Summary of Key Ideas

The Sun’s Atmosphere

• The thin shell of the Sun’s gases we see are from its photosphere, the lowest level of its atmosphere. The gases in this layer shine nearly as a blackbody. The photosphere’s base is at the top of the convective zone.
• Convection of gas from below the photosphere produces features called granules.
• Above the photosphere is a layer of hotter, but less dense, gas called the chromosphere. Jets of gas, called spicules, rise up into the chromosphere along the boundaries of supergranules.
• The outermost layer of gases in the solar atmosphere, called the corona, extends outward to become the solar wind at great distances from the Sun. The gases of the corona are very hot, but they have extremely low densities.

The Active Sun

• Some surface features on the Sun vary periodically in an 11-year cycle. The magnetic fields that cause these changes actually vary over a 22-year cycle.
• Sunspots are relatively cool regions produced by local concentrations of the Sun’s magnetic field protruding through the photosphere. The average number of sunspots and their average latitude vary in an 11-year cycle.
• A prominence is gas lifted into the Sun’s corona by magnetic fields. A solar flare is a brief, but violent, eruption of hot, ionized gases from a sunspot group. Coronal mass ejections send out large quantities of gas from the Sun. Coronal mass ejections and flares that head in Earth’s direction affect satellites, communication, and electric power, and cause aurorae.
• The magnetic dynamo model suggests that many transient features of the solar cycle are caused by the effects of differential rotation and convection on the Sun’s magnetic field.

The Sun’s Interior

• The Sun’s energy is produced by the thermonuclear process called hydrogen fusion, in which four hydrogen nuclei release energy when they fuse together to produce a single helium nucleus.
• The energy released in a thermonuclear reaction comes from the conversion of matter into energy, according to Einstein’s equation E = mc².
• The solar model is a theoretical description of the Sun’s interior derived from calculations based on the laws of physics. The solar model reveals that hydrogen fusion occurs in a core that extends from the center to about a quarter of the Sun’s visible radius.
• Throughout most of the Sun’s interior, energy moves outward from the core by radiative diffusion. In the Sun’s outer layers, energy is transported to the Sun’s surface by convection.
• The amount of energy the Sun emits has increased by about 30% since it first formed.
• Neutrinos were originally believed to be massless. The electron neutrinos generated and emitted by the Sun were originally detected at a lower rate than is predicted by our model of thermonuclear fusion. The discrepancy occurred because electron neutrinos have mass, which causes many of them to change into other forms of neutrinos before they reach Earth. These alternative forms are now being detected.