Universe Tenth Edition Chapter 16 Our Star, the Sun Roger Freedman Robert Geller William Kaufmann III

16-1 The source of the Sun ’ s heat and light 16-2 How scientists model the Sun ’ s internal structure 16-3 How the Sun ’ s vibrations reveal what lies beneath its glowing surface 16-4 How scientists are able to probe the Sun ’ s energy- generating core 16-5 Why the gaseous Sun appears to have a sharp outer edge By reading this chapter, you will learn

16-6 Why the upper regions of the solar atmosphere have an emission spectrum 16-7 The relationship between the Sun ’ s corona and the solar wind 16-8 The nature of sunspots 16-9 The connection between sunspots and the Sun ’ s magnetic field How magnetic reconnection can power immense solar eruptions By reading this chapter, you will learn

The Sun

The Proton- Proton Chain

16-1: The Sun’s energy is generated by thermonuclear reactions in its core

The Proton-Proton Chain

Hydrostatic Equilibrium 16-2: A theoretical model of the Sun shows how energy gets from its center to its surface

Hydrostatic Equilibrium

A Theoretical Model of the Sun’s Interior

The Sun’s Internal Structure 16-3: Astronomers probe the solar interior using the Sun’s own vibration

A Sound Wave Resonating in the Sun

A Solar Neutrino Experiment 16-4: Neutrinos reveal information about the Sun’s core – and have surprises of their own

The Photosphere 16-5: The photosphere is the lowest of three main layers in the Sun’s atmosphere

The Origin of Limb Darkening

Solar Granulation

Supergranules and Large Scale Convection

The Chromosphere 16-6: Spikes of rising gas extend through the Sun’s chromosphere

The Solar Atmosphere

Spicules from Above

The Solar Corona 16-7: The corona ejects mass into space to form the solar wind

Temperatures in the Sun’s Upper Atmosphere

The Ultraviolet Corona

Sunspots 16-8: Sunspots are low-temperature regions in the photosphere

Tracking the Sun’s Rotation with Sunspots

The Sunspot Cycle

Variations in the Average Latitude of Sunspots

Sunspots have Strong Magnetic Fields 16-9: Sunspots are produced by a 22- year cycle in the Sun’s magnetic field

Sunspots have Strong Magnetic Fields

Magnetic Fields Deflect Moving, Electrically Charged Objects

Mapping the Sun’s Magnetic Field

Babcock’s Magnetic Dynamo Model

Rotation of the Solar Interior

Magnetic Arches and the Magnetic Reconnection 16-10: The Sun’s magnetic field heats the corona, produces flares, and causes massive eruptions

Magnetic Arches and the Magnetic Reconnection

The Active Sun Seen through an H α Filter

The Sun in Ultraviolet Light

A Solar Prominence

Coronal Mass Ejection

The Sun-Earth Connection

The November 15, 1999 Transit of Mercury

Key Ideas Hydrogen Fusion in the Sun ’ s Core: The Sun ’ s energy is produced by hydrogen fusion, a sequence of thermonuclear reactions 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 according to Einstein ’ s equation E = mc 2. Thermonuclear fusion occurs only at very high temperatures; for example, hydrogen fusion occurs only at temperatures in excess of about 10 7 K. In the Sun, fusion occurs only in the dense, hot core.

Key Ideas Models of the Sun ’ s Interior: A theoretical description of a star ’ s interior can be calculated using the laws of physics. 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. 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.

Key Ideas Solar Neutrinos and Helioseismology: Conditions in the solar interior can be inferred from measurements of solar neutrinos and of solar vibrations. 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. These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun.

Key Ideas The Sun ’ s Atmosphere: The Sun ’ s atmosphere has three main layers: the photosphere, the chromosphere, and the corona. Everything below the solar atmosphere is called the solar interior. 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 5800 K. Convection in the photosphere produces granules.

Key Ideas Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere. Spicules extend upward from the photosphere into the corona. The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density. Activity in the corona includes coronal mass ejections and coronal holes. The solar corona blends into the solar wind at great distances from the Sun.

Key Ideas The Active Sun: The Sun ’ s surface features vary in an 11-year cycle. This is related to a 22-year cycle in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity. Sunspots are relatively cool regions produced by local concentrations of the Sun ’ s magnetic field. The average number of sunspots increases and decreases in a regular cycle of approximately 11 years, with reversed magnetic polarities from one 11-year cycle to the next. Two such cycles make up the 22- year solar cycle.

Key Ideas 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. 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.