Key Ideas and Terms

9-1 The Sun’s energy is generated by thermonuclear reactions in its core

• The Sun’s luminosity is the amount of energy emitted each second and is produced by the proton-proton chain in which four hydrogen nuclei combine to produce a single helium nucleus.
• The energy released in a nuclear reaction corresponds to a slight reduction of mass, as predicted by Einstein’s equation E = mc2.
• Thermonuclear fusion occurs only at very high temperatures; for example, hydrogen fusion occurs only at temperatures in excess of about 18,000,000°F (107 K). In the Sun, fusion occurs only in the dense, hot core.

9-2 Energy slowly moves outward from the solar interior through several processes

• A theoretical description of a star’s interior can be modeled using the laws of physics showing that it is in hydrostatic equilibrium where energy moving outward precisely balances its gravitational pull inward.
• The standard model of the Sun suggests that hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius and that our Sun is in thermal equilibrium.
• The core is surrounded by a radiative zone extending to about 0.71 solar radius. In this zone, energy travels outward through radiative diffusion.
• The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure. In this zone, energy travels outward primarily through convection.
• Neutrinos emitted in thermonuclear reactions in the Sun’s core have been detected, but in smaller numbers than expected. Recent neutrino experiments explain why this is so.
• Helioseismology is the study of how the Sun vibrates, which has been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun.

9-3 The Sun’s outer layers are the photosphere, chromosphere, and corona

• The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphere. Its spectrum is similar to that of a blackbody at a temperature of 10,000°F. Convection in the photosphere produces granules.
• Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere. Spicules extend upward from the photosphere into the chromosphere.
• The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density. A stream of particles making a solar wind emanates from thin regions called coronal holes.

9-4 Sunspots are low-temperature regions in the photosphere

• Sunspots are relatively cool regions produced by local concentrations of the Sun’s magnetic field.
• The average number of sunspots increases to a sunspot maximum and decreases to a sunspot minimum in a regular sunspot cycle of approximately 11 years, with reversed magnetic polarities from one 11-year cycle to the next. Two such cycles make up a 22-year solar cycle in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity.
• The magnetic polarity is measured by observing the Zeeman effect.
• The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field. These changes are caused by convection and the Sun’s differential rotation.

9-5 The Sun’s magnetic field also produces other forms of solar activity and causes aurorae on Earth

• Plasma on the Sun arranges itself into various observable features, one such being prominences.
• A solar flare is a brief eruption of hot, ionized gases from a sunspot group. A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona.
• When charged particles emitted by the Sun interact with Earth’s atmosphere, it causes an aurora where the upper atmosphere glows. When observed in the northern hemisphere it is called the northern lights or aurora borealis.