Chapter 10: The Sun – Our Favorite (and Ordinary) Star

Key Questions to Consider

• How does the mass of the Sun compare to the rest of the Solar System?
• Are there stars nearer to Earth than the Sun?
• Does the Sun have a solid and liquid interior like Earth?
• What is the surface of the Sun like?
• Does the Sun rotate?
• What makes the Sun shine?

Structure of the Sun

Interior of the Sun

• Core: Site of hydrogen fusion.
• Radiative Zone: Energy is transported outward by radiation.
• Convective Zone: Energy is transported by convection.

Atmosphere of the Sun

• Photosphere: Emits visible light; visible surface.
• Chromosphere: Middle, transparent layer; characterized by spicules.
• Corona: Upper transparent layer; source of the solar wind.

Sun's Surface Features

• Limb Darkening: Edges appear darker than the center.
• Granulation: Marbled pattern caused by convection.
• Spicules: Jets of gas in the chromosphere.

Solar Phenomena

• Differential Rotation: Sun rotates faster at the equator than at the poles.
• Sunspots: Areas of intense magnetic fields with umbra and penumbra regions.
• Prominences and Flares: Magnetic loops and eruptions releasing energy.
• Coronal Mass Ejections (CMEs): Release plasma at high speeds.

Sun's Energy and Stability

• Powered by thermonuclear fusion: Hydrogen to helium in the core.
• Hydrostatic Equilibrium: Balance between radiation pressure and gravity.

Summary: Key Facts

• The Sun contains 99.85% of the Solar System's mass.
• The Sun is the closest star to Earth.
• Composed of hot gases, not solid or liquid.
• Surface is a layer of hot gases, rotating differentially.
• Shines due to thermonuclear fusion.

Chapter 11: Characterizing Stars

Apparent and Absolute Magnitude

• Apparent Magnitude: Brightness as seen from Earth.
• Absolute Magnitude: True brightness, regardless of distance.
• Inverse-Square Law: Brightness decreases with distance.

Star Color and Temperature

• Hottest stars are blue, medium are orange/yellow, coolest are red.
• Spectral classes ordered by color and temperature.

Hertzsprung-Russell (HR) Diagram

• Plots star brightness against spectral type.
• Main Sequence stars fuse hydrogen to helium.
• Giants, supergiants, and white dwarfs have distinct positions.

Star Size and Brightness

• L = R²T⁴: Brightness depends on radius and temperature.
• Brighter stars can be either larger or hotter.

Summary: Key Facts

• Closest star other than the Sun is Proxima Centauri.
• Sun's luminosity varies greatly compared to other stars.
• Stars come in a wide range of colors.
• Brighter does not always mean hotter.
• Most stars are not isolated like the Sun; many are in pairs or groups.

Chapters 12 and 13: The Lives and Deaths of Stars - Stellar Evolution

Star Formation

• Form in giant molecular clouds from gas and dust.
• Stars of different masses evolve differently.

Stellar Evolution Stages

• Main Sequence: Hydrogen fusion in the core.
• Red Giant/Supergiant: Hydrogen shell fusion.
• Planetary Nebulae and White Dwarf: Final stages for Sun-like stars.
• Supernovae: Explosive end for massive stars.

End Stages of Stars

• Neutron Stars: Result from supernova of big stars.
• Black Holes: Result from supernova of huge stars.

Key Evolutionary Pathways

• Stars like the Sun become planetary nebulae and white dwarfs.
• Big stars become neutron stars.
• Huge stars become black holes.

Material Recycling in the Universe

• Material from dead stars forms new stars.

Summary: Key Facts

• Stars like our Sun will end as white dwarfs and eventually black dwarfs.
• Neutron stars and black holes form from more massive stars.
• Heavy elements on Earth originated from stellar evolution, supernovae, and neutron star collisions.