Essay: Solar Anatomy

Like all stars, the Sun is an immense and glowing mass of neutrons, protons and other subatomic particles, all bound by gravity and a roiling magnetic field. As a cosmic neighbor, it is immensely influential: every second, the Sun produces an amount of energy equivalent to the detonation of about 10 trillion Hiroshima-scale atomic bombs. The light it generates burns with a brightness equivalent to 100 trillion trillion 100-watt light bulbs shining simultaneously.

As stars go, though, the Sun is merely typical: about 850,000 miles wide, or roughly 200 times the width of Earth; with a mass roughly 300,000 times that of Earth and a density equal to Jupiter's, or about one-fourth Earth's. That mediocrity serves science well. Through close study, solar researchers have learned a great deal not only about the Sun, but also about the workings of similar, more distant stars. "There is no other star we can observe in such detail," says Harrison Jones, a solar physicist with NASA's Goddard Space Flight Center.

Advances in technology have revealed that the Sun is comprised of several layers, rather like a giant flaming onion. Its power derives entirely from nuclear reactions at the core. There, under tremendous pressure, hydrogen atoms fuse to form helium atoms, releasing vast amounts of energy; the temperature at the core is about 15 million degrees Celsius. This energy travels outward through the various layers of the Sun and onward into space. It is a slow process: a photon of light takes more than 10,000 years to travel the quarter-million miles from the core to the Sun's surface.

In the core and the surrounding layer, called the radiative zone, the gas particles are so densely packed that the overall gas, or plasma, barely moves. In these innermost regions, solar energy travels by radiation: photons of light bounce from one gas particle to the next and slowly leak outward. In the convective zone, the outermost of the Sun's interior layer, the gas particles have a little more elbow room. They collide less frequently and less violently, so the temperature falls, and the plasma begins to move, boiling upward on vast thermal plumes, like soup in a kettle (onion soup, presumably). The plumes, or convection currents, carry energy to the Sun's surface and cool as they rise. The temperature at the surface, or photosphere, is "only" 6,000 degrees Celsius.

The photosphere is not a hard surface, but a thin, roiling layer of magnetically charged plasma. Only about 60 miles thick, the photosphere is the source of much of the visible radiation, or sunlight, that leaves the Sun. The Sun that the casual viewer sees--that white-hot sphere suspended in the sky--is really only the photosphere, the thinnest layer of the Sun's true self.

The photosphere is a frenzy of magnetic activity. Some regions are centers of positive polarity, others are regions of negative polarity, like positive and negative ends of bar magnets. These regions are linked by magnetic field lines that may loop high into the Sun's atmosphere. The regions of positive and negative polarity constantly shift, however, and as they do, the field lines break and reform. This process can produce violent storms in the Sun's outer regions.

The average temperature of the photosphere is 6,000 degrees Celsius. In regions where magnetic activity is particularly intense, temperatures are slightly cooler. Against the bright, hot photosphere, these regions look comparatively dark and are known as sunspots. Sunspots are a useful gauge of solar activity. During the quieter phase of a solar cycle, scientists may detect only a handful of sunspots at any given time. During solar maximum, as many as 250 might exist.

The next layer above the photosphere is the chromosphere; after that is the transition region. Magnetic activity near the surface generates violent storms in these outer regions: solar flares, coronal holes, coronal mass ejections and other phenomena that blast hot plasma outward into space. Due to all the activity, the temperature skyrockets from about 100,000 degrees Celsius at the inner edge of the transition layer to several million degrees at its outer perimeter.

Finally, the outermost region of the Sun, the corona, extends millions of miles into space. Temperatures in the corona constantly fluctuate, so the strength of the solar wind--a breeze of solar particles that blows outward in all directions--is always changing.

Space-weather scientists have benefited greatly from studies of the Sun's internal structure. Understanding its complexities helps forecasters better predict where and when solar storms are likely to develop.