The Sun's Energy and Structure

• The sun shines not due to fire (as it would burn out in about 10,000 years) or contraction (which would last around 25 million years).
• Nuclear fusion is the actual energy source.
• The sun's luminosity could be sustained for 15 billion years through nuclear potential energy from its core.

Gravitational Equilibrium

• The energy from fusion balances gravity's inward crush.
• The inward gravitational force is balanced by the outward pressure.

Energy Balance

• The rate of energy radiated from the sun's surface equals the rate of energy released by fusion in the core.
• The energy created equals the energy released.

Gravitational Contraction

• This process heated the sun's core during its formation.
• Contraction ceased once fusion began.

The Sun's Structure

• The sun has various components.
• Solar wind is a flow of charged particles from the sun's surface.
• The corona is the outermost layer of the solar atmosphere, with a temperature of about 1 million Kelvin; it's visible during total solar eclipses.
• The chromosphere is the middle layer of the solar atmosphere, ranging from 104 to 105 Kelvin.
• The photosphere is the visible surface of the sun, around 6000 Kelvin.
• The convection zone transports energy upwards via rising hot gas.
• The radiation zone transports energy upwards by protons.
• The core is where energy is generated through nuclear fusion, reaching approximately 15 million Kelvin.

Nuclear Fission

• A large nucleus splits into smaller pieces.
• The mass of resulting pieces is slightly less than the original, with the difference converted to energy.

Nuclear Fusion

• Small nuclei combine to form a larger nucleus.
• The mass of the resulting nucleus is slightly less than the original nuclei, with the difference converted to energy.
• High core temperatures required due to increased density and velocity.
• Electromagnetic repulsion prevents nuclei from colliding at low speeds.
• Nuclei must come close enough for the strong force to bind them together at high speeds.
• The sun generates energy by fusing four hydrogen nuclei into one helium nucleus via the proton-proton chain.
• Two protons fuse into a deuterium nucleus (1 proton, 1 neutron).
• A deuterium nucleus and a proton fuse into helium-3.
• Two helium-3 nuclei fuse into helium-4, releasing two excess protons.
• Four protons yield a helium-4 nucleus, two gamma rays, two positrons, and two neutrinos, with a total mass decrease of 0.7%.

Solar Thermostat

• A drop in core temperature decreases the fusion rate, which causes the core to contract and heat up.
• An increase in core temperature elevates the fusion rate, causing the core to expand and cool down.

Energy Transport

• Energy gradually leaks out of the radiation zone through randomly bouncing photons.
• Convection carries energy to the surface.
• Hot gas reaching the surface is indicated by bright blobs on the photosphere.

Studying the Sun's Interior

• Mathematical models can be created.
• Observations of solar vibrations reveal details of the sun's interior; mathematical models are created and compared to vibrations.
• Solar neutrinos, created during fusion, pass directly through the sun; their observations provide insights into core processes.

The Sun-Earth Connection

• "Weather" on the sun is caused by its magnetic field.

Sunspots

• Sunspots are cooler areas on the sun's surface (about 4000 Kelvin).
• They are regions of strong magnetic fields.
• Loops of bright gas often connect sunspot pairs.

Solar Flares:

• Solar flares emit bursts of X-rays and charged particles into space.

Solar Prominences

• Solar prominences are eruptions high above the sun's surface.

Solar Activity Variation

• Sunspot activity rises and falls in an approximately 11-year cycle.
• The sunspot cycle is related to the winding and twisting of the sun's magnetic field.