Our Star: Structure, Energy, Atmosphere, and Activity

Electromagnetic Spectrum and Telescopes

• The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet (UV) radiation, X-rays, and gamma rays.
• Different types of telescopes are used to observe different parts of the spectrum:
  • Infrared telescopes
  • Optical telescopes
  • Ultraviolet telescopes
  • X-ray telescopes
• The Sun emits radiation across the electromagnetic spectrum, including:
  • Infrared: $λ = 1,083$ nm
  • Visible light: $λ = 750-380$ nm
  • Ultraviolet: $λ = 13.1$ nm
  • X-rays: $λ = 6-0.6$ nm

Structure of the Sun

• The Sun's structure is governed by:
  • Hydrostatic equilibrium: balance between pressure and gravity.
  • Nuclear fusion: energy generation in the core.
  • Forces: Strong nuclear force, weak nuclear force.
  • Hydrogen burning: proton-proton chain.
  • Particles: Isotopes, positrons, neutrinos.

Hydrostatic Equilibrium

• Hydrostatic equilibrium is the balance between pressure and gravity within the Sun.
• Outward pressure precisely balances the inward pull of gravity.
• Pressure is greatest deep within the Sun due to the overlying weight.

Fundamental Forces

• Four fundamental forces in nature:
  • Strong force: binds the nucleus (strongest force).
  • Electromagnetic force: binds atoms (infinite range).
  • Weak force: involved in radioactive decay.
  • Gravitational force: binds the solar system (weakest force, infinite range).
• Strong and Weak force are short range acts only on subatomic particles.

Strong Nuclear Force

• The strong nuclear force binds protons and neutrons together in the nucleus.

Atoms and Isotopes

• Atom: The smallest unit of an element in the periodic table.
• Atoms consist of protons, neutrons, and electrons.
• Protons and neutrons are made up of quarks. Electrons are indivisible.
• Isotopes: different forms of the same element with varying numbers of neutrons.
  • Hydrogen-1: mass number 1
  • Hydrogen-2 (deuterium): mass number 2
  • Hydrogen-3 (tritium): mass number 3

Particles and Antiparticles

• Matter Particles: Quarks (u, d, c, s, t, b) and leptons electrons $e$, muon $\mu$, tau $\tau$
• Higgs Boson: The quantum excitation of the Higgs field, a necessary component of the Standard Model.
• Forces: V. ${\scriptscriptstyle e}$, V. ${\scriptscriptstyle \mu}$, V. ${\scriptscriptstyle \tau}$, W, Y, Z⁰, g
• Particles have corresponding antiparticles.
• Examples:
  • Electron - Positron (antielectron)
  • Proton - Antiproton
  • Neutrino - Antineutrino

Nuclear Fusion

• Nuclear fusion is the combining of two light nuclei to form a single heavier nucleus.
• Heat and light energy from the Sun are produced by nuclear fusion in the core.

Proton-Proton Chain

• Step 1: Two hydrogen nuclei (protons) collide.
  • A proton transforms into a neutron, producing a positron and an electron neutrino and deuterium.
• Step 2: A proton collides with deuterium, producing helium-3 and a gamma ray.
• Step 3: Two helium-3 nuclei collide, producing a helium-4 nucleus; two protons escape.

Observing the Heart of the Sun

• Neutrinos escape the Sun faster than photons because they interact less with other particles.

Energy Transport

• Energy in the Sun moves by radiation and convection through different zones:
  • Core
  • Radiative zone
  • Convective zone
  • Photosphere

Radiative and Convective Zones

• Radiative Zone: Inner part of the Sun where opacity is low; radiation transports energy from the core outward.
• Convective Zone: Outer layer of the interior; transports energy from the edge of the radiative zone to the surface through convection cells.

Helioseismology

• Helioseismology is the study of the Sun using waves from its interior.

Atmosphere of the Sun

• Photosphere: the apparent surface of the Sun (approximately 100 km thick, 4000 K to 6500 K).
• Limb Darkening: The Sun appears darker near the edge of the solar disk than near the center.
• The solar spectrum consists mostly of hydrogen and helium, with some heavier elements like iron, nickel, calcium, and sodium.
  • Heavier elements are remnants of previous stars.

The Sun’s Outer Atmosphere

• Chromosphere: The region above the photosphere (approximately 4000 K to 8000 K).
• Transition Region: A narrow layer between the chromosphere and the corona where the temperature rises abruptly.
• Corona: The outermost layer of the Sun (approximately 500,000 K up to a few million K).

Activity in the Sun’s Atmosphere

• Solar activity is related to magnetic fields.
• Features include: Coronal loops, coronal holes, sunspots, and prominences.

Solar Wind

• A continuous stream of electrically charged gas propelled away from the Sun in all directions.
• Average speed: 400 km/s
• Temperature: 1 million degrees Celsius
• Disrupts satellites, power grids, and communications on Earth.

Coronal Loops

• Plasma flows along curving lines produced by the magnetic field.
• Can last for days or weeks, but change quickly.
• More common around solar maximum.
• Project up into the corona.

Coronal Holes

• Large dark regions of the corona.
• Cool and lower in density than surroundings.

Sunspots

• Sunspots are dark blemishes in the solar photosphere.
• Consist of an umbra (darker region) and penumbra (lighter region).
• Sunspots appear darker because they are cooler regions of the photosphere.

Differential Rotation

• The Sun rotates at different rates at different latitudes.
• Example rotation rates: 25 days and 35 days

Sunspot Cycle

• The number of sunspots varies from year to year.
• The activity of the Sun follows an 11-year cycle, with a full magnetic cycle being 22 years.

Solar Maximum and Minimum

• Solar Maximum: Peak in sunspots on the surface of the Sun; corresponds to high solar activity.

Zeeman Effect

• Occurs when a spectral line is split into varying frequencies/wavelengths as light enters a magnetic field.

Prominences

• Red glowing loops of plasma that extend outward from the Sun’s surface, anchored in active regions.
• Can last for months.

Solar Flares

• Violent eruptions that release enormous amounts of magnetic energy in minutes to hours.
• Source of intense X-ray and gamma-ray radiation.
• Typically associated with sunspot groups.

Coronal Mass Ejections (CMEs)

• High-energy bursts of plasma that travel through interplanetary space.
• Can produce geomagnetic disturbances, auroras, and disrupt satellites and power grids.
• Flares are the most powerful explosions in the solar system.
• Energetic particles can travel from the Sun to Earth in less than 20 minutes.

Solar Flare Classification

• Solar flares are classified based on X-ray flux (I in W/m²):
  • B: I < 1 \times 10^{-6}
  • C: 1 \times 10^{-6} < I < 1 \times 10^{-5}
  • M: 1 \times 10^{-5} < I < 1 \times 10^{-4}
  • X: $I \geq 1 \times 10^{-4}$
• Potential effects on Earth and space missions vary by class.

Sirius Binary System

• Sirius is a binary star system: the brighter "Dog Star" and the fainter "Pup Star."
• The Dog Star has a temperature of approximately 9940 K and a radius of $1.2 \times 10^{9}$ m.
• The Pup Star has a temperature of approximately 28,400 K.

Sunspot Temperature Calculation

• Sunspots appear 70% as bright as the surrounding photosphere.
• The photosphere temperature is approximately 7777 K. Use the flux to determine the temperature of the sunspot.

Mass-Energy Conversion

• The Sun converts mass into energy according to Einstein’s equation: $E = mc^2$
• The Sun produces $3.85 \times 10^{26}$ joules of energy per second, converting $4.3 \times 10^{9}$ kg of mass per second into energy.

Solar Mass Loss Over Lifetime

• Assume the Sun has been producing energy at a constant rate for 4.5 billion years.
• Calculate the total mass lost by the Sun over its lifetime.
• The current mass of the Sun is $2 \times 10^{30}$ kg. Determine what fraction of the Sun’s current mass has been converted into energy.