Start with the Virtual Tour of the Sun. Then go to The Sun, by the University of Arizona. Then have a look at Views of the Sun, which has a spectacular gallery of images and movies of the Sun.

Distance from Sun to Earth: 1 AU = 1.5 x 10⁸ km = 8.3 light-minutes
(Denver ® NY = 0.01 light-seconds; Earth ® Moon = 1.3 light-sec)
Diameter: D_Sun = 1.4 x 10⁶ km = 5 lt-sec = 100 x Earth's Diameter
Mass: M_Sun = 2 x 10³⁰ kg = (1/3) x 10⁶ Mass of Earth
Density: Average = 1.4 g cm^-3 (slightly more than water); Central = 150 g cm^-3; Photosphere = 3.5 x 10^-7 g cm^-3 = 1/3000 x air.
Temperature: Central: 1.6 x 10⁷ K; Photosphere: 6000 K; Sunspots: 4800 K; Corona: 10⁶ - 10⁷ K.
Composition (by mass): 72% Hydrogen, 26% Helium, 2% all other elements. Gaseous
Rotation period: 25 days (equator), 28 days (higher latitudes).

We are burning fossil fuel fast, and the petroleum may run out in another century or so at this rate. As you will read below, the Sun's energy comes from nuclear fusion, by which four atoms of hydrogen are converted into one atom of helium. Mankind has found a way to produce energy from fusion -- the hydrogen bomb! (Actually, the reactions in the H-bomb do not convert ordinary hydrogen into helium; they are fusion reactions between an element called lithium and rare isotopes of hydrogen called deuterium and tritium.) H-bombs produce a lot of energy, but nobody knows how to use this energy except to do damage. A great dream of scientists is to develop a technology for controlled thermonuclear fusion -- some kind of reactor that could produce electrical energy from fusion reactions that take place in a non-explosive way. The most promising fuel for such reactions is deuterium - a rare isotope of hydrogen. In ordinary water, only one hydrogen atom in 6000 is deuterium. But even so, the energy that is released by the fusion of deuterium atoms into helium is so great that the water on Earth would provide an inexhaustible source of energy -- if we could figure out how to make such a reactor. For example, I estimate that fusion of the deuterium in one gallon of ordinary water would produce about as much energy as burning 1000 gallons of gasoline! But, despite substantial investment in controlled thermonuclear fusion research (by the US and several other nations) for more than 30 years, scientists have not found a way to build a reactor that will produce useful energy from fusion at a reasonable price.

The Sun's interior structure is determined by an exact balance between the inward force of the Sun's own gravity and the outward force of the pressure of the hot gases (mostly hydrogen and helium) in its interior. This balance is called hydrostatic equilibrium. If the Sun's interior were much colder than it is, it would collapse within an hour; if much hotter, it would expand within an hour.

Heat leaks out from the Sun's hot interior to its relatively cool photosphere by two mechanisms. Most of the interior of the Sun is stable, so the heat energy is carried out through the matter by photons (mainly X-rays). Moving at the speed of light, a photon could travel from the center of the Sun to the photosphere in only about 2 seconds if there was nothing to stop it. But actually, a photon can travel only a few centimeters through the Sun's interior before it will be deflected or absorbed by an electron or atom. Thus, instead of travelling straight out from the Sun's interior, a photon will rattle around for thousands of years before it eventually finds its way out. This process is called radiative diffusion. Thus, the Sun's envelope serves as a very effective insulating blanket that lets the intense heat of the Sun's core leak out only very slowly. That's a good thing, because without this insulation, the heat from the Sun's core would melt the Earth's surface in a very short time.

In the outer 20% of the Sun's radius, the envelope is literally boiling. Hot gases at the bottom become buoyant and rise to the top, causing an overturning motion called convection. The heat is carried from the bottom to the top of the convective layer by the motion of the rising hot gas. The top of the convective layer is the photosphere, where we can see the overturning motion of the convective cells. (Click here for a movie showing a simulation of solar convection on a supercomputer.)

As we described above, the Sun is constantly radiating energy into space. What is the ultimate source of this energy? The first theory, developed near the end of the 19th C by Lord Kelvin and a German physicist named Helmholtz, was that the Sun's energy must come from gravity. Their idea, called the Kelvin-Helmholtz contraction hypothesis, begins with the fact that the heat energy stored in the hot gases in the Sun's interior produces radiation, which slowly leaks out by radiative diffusion and convection. But that leakage of heat energy would cause the interior gas to lose pressure. Then, the outward force due to gas pressure would become less than the inward force of gravity, and so the Sun would begin to contract, compressing the interior gas. But compression causes gas to become hotter. This line of reasoning leads to the paradoxical but true result that when the Sun loses heat by radiation, it will actually get hotter.

They were wrong! But I want you to read the full story as it was originally told. So, be sure to read highlights from The Internal Constitution of the Stars, by Sir Arthur S. Eddington -- one of the greatest scientific papers ever written.

Eddington was right in every respect: the Sun's energy comes from fusion reactions that convert hydrogen into helium in the inner core of the Sun (the inner 25% of its radius). But, in 1920, when Eddington published his paper, nobody knew exactly how such reactions could work. It was another 30 years before Professor Hans Bethe of Cornell University finally explained the details of the nuclear reactions by which four hydrogen atoms are converted into helium in the Sun (for which he was awarded the Nobel Prize). The conversion actually takes place by a sequence of nuclear reactions, called the proton-proton chain, described on pp. 350 - 351 of your text (pay particular attention to Figures 16.27 & 16.28).

One interesting consequence of Bethe's theory is that about 5% of the energy produced by fusion of hydrogen into helium in the Sun's core escapes directly in the form of solar neutrinos. (The other 95% produces heat that ultimately escapes as radiation.) Neutrinos are remarkable subatomic particles that hardly interact at all with matter. As a result, they can pass right through the Sun, right through the Earth, and not be stopped or deflected at all. (In every second, approximately 10¹⁵ neutrinos pass through your body, without disturbing a single atom!) For the same reason, it is very difficult to detect neutrinos (they will pass right through an apparatus without leaving a trace). Despite these difficulties, scientists have built devices that can detect neutrinos from the Sun. But, surprisingly, when they began to take measurements, they found that the flux of neutrinos was somewhat less than 50% of the value predicted by the theory. This discrepancy between theory and experiment remains a mystery. See your text, Section 16.6. You can read more about the neutrino here, and details of the experimental results here.

You can find an excellent review of energy generation in the Sun here.

The discrepancy between theory and observation of solar neutrinos noted above makes us worry that we don't understand the Sun's interior structure as well as we think. That raises the stakes in finding a new way to measure the density and temperature inside the Sun. And, just in time, scientists have found a method to do that with unprecedented accuracy. This method is called helioseismology. It is the study of the natural vibrations of the Sun through observations of motions of the photosphere. (Remember, we measure motions through the Doppler shifts of spectral lines.)

In the early 1960s, scientists observed the Sun's surface carefully and noticed that the convective cells seemed to be bobbing up and down with a period of about 5 minutes. They measured the frequencies of these oscillations more and more accurately, and found that the Sun was virtually ringing like a bell, and that, like a musical instrument, it didn't oscillate at just one frequency, but at several precise frequencies, called resonances. The oscillations seen at the surface have their roots in the Sun's interior. The frequencies of the Sun's oscillations are sensitive to the sound speed inside the Sun, which depends on temperature and composition. The higher the temperature inside the Sun, the higher the frequencies of the oscillations; the higher the ratio of helium to hydrogen abundance, the lower the frequencies.

We have a problem counting pulses from the Sun's surface for a long time: the Sun sets every day! That means, with one telescope on the ground, we can't measure the Sun's oscillations for much longer than 12 hours without the interruption of night. There are two ways to get around this problem. One is to build a network of telescopes around the Earth all designed to observe the Sun's oscillations. (Midnight in Colorado is noon in India.) By gathering together the data from all the telescopes, we can count the oscillations without interruption for many days. A consortium of scientists, supported by several nations, has built just such a network, called the Global Oscillations Network Group, or GONG.

The other way is to watch the Sun's oscillations from a spacecraft, and an international consortium of scientists has built just such a spacecraft, called the Solar and Heliospheric Observatory, or SOHO. Besides for an instrument specifically designed to measure the Sun's oscillations, SOHO carries many other instruments designed to measure ultraviolet and optical radiation from the Sun's upper atmosphere and corona and atomic particles in the Sun's wind, as we describe below.

In addition to measuring the Sun's internal density and temperature, the GONG and SOHO observations enable us to measure the rotation speed of the Sun's interior. A few years ago, many scientists speculated that the interior of the Sun might be rotating faster than the outside. But the new observations from GONG and SOHO now rule out that possibility. But just a few weeks ago, scientists discovered "rivers" flowing beneath the surface of the Sun from SOHO data. You can find an excellent description of SOHO observations and discoveries in the Stanford Solar Center.

The gas and radiation beneath the Sun's surface are so hot that electrons are being knocked free from the hydrogen and helium atoms constantly, by atomic collisions and by photons. As a result, the gas is full of free electrons and positively charged ions (atoms lacking electrons), and so becomes a very good conductor of electricity. We call such an ionized gas a plasma. The physical behavior of plasmas is a very interesting and complicated subject. One very important property of plasmas is that they can cause magnetic fields to increase when they flow. That certainly happens inside the Sun. There are two obvious kinds of motions that can increase the Sun's magnetic field. The first is the differential rotation seen at the Sun's surface: the gas at the equator rotates faster than that the gas at higher latitudes (see text, Fig. 16.19).

When the magnetic fields within the Sun's interior become strong enough, they become buoyant and tend to rise toward the photosphere. They eventually break through the photosphere, forming loops of magnetism with relatively cool sunspots at their footprints. Some sunspots are larger than the Earth.

Galileo was the first to observe sunspots with a small telescope in 1610 (he must have put very dark glass in front of the telescope; otherwise he would have been blinded). Since then, people have been keeping records of the number of sunspots at any given time. The number varies with cyclically with time, reaching a solar maximum every 11 years and a solar minimum of almost no sunspots between maxima. Click here for historical records of this sunspot cycle, reaching back to Galileo's time. They show that there was a period, from about 1645 to 1715, called the Maunder minimum, when there were almost no sunspots. Click here for modern records of this sunspot cycle, which shows that the last solar maximum occurred in about 1991, and that the Sun is just now beginning to emerge from a solar minimum. It's easy to predict that the next solar maximum will occur around 2002. The average latitude on the Sun's surface where sunspots appear tends to move from about +/- 30^o at solar maximum toward the equator at solar minimum. We can see this behavior in a plot called a butterfly diagram (same as Fig. 16.21 of your text).

We do, however, know a great deal about what the magnetic field does after it emerges from the photosphere, because then we can observe it. Magnetic fields can make some spectral lines split into two, by a mechanism called the Zeeman effect. By measuring the amount of splitting, we can map the magnetic field strength. Click here for a solar magnetogram, which is a map of the magnetic field strength at the Sun's photosphere as measured by the Zeeman effect. Note that the regions of strongest magnetic field are associated with sunspots.

Violent activity occurs above the Sun's photosphere. It is caused both by solar convection and by instabilities in the solar magnetic field. The convection, which is a relatively steady rolling motion below the photosphere, turns into a violent splashing motion above the photosphere, just as relatively smooth waves in the open sea become violent breakers when they reach the shore. We can see the splashing motion above the Sun's photosphere as spicules (see text, Fig. 16.11). Moreover, the magnetic field often becomes unstable above the Sun's photosphere and erupts outward, causing solar flares, which can be seen with radio telescopes as well as with optical telescopes. Click here for a spectacular picture of such a flare. These violent motions heat the very tenuous gas above the photosphere to temperatures of millions of degrees, creating the corona, which extends far beyond the optical disk of the Sun. We can see the relatively faint optical radiation from the corona during solar eclipses, when the Moon blocks the much greater optical light from the Sun's photosphere.

Ultraviolet and X-ray images of the solar corona give us very detailed information about the distribution and of temperature and density in the solar corona. The best ultraviolet images of the corona come from the SOHO satellite, while the best X-ray images come from a Japanese satellite called Yohkoh. Click here for a gallery of such images and time-lapse movies. One of the best movies is one from Yohkoh showing the coronal X-ray emission throughout a 27 day rotation period of the Sun. (If you want to watch movies over the web, better use a computer on campus that has a high-speed link. They make take a half-hour or more to load over a modem.) Note that the X-ray emission is strongest above sunspots, where the magnetic fields help to retain higher densities of the X-ray emitting gas.

A few solar radii above the photosphere, the Sun's gravity is no longer strong enough to hold in the hot gas of the corona, and the corona turns into the solar wind, which flows outward through the solar system. Moving at velocities of 400 - 500 km/s, the solar wind takes about four days to travel from the Sun to the Earth.

Disturbances in the corona resulting from solar flares propagate out through the solar wind as coronal mass ejections, which may reach the Earth a couple of days after the flare. Click here for a spectacular movie of such an event, taken with the optical coronagraph on SOHO. The size of the Sun's photosphere is represented by the circle at the center of the disk, which blocks the light of the inner corona so that the instrument can see the outer corona. You can see many stars moving from left to right across the frame, including the center of the Milky Way toward the bottom of the movie. You can also see many rapidly moving streaks of light, which are caused by cosmic rays that strike the detector. Don't miss the little comet that enters from the lower left early in the movie. It obviously must have hit the Sun, because it doesn't come out again. Most important, notice the repeated coronal mass ejections from the two coronal streamers on the right side.

Because the disturbances in the solar wind can cause trouble with satellites and radio communications, especially military communications, the US Air Force and the National Oceanic and Atmospheric Administration (NOAA) constantly monitor the weather on the Sun and interplanetary space with instruments on the ground and in space. The NOAA Space Environment Center in Boulder puts out daily reports to inform those agencies whose facilities may be affected by these disturbances. Click here for today's Space Weather Report.

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