The Sun: Structure, Sunspots, and Solar Activity

Core Idea

The Sun is a ball of hot plasma that makes energy by nuclear fusion in its core. That energy moves outward first by radiation and then by convection, and escapes into space from the visible surface, the photosphere. Above the photosphere lie the thin chromosphere and the very hot corona. The Sun's magnetic field, twisted by its uneven rotation, produces sunspots, flares, and coronal mass ejections, and this activity rises and falls in a cycle of about 11 years. Solar activity drives space weather that can trigger auroras and disrupt satellites and power grids on Earth.

How It's Best Learned

Draw the Sun as a set of nested shells, labeling the core, radiative zone, convection zone, photosphere, chromosphere, and corona, and write how energy moves through each. Then plot the yearly sunspot number for the past century to find the 11-year cycle. Use safe solar images, such as those from NASA's Solar Dynamics Observatory, to compare the photosphere in visible light with the corona in ultraviolet.

Common Misconceptions

• The Sun burns like a fire. (Combustion needs oxygen and could not last billions of years; the Sun's energy comes from nuclear fusion of hydrogen into helium.)
• Sunspots are holes or cold, black patches. (Sunspots are still several thousand kelvin; they look dark only because the surrounding photosphere is hotter and brighter.)
• The Sun is solid and rotates as one body. (The Sun is plasma, and its equator rotates faster than its poles, in about 25 days compared with about 35 days.)
• Temperature must fall steadily with distance from the Sun's core. (The corona is over a million kelvin, much hotter than the photosphere below it; how it is heated is still an active research question.)

Explainer

The Sun is a typical star, and it is the only one close enough to study in detail. It has shone for about 4.6 billion years by fusing hydrogen into helium in its core, where the temperature is about 15 million kelvin.

Energy leaves the core in stages. In the radiative zone, which extends to about 70 percent of the Sun's radius, the gas is so dense that light is absorbed and re-emitted over and over in random directions. The energy takes many thousands of years to work its way out. In the outer 30 percent, the convection zone, hot gas rises, cools, and sinks again, like boiling water. The tops of these convection cells show on the surface as granulation, a pattern of bright cells roughly 1,000 km across.

The photosphere is the layer we see. Its temperature is about 5,800 K, and it emits the sunlight that reaches Earth. Above it, the chromosphere glows red and is visible during total solar eclipses. Higher still is the corona, a thin halo of plasma at over a million kelvin. The corona keeps expanding outward as the solar wind, which flows past the planets at hundreds of kilometers per second.

Much of the Sun's changing behavior comes from its magnetic field. The Sun does not rotate like a solid ball. Its equator turns once in about 25 days and its poles in about 35 days. This uneven rotation, combined with convection, twists and concentrates the field. Where bundles of field break through the surface, they block convection and create cooler, darker sunspots. Sunspot numbers rise and fall in a cycle of about 11 years. Between about 1645 and 1715, very few sunspots were seen, a period called the Maunder Minimum.

Twisted magnetic fields can also release energy suddenly. A solar flare is a burst of radiation from a small region. A coronal mass ejection (CME) hurls a vast cloud of magnetized plasma into space. If a CME reaches Earth, it can disturb the magnetosphere and cause a geomagnetic storm. Auroras then appear farther from the poles than usual, satellites can be damaged, and long power lines can carry damaging currents. The 1859 Carrington Event and the 1989 Quebec blackout are well-known examples. Forecasting this "space weather" is now a practical job for agencies that watch the Sun.