foot" at the bottom, where such electrons enter the denser layers of the Sun's atmosphere.

Radio and Microwaves

For instance, waves emitted by electron and ion beams traveling outwards from the Sun are regularly tracked. Also, rising microwave radiation from above sunspot groups is often a good warning that "something is brewing. "

Electromagnetic Waves from the Universe

He then showed that a large fraction of the discoveries in astronomy were associated with some sort of improved coverage of the electromagnetic spectrum: new wavelength ranges (e.g. radio, x-rays) or better resolution (e.g. larger, better telescopes). He therefore recommended to NASA to concentrate its space astronomy efforts on extending that coverage, and NASA has largely followed his lead. Each of NASA's "great observatories"--e.g. Compton for gamma rays, Hubble for the visible and near-visible spectrum, Chandra for x-rays--has targeted a certain spectral region and tried to extend its coverage. The results have been very rewarding, but they are beyond the scope of this presentation, which is focused on the Sun.

The Sun is the source of most of the energy on Earth--the power source for plants, the cause of flows of atmosphere and of water, the source of the warmth which makes life possible. None would exist without it. At the Earth's orbit, neglecting absorption by the atmosphere, each square meter of area facing the Sun receives about 1380 joules per second (nearly 2 horsepower). That quantity is known as the solar constant and sensors aboard NASA's satellites over the 1979-99 interval suggest it varied by only about 0.2% (see graphs in Nature, vol 401, p. 841, 28 October 1999) .

But what powers the Sun itself? How much longer will it shine, before its fuel runs out? For how long has it given out its energy?

The first to consider these questions seriously was the great German physicist Hermann von Helmholtz, who noted in 1854 that the Sun's own gravity could supply an appreciable amount of energy. If the Sun were gradually shrinking--if all its matter was gradually falling towards its center--enough energy could be released to keep it radiating for a fairly long time. He calculated that this source could provide the Sun's energy for times of the order of up to 20 million years.

Then radioactivity was discovered, the decay of heavy elements into lighter ones through the emission of fast particles, containing a great deal of energy. As it turned out, it was this energy, from radioactive elements in rocks, that provided the internal heat of the Earth. Radioactivity also allowed new estimates of the age of the Earth, since the amount of accumulated decay products in ores indicated how long the process had been going on. This suggested the Earth was much older than Helmholtz'es estimate, perhaps billions of years old. Could perhaps the new source of internal energy also supply the Sun's needs for such a long time?

Nuclear Physics

[Sorry for just reciting the dry facts--to explain how they were established would be asking too much from such a brief review. A few of the benchmarks in the discovery of atoms and nuclei are listed here.]

Electrons and nuclei were kept together by electric attraction (negative attracts positive). Furthermore, electrons were sometimes shared by neighboring atoms or transferred to them (by processes of quantum physics), and this link between atoms gave our world its many chemical compounds.

But something else was needed to hold nuclei together, since all protons carried positive charges and repelled each other. Electric forces are definitely not the glue that holds nuclei together, they act in the wrong direction! Besides, binding neutrons to nuclei clearly requires a non-electrical attraction.

All that suggested a different kind of force, a nuclear force, was holding nuclei together. That force had to be stronger than the electric repulsion at short distances, but weaker far away, or else different nuclei might have tended to clump together, too. In other words, it had to be a short-range force, like the force between two small magnets--very hard to separate when stuck together, but once pulled a short distance apart, the force between them drops almost to zero (please do not take this analogy too literally!).

Actually two kinds of force are active in the nucleus, known simply as the "strong force" and the "weak force," or more commonly the "strong interaction" and the "weak interaction" (because their main effect is in converting and creating particles). The weak interaction also affects electrons and other particles, but in the nucleus its main role is to maintain a balance between protons and neutrons, which except for their electric charge are very similar particles (diferent kinds of "nucleons"). Nuclear structure (in light nuclei, at least) favors nuclei containing equal numbers of protons and neutrons, and although moderate inequalities can also exist (in "isotopes"), when they get too big, the weak interaction can convert nucleons of one kind to the other, emitting an electron (or a positron, its positive counterpart) in the process. That is known as beta radioactivity and will not be discussed any further.

The strong nuclear force (the only nuclear force considered from here on) can bind protons and neutrons into bigger nuclei. Being positively charged, all these nuclei repel each other, and therefore, except in the presence of extreme temperatures and pressures--such as exist in the core of the Sun--two different nuclei are not likely to combine into one. Their electric repulsion does not allow them to get close enough for the nuclear force to take over.

The Binding Energy of Nuclei

As we go on to elements heavier than oxygen, the energy which can be gained by assembling them from lighter elements decreases, up to iron. For nuclei heavier than iron, one actually gains energy by breaking them up into 2 fragments. That, of course, is how energy is extracted by breaking up uranium nuclei in nuclear power reactors.

The reason the trend reverses after iron is the growing positive charge of the nuclei. The electric force may be weaker than the nuclear force, but its range is greater: in an iron nucleus, each proton repels 25 other nuclei, while (one may argue) the nuclear force only binds close neighbors.

As nuclei grow bigger still, this disruptive effect becomes steadily more significant. By the time uranium is reached (92 protons), nuclei can no longer accomodate their large positive charge, but emit their excess protons in the process of alpha radioactivity--the emission of helium nuclei, each containing two protons and two neutrons. (Helium nuclei are an especially stable combination, as evidenced by the peak they form on the curve of binding energy above!) Still heavier nuclei are not found naturally on Earth.

The Sun's Energy Source

Heat is the motion of atoms and molecules: the higher the temperature, the greater is their velocity and the more violent are their collisions. When the temperature at the center of the newly-formed Sun became great enough for collisions between nuclei to overcome their electric repulsion, nuclei began to stick together and protons were combined into helium, with some protons changing in the process to neutrons (plus positrons, positive electrons, which combine with electrons and are destroyed). This released nuclear energy and kept up the high temperature of the Sun's core, and the heat also kept the gas pressure high, keeping the Sun puffed up and stopping gravity from pulling it together any more.

That, in greatly simplified terms, is the "nuclear fusion" process which still takes place inside the Sun. Different nuclear reactions may predominate at different stages of the Sun's existence, including the proton-proton reaction and the carbon-nitrogen cycle which involves heavier nuclei, but whose final product is still the combination of protons to form helium. A more detailed qualitative account, by astrophysicists of the University of California at Berkeley, can be reached here.

A branch of physics, the study of "controlled nuclear fusion," has tried since the 1950s to derive useful power from "nuclear fusion" reactions which combine small nuclei into bigger ones--power to heat boilers, whose steam could turn turbines and produce electricity. Unfortunately, no earthly laboratory can match one feature of the solar powerhouse--the great mass of the Sun, whose weight keeps the hot plasma compressed and confines the "nuclear furnace" to the Sun's core. Instead, physicists use strong magnetic fields to confine the plasma, and for fuel they use heavy forms of hydrogen, which "burn" more easily. Still, magnetic traps can be rather unstable, and any plasma hot enough and dense enough to undergo nuclear fusion tends to slip out of them after a short time. Even with ingenious tricks, the confinement in most cases lasts only a small fraction of a second.

The Sun today still consists mostly of hydrogen. The fuel supply which has seen it through its first 5 billion years should be good for about as long in the future.

The Evolution of Stars

The Earth keeps its size because its gravity is not strong enough to crush the minerals of which it consists. Not so with a star massive enough to sustain nuclear burning. A small star may crush all its atoms together, creating a "white dwarf"--e.g. of half the mass of the Sun, but only as big as the Earth. Some energy release continues (hence "white") but ultimately, the star probably becomes a dark cinder.

This may be the fate of our Sun, too. In the final transition strange changes occur--the star becomes a "red giant," diffuse and enormously large, and later much of the material is blown to space where it forms a "planetary" nebula, but there is no explosion. See "The Complexity of Stellar Death" by Yervant Terzian, "Science" vol. 256 p. 425-6, 15 October 1999.

Supernovas

That catastrophic event is known as a supernova explosion (technically, a "type 2 supernova"). Tycho Brahe was fortunate to have seen one that occured in our galaxy, outshining Venus and visible even in the daytime. The Chinese observed one in the year 1054, in the Crab constellation of the zodiac, and still another occured in Kepler's lifetime. Since then, however, none seemed to have occured close to Earth. The most notable event of this type was observed (quite extensively) in 1987 in the Large Magellanic Cloud, a small galaxy neighboring ours (see image above; the inner cloud is the one produced in the explosion, the rings seem older). For more about supernovas, see here

The material blown off by a supernova explosion ultimately scatters throughout space, and some of it is incorporated in clouds of dust and gas which later form new suns and planets. All elements on Earth heavier than helium (except, possibly, a small amount of lithium) must have arrived that way: products of nuclear burning in some pre-solar star, released or created in the explosion accompanying its final collapse. Our bodies are made of star stuff--carbon, oxygen, nitrogen and the rest have all been produced by nuclear fusion.

As for the "supernova remnant" left over from the collapse, its fate depends on its mass. If the star was not too massive, the remnant (as explained) is a neutron star. It that star originally rotated around its axis, that rotation is enormously speeded up; the remnant of the supernova of the year 1054 (its ejected cloud, the "Crab Nebula," is shown on the left) is spinning at about 30 revolutions per second! Any magnetic field of the original star is also enormously amplified, and associated phenomena can make it beam radio waves. Pulsars, pulsed radio sources with remarkably stable pulsation periods, are produced that way.

Added 20 October 1999: The new Chandra orbiting X-ray telescope has taken a high-resolution picture in X-rays of the central region of the Crab nebula. Before this, astrophysicists guessed the remnant star might be surrounded by orbiting debris, with high-energy particles shooting out along its magnetic axis, the one direction in which magnetic field lines do not confine them. The picture on the right suggests something like that might indeed be happening. For more about this image, see here.

Theory suggests that a star much more massive than the Sun will collapse even further and become a black hole. What happens then can only be guessed and calculated, not observed, for the star's gravity in the collapsed state is so strong that no light and no information can return from it to the outside world. One therefore expects such objects to be completely black; they are called "black holes" because the general theory of relativity suggests that the matter in such a star keeps falling indefinitely, as the star contracts to a point. Thus in theory such stars are like the proverbial bottomless pit, although no observation could ever confirm it.

Although astronomers cannot see such objects, they have considerable evidence that they exist, at least in a number of locations. A very massive black hole may exist at the center of our galaxy, and if so, probably also at the centers of other galaxies, helping hold them together.

This concludes our discussion of the Sun. "From Stargazers to Starships" continues with sections dealing with spaceflight and spacecraft, starting with The Principle of the Rocket

However... you may want to extend what you have learned
about the atomic nucleus to find out how nuclear power is
comercially obtained. If so, go next to (S-8) Nuclear Energy.

    Also, a brief historical review of the evolution of our knowledge about atoms and nuclei is provided in (LS-7A)   The Discovery of Atoms and Nuclei

What follows is a brief list of benchmarks and discoveries that shaped our ideas on atoms and nuclei. It supplements lesson plan Lsun7erg, attached to section S-7 "The Energy of the Sun""From Stargazers to Starships."

• The notion of atoms started from a fundamental problem in chemistry (#1): why did (say) one gram of hydrogen always combine with 8 grams of oxygen, never more, never less? Because each molecule of the resulting compound--water--always contained a fixed number of atoms of each kind. By comparing different reactions, Dalton concluded that (for instance) 2 atoms of hydrogen combined with one of oxygen, to create H₂O.

• Avogadro (#2, #5), in Italy, meanwhile noted a simple relation between the volume and weight of gases. Quantities which from chemistry seemed to combine naturally--1 gr. of hydrogen, 16 gr. of oxygen, 35.5 gr. of chlorine etc.--had the same volume, and Avogadro proposed that they contained the same number of molecules or atoms.

  (He also proposed that atoms in gases often combined in pairs, to create molecules such as H₂, O₂ and Cl₂, explaining some strange factors of two. We thus should talk about the number of molecules in 2 gr. of hydrogen, 32 gram of oxygen, 71 gram of chlorine, etc. That number is now known as Avogadro's number, and is truly enormous.)

  Avogadro's work was neglected for many years, while chemists struggled to understand the way atoms combined. At a conference of chemists in 1860. Cannizzaro (#5) again drew attention to Avogadro's results, and after that progress was rapid.

• Meanwhile, clues accumulated suggesting that atoms carried electric charges. Humphrey Davy (#3) used an electric current to separate new elements out of molten salts (a process called electrolysis). He obtained sodium and then potassium, soft metals which burned violently.

• Faraday, who started as Davy's assistant, derived in 1833 the laws of electrolysis (#4), which suggested that in a water solution (and also a molten salt), each atom or molecular fragment carried a fixed electric charge.

• Other researchers studied the flow of electricity in rarefied gases, under the influence of high voltage (fluorescent lamps are one outgrowth of such experiments). It became evident that such currents were carried by positive and negative particles in the gas. Joseph ("J.J.") Thompson isolated one type, a very light negative particle, measured its properties and named it the electron. (#7).

• The conducting gases also contained positive "ions" (wanderers) which J.J. Thompson studied as well. Such ions ("alpha particles," actually nuclei of helium) were also emitted by heavy radioactive elements, discovered in 1895 (#6).

• Ernest Rutherford, born on New Zealand, showed in 1911 that alpha particles were sometimes very strongly scattered by the positive charges of the atom, in a way that could only be explained if such charges were concentrated in a very small volume, practically a point in space. He therefore suggested that every atom had a compact nucleus, with negative electrons floating around it (#8).

  Rutherford's picture suggested that the nucleus was like a miniature Sun, with electrons orbiting it like planets. If Newton's laws were valid on the atomic scale, that might indeed be so, but as later research showed, on the atomic distance scale Newton's laws change into other forms. By these new laws of "quantum mechanics," electrons do not move in precisely defined orbits, but are distributed in space in a way that only allows the probability of finding them anywhere to be calculated. Similarly, energized atoms are only allowed to exist in one of a number of energy levels.

• There remained a problem: nuclei were too heavy. Helium nuclei had twice the charge of the proton but 4 times the mass. For a while scientists wondered whether helium nuclei contained 4 protons and 2 electrons. Then in 1932 Chadwick discovered the neutron and it was realized that helium nuclei contained two protons, two neutrons and no electrons.

• In 1938 Hans Bethe proposed that the Sun obtained its energy by fusing hydrogen nuclei to form helium. He also showed how fusion could proceed by means of a cyclical process, involving nuclei of carbon and nitrogen; the carbon and nitrogen nuclei are recovered and the only change is that 4 hydrogen atoms combine into helium. Today Bethe's cycle is believed to operate mainly in stars somewhat hotter than the Sun.

Note: This is a side-excursion into the basics of nuclear energy, beyond the main scope of "from Stargazers to Starships." It is included because nuclear energy is important to modern society, and because section S-7 "The Energy of the Sun" has already provided many of the basic ideas. Bear in mind that even without math, this can be a fairly difficult subject and that the discussion is rather lengthy.

The ideas from section S-7 are reviewed in what follows next. The rest of the section is a qualitative discussion of all key processes involved in the practical use nuclear energy.

A Review of Nuclear Structure

The way the Sun generates its energy helps understand the way a nuclear power station does so. The two processes are however quite different.

Here some facts about the way protons and neutrons combine to form nuclei, as covered in section S-7 about the Sun:

1.   Apart from their electrical charge, protons and neutrons ("nucleons") are quite similar. They can attract other nucleons and combine with them to form heavier nuclei, a process which releases energy. For instance, on the Sun pairs of protons combine with pairs of neutrons to form helium nuclei. In the process atomic particles gain great speed, and that is how the Sun's heat is generated.

2.   Unlike gravity or electrical forces, the nuclear force is effective only at very short distances. At greater distances, the protons repel each other because they are positively charged, and charges of the same kind repel.

     For that reason, the protons forming the nuclei of ordinary hydrogen--for instance, in a balloon filled with hydrogen--do not combine to form helium (a process which also would require some to combine with electrons and become neutrons). They cannot get close enough for the nuclear force, which attracts them to each other, to become important! Only in the core of the Sun, under extreme pressure and temperature, can such a process take place.

3.   Other small nuclei can similarly combine into bigger ones and release energy, but in combining such nuclei, the amount of energy released is much smaller. The reason is that while the process gains energy from letting the nuclear attraction do its work, it has to invest energy to force together positively charged protons, which also repel each other with their electric charge.

4.   Once iron is reached--a nucleus with 26 protons--this process no longer gains energy. In even heavier nuclei, we find energy is lost, not gained by adding protons. Overcoming the electric repulsion (which affects all protons in the nucleus) requires more energy than what is released by the nuclear attraction (effective mainly between close neighbors). Energy could actually by gained, however, by breaking apart nuclei heavier than iron.

5.   In the biggest nuclei (elements heavier than lead), the electric repulsion is so strong that some of them spontaneously eject positive fragments--usually nuclei of helium, which form very stable combinations ("alpha particles"). This spontaneous break-up is one of the forms of radioactivity found in nuclei.

6.   Nuclei heavier than uranium break up too quickly to be found in nature, although they can be produced artificially. The heavier they are, the faster is their spontaneous decay.

In summary, then: iron nuclei are the most stable ones, and the best sources of energy are therefore nuclei as far removed from iron as possible. One can combine the lightest ones--nuclei of hydrogen (protons)--to form nuclei of helium, and that is how the Sun gets its energy. Or else one can break up the heaviest ones--nuclei of uranium--into smaller fragments, and that is what nuclear power companies do.

How many Protons, how many Neutrons?

The nuclear forces apparently prefer equal numbers of each kind, and light nuclei--helium, carbon, nitrogen, oxygen--usually maintain a 50:50 ratio, although nuclear variants ("isotopes") with small deviations from equality may exist and may be stable.

In heavier nuclei, because of the electric repulsion between protons, this equality no longer holds. Imagine a nucleus with 56 nucleons, and suppose we could choose how many of this total would be neutrons and how many protons. What would be the most stable combination?

Choosing 28 of each kind might provide the most stable nuclear binding, but that is offset by the energy required to hold in close quarters 28 positive protons. So nature compromises: the preferred combination--the nucleus of the most common form of iron--has 30 neutrons but only 26 protons.

As nuclei get heavier, the fraction of protons drops still further--about 45% in mid-range nuclei, and less than 40% in the heaviest ones, those of uranium. Ordinary uranium ("U-238") has 92 protons but 146 neutrons, for a total of 238 nucleons. As will be seen, this gradual change in the proton/neutron ratio is essential to the nuclear chain reaction.

Nuclear Fission

Uranium nuclei in nature are unstable. Each of their 92 protons repels the rest, and sooner or later (half of them within 4.5 billion years) they eject a positive fragment, an "alpha particle" which is another term for a rapidly moving nucleus of helium. Almost all the helium atoms we extract from natural gas and from rocks--for filling balloons and for other uses--were originally created as alpha particles.

    But there exists a way of speeding up this break-up, by exposing the material to free neutrons.

Free neutrons are not found in nature (they would decay into protons and electrons), but they can be released from atoms of beryllium by bombarding them with alpha particles from radioactive materials, or by other methods. Since they are only attracted by nuclei (while protons are repelled before they get close enough for the nuclear attraction to do its job), they can easily enter a nucleus and stick to it--a bit like small magnets clicking onto a chunk of steel.

Such an attraction releases energy. If the nucleus is heavy and unstable, like that of uranium, adding energy destabilizes it even more, to where it may break up immediately. The interesting thing about such a break-up is the spectacular way in which it sometimes happens, as was discovered by Hahn and Meitner in 1939. Instead of breaking off a little chip, a helium nucleus containing less than 2% of the mass, the entire nucleus splits into 2 comparable fragments, typically containing 1/3 and 2/3 of the mass.

This process is called nuclear fission and besides the speed with which it can take place, it has at least two other remarkable features:

1.   It releases much more energy then the chipping-off of a helium nucleus ("alpha radioactivity").
2.   The fragments themselves are unstable. If you break up a nucleus which has (say) 40 protons for every 60 neutrons, into fragments whose optimal ratio is only (say) 45 to 55, there is bound to be some adjustment.

Ordinarily, such an adjustment calls for the conversion of some neutrons into protons (plus emitted electrons, "beta rays"), a process known as "beta radioactivity." Such an adjustment does in fact occur, making such fragments fiercely radioactive, and turning their disposal into a major issue for the nuclear power industry.

But at first, when the nucleus breaks up, the fragments are too unstable for such a gradual process. A quicker adjustment is needed, and the fragments achieve it by each emitting one or sometimes more free neutrons.

The Chain Reaction

On the average, about two neutrons are released this way per fission event. But it takes only one neutron to initiate another fission! Thus if fissionable nuclei are so densely packed that each neutron is bound to produce a new fission, the number of fission events quickly multiplies: 2, 4, 8, 16, 32, 64, 128... Since the energy release is proportional to the rate of fission, it also grows--very quickly!

This chain reaction is what makes a nuclear bomb (or "atomic bomb") function. The material with nuclei prone for fission--usually plutonium, an artificial heavy elements with 94 protons--must be compressed tightly and at the appropriate moment, exposed to a blast of neutrons. A variety of tricks, all of them top secret, is used to make sure that at least an appreciable fraction of its atoms undergoes fission before the whole thing blows apart.

Commercial nuclear power is produced somewhat differently, in a more controlled fashion. The fuel is uranium 235 (U-235)--a variant ("isotope") with 92 protons but only 143 neutrons, not 146, an odd number which makes it less stable. Natural uranium consists mostly of U-238, and it can absorb a neutron without undergoing fission (it ultimately turns into plutonium). U-238 therefore will not support a chain reaction. However, 0.7% of uranium is U-235, which can fission as soon as it absorbs a neutron.

By using a clever design, one can actually build a reactor fueled by natural uranium. The trick is to form the fuel into rods, and put between them some material ("moderator") which slows down neutrons but does not absorb them, e.g. pure carbon or "heavy water" containing the heavy isotope of hydrogen. Neutrons produced in a rod generally escape into the moderator, and by the time they hit another rod, they are moving very slowly: such slow neutrons are gobbled up much more avidly by U-235 than by U-238, so that even in a rod containing only 0.7% U-235, the U-235 atoms make most of the "catches. "

The Critical Mass

It should be added that many neutrons are also lost--escaping from the edges of the reactor into the surrounding material, or being absorbed inside it by the " wrong " nuclei, the ones which do not undergo fission. In fact, a reactor needs to be carefully designed to sustain a chain reaction in the first place: but it can be done.

From the beginning, complex and very expensive methods were devised to separate U-235 or to enrich its percentage past 0.7%. Today all commercial power reactors use enriched fuel, which makes the design of reactors easier and more controllable. With enriched fuel, ordinary water can serve as moderator, and it is even feasible to combine moderator and fuel, dissolving some uranium compound in water which acts both as moderator and as a distributor of heat.

Such a reactor--or a chunk of plutonium--will not support a chain reaction if it is too small. If the amount of fissionable material is less than a critical mass, the average fission occurs too close to the surface. Even though (say) 2 neutrons are produced in each fission, on the average 1.2 of them escape to the outside before hitting another nucleus, leaving only 0.8 neutrons to continue the process, whereas one or more are needed.

When processing nuclear fuel, or reprocessing fuel rods, it is therefore essential to work only with small quantities to prevent any accidental chain reaction. On 30 September 1999, at the nuclear processing plant in Tokaimura, Japan, workers thought they would save time by combining several batches of a solution of uranium. With a flash of blue light, a chain reaction began, giving three workers very bad doses of radiation and lasting 18 hours. After 3 months one worker died (in spite of extreme measures), one was discharged from the hospital and one is still (as of 12/99) in intensive care.

A detailed report on the accident ("What Happened in Tokaimura?") appeared in "Physics Today", December 1999, p. 52-4. A similar accident occured in the US in the 1950s, when a worker extracting plutonium from a solution in one liquid into another took a shortcut and combined several batches. He died of radiation exposure within two days.

[A note about history: The first nuclear reactor was designed by Enrico Fermi and was built under the stands of an old stadium at the University of Chicago. Rather than rods, it used cylindrical pellets of uranium, and these were embedded in a big "pile" of bricks of pure carbon, the moderator. It achieved a self-sustaining chain reaction on 2 December, 1942, and the name "atomic pile" for a nuclear reactor remained in use for about a decade afterwards. ]

The Controlled Nuclear Reactor

Because a nuclear reactor requires neutrons that have slowed down, it has a built-in delay and cannot explode like a nuclear bomb (even if scary films claim otherwise). Still, the chain reaction can grow very rapidly, and unless it is controlled, the reactor could in principle heat up to where it melts down. The usual method of control is to insert among the fuel "control rods" which strongly aborb neutrons--e.g. of the metal cadmium, also used in electroplating. By absorbing free neutrons, these rods slow down or stop the chain reaction.

Fortunately, nature has been helpful here. About 1% of the neutrons released in fission are not emitted promptly but are delayed, by a fraction of a second. Reactors are always operated to produce just barely enough neutrons to sustain a chain reaction. If for some reason the heat output starts to rise, the delayed neutrons slow down the rate of increase to where an automatic mechanism lowering or raising the control rods is fast enough to stop it.

Power reactors in the US use ordinary water as a moderator, inside a "pressure vessel" made of thick steel, with rod-like fuel elements and control rods fitted through openings in its lid. To start the chain reaction

• the control rods are withdrawn part of the way,
• the fast fission fragments heat the fuel elements,
• the fuel elements heat the water, steam is produced (usually, "clean" steam in pipes separated from the radioactive reactor water), the steam turns turbines,
      and
• generators attached to the turbines produce electricity.
That is the basic process --the details are many and much more complicated.

Is this the energy of the future? As of this writing (1999) France gets 75% of its energy from nuclear power, and many industrial countries, short on coal and oil, also obtain an appreciable fraction of their energy this way--e.g. 1/3 of the energy used in Japan and Spain. In the US, after an enthusiastic start, the use of nuclear power has leveled off to about 20% of the power generated, mainly due to public resistance to nuclear energy.

The US however is fortunate in having large reserves of coal: its growing energy consumption is largely met by these fuels. Environmentally, the choice is between two alternatives:

•   Burning coal and natural gas, which produces carbon dioxide (CO₂) and other pollutants, and which may therefore amplifies the "greenhouse effect" and accelerate global warming; or else
•   Using nuclear power stations, with the associated production of nuclear waste.

It is not easy to choose, and if we reject both options, we can expect much higher power costs and much less available power.

Nuclear Waste

The trouble with fission power is that the "fission fragments" from the break-up of uranium or plutonium are very "hot, " extremely radioactive. This creates two serious problems:

1.   The problem of waste storage, arising from the long "lifetime" of these substances, the time over which their activity persists.

   Nuclear waste contains a wide range of substances. Some have short lifetimes: their radioactivity is intense, but it "burns out" after hours, days, weeks or months (their hazard is different--see further below). However, some waste substances stay "hot" (radioactive) for decades and centuries, and their radioactivity is still so intense that they need to be kept away from human contact for a thousand years, maybe several thousand. At least initially, nuclear wastes also need to be cooled, because their radioactivity still generates heat.

   It has been proposed to cast nuclear waste into a glassy slag and isolate it in underground caverns, but fears remain--no human activity in past history has required such an unfailing long-term commitment (toxic chemicals also may do so, if they are buried instead of destroyed). Luckily, the amount of nuclear wastes is relatively small. As waste products of nuclear fuel, they have about the same weight as the fuel itself, of which a few tons can supply a city with electricity for years.
       Still, they must be handled by remote control, and must have no chance to contaminate ground water.

2.   The possibility of reactor meltdown. In the regular operation of a nuclear power station, fuel rods accumulate an appreciable amount of fission fragments. On a rotating schedule, each rod is replaced with a new one and its radioactive waste is removed and stored; but at any time, the rods contain enough of such waste to generate a lot of heat--enough, in fact, to melt the rod itself, if for some reason the flow of steam (or hot water) which removes its heat were to fail.

Nuclear Accidents

Suppose something went wrong with the cooling mechanism. Automatically, of course, control rods are lowered and any chain reaction stops immediately. But the radioactive waste in the reactor core continues to generate heat, so that cooling must still be provided for some hours, if not days. On 28 March 1979, at the nuclear power station at Three Mile Island, outside Harrisburg, Pennsylvania, a minor malfunction led to a series of errors, shutting down for a while both the main and emergency cooling systems.

The residual heat of the nuclear wastes melted part of the core and created (by chemical reaction) free hydrogen, which further complicated the situation. The billion-dollar reactor was a total loss, but the worst damage was probably to the public's trust in the safety of nuclear power. Still, the reactor vessel was not breached, and the second line of defense, the heavy concrete "containment building" inside which it was housed, also remained intact.

The power reactor at Chernobyl, near Kiev, capital of the Ukraine, had a different design--like Fermi's original reactor, it used a pile of carbon (graphite) to slow down neutrons, with pipes inside it carrying the fuel rods, control rods and cooling water. It was a big reactor, and was not enclosed in a containment building.

On 25 April 1986, an unwise engineering experiment at low power got out of control. The power level surged, the reactor vessel burst, and the hot steam and graphite (as well as combustible zirconium metal used in the fuel rods) reacted with hot steam and with oxygen of the atmosphere to produce an intense fire, whose plume rose to high altitudes and spread radioactive debris over a wide area. Of the fire-fighters called to extinguish the fire, many later died from radiation. Towns and villages near Chernobyl had to be evacuated (they remain empty as of 1999) and agricultural produce over much of Europe was contaminated. The remains of the reactor were later encased in a thick cover of concrete, entombing the radioactive waste left inside it.

Since these accidents, nuclear power generation in the US has proceeded without any major mishap. However, nuclear waste is still kept in temporary storage, as the national policy for its treatment and disposal continues to be debated. The other reactor at at Three Mile Island (and even the ones at Chernobyl) have been restarted to supply power again.

As the example of France suggests, nuclear power can be the main energy source of an industrial nation, though it calls for a high level of professional competence and carefully designed safety features. At the same time, the accidents at Three Mile Island and Chernobyl are a reminder that this power source carries unique risks of its own.

Further Exploring

"From Stargazers to Starships" continues with sections dealing with spaceflight and spacecraft, starting with The Principle of the Rocket