Chapter+10+Lecture

Chapter 10: The Sun as a Star

The Nature of the Sun

• The Sun is a mass of incandescent gas, as popularized in the song by They Might Be Giants.

Basic Characteristics

• Radius: 6.9 × 10^8 m (109 times Earth’s radius)

• Mass: 2 × 10^30 kg (equivalent to about 300,000 Earths)

• The Sun accounts for approximately 99.86% of the Solar System’s total mass.

• Volume: Over 1.3 million Earths can fit inside the Sun.

• The Moon’s orbit around Earth (about 385,000 km) would fit well within the Sun.

Physical Properties

• Luminosity: 3.8 × 10^26 watts

• Rotation Period: 25 days at the equator (faster) and 36 days at the poles (slower) which contributes to its magnetic fields.

• Composition: 71.5% Hydrogen, 27.1% Helium, and 1.4% other elements.

Structure of the Sun

• Core:

  • Temperature ~ 15 million K

  • Energy generated by nuclear fusion.

  • All matter exists in a plasma state.

• Radiation Zone:

  • Energy is transported upward by photons.

  • Opaque to visible light.

• Convection Zone:

  • Energy moves upward through rising hot gas.

  • Opaque to visible light.

• Photosphere:

  • The visible surface of the Sun, with a temperature of ~6000 K.

• Chromosphere:

  • The middle layer of the solar atmosphere, with temperatures ranging from ~10^4 to 10^5 K.

  • Invisible to the human eye; only seen in UV wavelengths.

• Corona:

  • The outermost layer of the solar atmosphere, with temperatures reaching ~1 million K.

  • The solar wind is a flow of charged particles from the Sun’s surface, contributing to its radiation pressure.

Solar Activity

• Solar phenomena include:

  • Sunspots: Regions with strong magnetic fields, appearing cooler (~4000 K) than the surrounding areas (~5800 K).

  • Solar Flares: Magnetic activity that emits X-rays into space.

  • Solar Prominences: Loops of bright gas that connect sunspot pairs.

Nuclear Reactions

• Fission: Big nuclei split into smaller pieces (e.g. nuclear power plants).

• Fusion: Small nuclei combine to form larger ones (e.g. the sun).

Proton-Proton Chain Reaction

• Involves the transformation of four protons to form one helium nucleus plus energy and other particles.

• Total mass after the reaction is about 0.7% less than the starting mass, resulting in energy release due to mass-energy equivalence.

Solar Thermostat

• Maintains a consistent temperature:

  • A decline in core temperature causes the fusion rate to drop, the core contracts, heats, and vice versa.

Solar Cycle and Climate Impacts

• Sunspot activity peaks in cycles, impacting Earth's climate (heating during peaks and cooling during troughs).

• The last ice age correlated with low sunspot activity, suggesting a connection between solar activity and climate.

Magnetic Activity

• Solar flares and coronal mass ejections (CMEs) result from the twisting of magnetic field lines due to differential rotation.

• Coronal Mass Ejections (CMEs): High-energy particle bursts sent from the Sun, taking 1-3 days to reach Earth.

Interaction with Earth

• Charged particles from the solar wind can disrupt electrical power grids and disable communication satellites.