Chapter 12: The Sun, Our Star

12.1 Size and Structure

Measuring the Sun’s Properties

• The sun has enormous gravity that crushes mater in its interior.

  • prevent its own collapse, sun must be very hot. it replensihes its own energy at the cost of consuming itself.

• Sun is gaseious throughout because its high temp breaks most molecular bonds, vaproizing even iron, and allowing atoms to move freely as gas.

The Solar Interior

• Human vision blcoked as we peer deepr into the sun.

  • Dense material, atoms so close together that they absorb light from deeper layers

  • above these layers, absorption of the light = weaker.

• Photosphere: The visible surface of the sun. Helps Sun retain heat, thus reducing the amount of fuel it consumes. lowest layer of the atmosphere

• Density and temperature rise steadily neared its core.

• Despite density, Sun is gaseous because its hgih temp gives atoms so much energy of motion they’re unable to bond with one another to form liquid or solid.

Energy Flow in the Sun

• Understanding of Sun’s properties comes from theory and measurement.

• Heat will flow outward from sun center.

  • near core, energy moves by radiation carried by photons through radiative zone.

• Radiative Zone:

  • gas very dense a photon travels very little and absorbed by an atom and stopped.

    • 16 million to escape; today’s sunshine was born in core before humans.

    • method of energy transport: radiation.

      • Radiation: energy generated in core is carried by photons that bounces from particle to particle.

• Convection Zone:

  • rising and sinking gases. Granulation: tiny bright regions surrounded by narrow darker zones.

    • Energy is transported through convection.

  • Bright area = bubbles of hot gas rising from sun.

  • releases heat and cools and sinks back down.

The Solar Atmosphere

• Immediately above photosphere, temp decreases, but at higher altitude gas grows hotter. Magnetic field plays a role.

• Chromosphere:

  • lower atmosphere

  • contains millions of spicules: jet of hot gas thousands of km long.

  • color (red), comes from strong emission line of hydrogen.

• Corona:

  • outer atmosphere

  • temperature shoots up.

  • super low density, just look through it. Not uniform, hot streamers coming form the sun along sun’s magnetic field.

  • contains very little energy despite high temp.

12.2 How the Sun Works

Internal balance (Hydrostatic Equilibrium)

• One force holds sun together, other prevents sun from collapsing. aka Hydrostatic Equilibrium.

• Inward Force = sun’s own gravity. Outward force = rapid motion of its atoms, at motion gives rise to a pressure.

  • if pressure too weak (inward force) sun would collapse on its own gravity.

Powering the Sun

• Sun’s energy needs to be replenished.

  • Sun energy flow outward as sunshine. Decreasing its energy source.

    • energy lost must be replaced or Sun’s core would cool

      • cooler temperature = less pressure and sun would shrink under force of its own gravity.

• Eventually came around with the process called nuclear fusion.

Nuclear Fusion

• Fusion possible because sun interior very hot.

  • At hot temps: nuclei move so fast when they collide, they’re very close and thus the eletrical repulsion between their protons is overwhelmed by strong force (the force that binds protons and neutrons together).

    • Thus separate nuclei can merge or fuse into single new nucleus.

• Hydrogen and helium is ionized. mission electrons.

The Proton-Proton Chain

• Step 1: Proton-Proton Fusion

  - Two protons (hydrogen nuclei) collide and fuse.

  - One proton turns into a neutron, forming a deuterium nucleus (a proton and a neutron).

  - A positron (the antimatter counterpart of an electron) and a neutrino are released.

• Step 2: Formation of Helium-3

  - The deuterium nucleus (a proton and a neutron) fuses with another proton.

  - This forms a helium-3 nucleus (two protons and one neutron).

  - Energy is released in the form of gamma rays.

• Step 3: Helium-3 Fusion

  - Two helium-3 nuclei collide and fuse.

  - They form a helium-4 nucleus (two protons and two neutrons) and release two protons.

  - The cycle can start over again with the newly released protons.

• Energy Release

  - During the entire process, energy is released in the form of light and heat, which powers the Sun.

  - The total energy comes from the slight loss of mass during fusion, converted into energy according to Einstein's equation \( E = mc^2 \).

12.3 Probing the Sun’s Core

Solar Neutrinos

• Number of released neutrinos tells how rapidly hydrogen is being converted into helium.

  • from this can deduce temp and density in Sun’s Core.

• Counting Neutrinos very difficult

  • no electric charge, very tiny mass, penetrating power.

  • Pass through anything

• Cosmic Rays: Particles (protons, electrons, variety of other subatomic particles) that constantly bombarding earth. That travel through the universe nearly at the speed of light.

• Neutrinos can change form as they travel through matter.

Solar Seismology

• studying the Sun’s interior by analyzing waves in Su'n’s atmosphere.

• analyzing waves in its gases

• Sun mkes gas waves and make regular pattern that can be detected as a Doppler shift of the moving material.

• Using this can measure the denisty and rotations deep within the sun from the pattern and speed of waves in atmosphere.

12.4 Solar Magnetic Activity

Solar Magnetic Fields

• magnetic field similar to Earth’s, only 2 times stronger.

  • crearted by rotation of electrically conductive material deep within (hot ionized gas)

• Electrons/other charged particles encounter magnetic field, spiral around field lines. Stronger magnetic filed is, tighter electrically charged particles spiral around lines.

Sunspots, Prominences, and Flares

• Sunspot: large, dark appearing region.

  • last from few days to over a month.

  • darker than surrounding gas because they’re cooler.

  • Regions wehre Sun’s magnetic field is concentrated to thousand times its normal strength. This strong magnetic field blocks normal convection beneath Sun’s surface, preventing hot gas from rising in reigon of sunspot.

• why magnetic fields grow so concentrated in sunspots?

  • tight connection between magnetic fileds and gas.

  • draws surrounding gas toward it, cause magnetic field is frozen into gas, magnetic filed is dragged inward and grows more concentrated.

• Prominences: huge plumes of glowing gas that jut from lower chromosphere into the corona.

  • form where Sun’s magnetic field reduces heat flow to a region.

    • cooler than gas around them

      • according to perfect gas law, pressure inside is less than pressure outside.

      • Hot external gas “bottles up” cooler gas of prominence.

• Solar Flares (Sunspots give birth to these): brief but bright eruptions of hot gas in the chromosphre.

  • magnetic fields appear to play important role

    • field near spot gets twisted by gas motions, twisting gets too much, field suddenly readjusts whipping gas out.

• CMEs are huge clouds of plasma (electrically charged gas) that are ejected from the Sun's outer atmosphere, known as the corona.

  • Build-Up: CMEs usually occur when magnetic field lines in the Sun's corona become twisted and tangled due to the Sun's rotation and other dynamic processes.

  • Release: When these magnetic field lines snap or reorganize themselves (a process called magnetic reconnection), a large amount of energy is released, propelling plasma into space at high speeds.

The Solar Wind

• Solar Wind: tenuous flow of mainly hydrogen and helium that sweeps across Solar System.

  • arises because corona is not in hydrostatic equilibrium.

  • Corona’s high temp creates pressure and can overcome Sun’s gravitational froce.

  • It pushes that material outward into space.

12.5 The Solar Cycle

• The number of sunspots changes from year to year in what is called solar cycle.

Cause of the Solar Cycle

• Sun rotates, gas near its equator circles sun faster than gas near its poles (spins differentially).

  • equator rotates in 25 days, poles rotates in 35 days.

• Differential rotation distorit sun’s magnetic field (winding it up)

  • winding of SUn’s magnetic field may cause solar cycle.

  • Sun’s rotation wraps the solar magnetic filed into coils below surface, increasing solar activites. Wrapping occur scause Sun’s magnetic filed is “frozen” into gas.

• Sunspots form whn kinks in magnetic filed rise to Sun’s surface and break through the photosphere.

Extra Info/more Info

• Size: The Sun's diameter is measured using its apparent size and known distance from Earth.

• Mass: The Sun's mass is calculated from its gravitational influence on Earth and other planets.

• Temperature: The Sun's surface temperature is determined from its spectrum of light, while the core temperature is estimated using models of nuclear fusion.

Chapter 1: The Cycles of the Sky

1.1 The Celestial Sphere

• Celestial sphere: dome of the heaves. It’s just a model to learn about the stars etc.

Constellations

• Constellations: fixed patterns on celestial sphere

  • constellations stars generally have no physical relations.

• Stars move very slowly, barely changing positions.

Daily Motions of the Sun and Stars

• Celestial Poles: directly above the North/South pole

• Celestial Equator: directly above earth’s equator.

  • For northern observer, some circumpolar stars near north never cross below horizon.

Annual Motion of the Sun

• The change of constellations with season cause of motion around Sun.

  • The Sun’s glare blocks view of part of the celestial sphere lies towards the Sun. Makes stars lie beyond Sun invisible

The Ecliptic and the Zodiac

• Ecliptic: Path traced by the Sun across the celestial sphere. (Just an extension of Earth’s orbit)

  • only when new or full moon crosses this line can eclipse occur.

1.2 The Seasons

• Earth=closest to sun in early Jan, when N.Hem is coldest

• Rotation Axis: 23.5 from the vertical. Earth spins around this

• The seasons are caused by the tilt of Earth’s rotation axis

  • it’s tilted, more sunbeam on N.Hem slanted amount on S.hem

Solstices, Equinoxes, and Ecliptic’s Tilt.

• Tilt of rational axis = sun’s path across celestial sphere is also tilted with respect to celestial equator.

• Equinoxes: sun on celestial equator, days/nights are equal length.

  • vernal (spring): 3/20

  • autumnal (fall): 9/22

• Solstices: beginning of summer/winter when Sun pauses in north or south directions. Sun furthest from equator north or south

Tracking the Sun’s Changing Position

• Sun follow different paths across sky each day.

  • zenith: point in sky straight overhead.

  • northern observer; sun high at noon on summer low in sky at noon on winter.

• Sun don’t rise due east or set due west on most day

  • vernal equinox: sun on celestial equator: rises due east and sets west

    • from vernal till summer sol, Sun’s rising and setting shift southward. rise/set due east/west on autumnal equinox

  • After winter sol,

  • Sun move north again.

  • Sun position obvious near equinoxes.

• Sun at north pole remain above horizon highest point at equinox. At equator Sun up for 12 hour every day of year, highest point at sol.

1.3 The Moon

• Moon: rise east set west. shift from west to east.

• In 29.5 days for— from new, full

  • phase caused by moon’s orbital motion around Earth. moon through constellations of zodiac

  • new: moon between earth and sun

  • full: moon on other side of earth from sun

• Phase-as moon move around, see differnece amounts of its illuminated half.

• 27.3 days to rotate around Earth.

1.4 Eclipses

• eclipse: moon exactly between Earth and Sun, or Earth exactly between Sun and Moon

  • Solar eclipse: moon directly between Sun and Earth, block view of Sun

  • Lunar eclipse: Earth between Sun and Moon cast shadow on moon

Appearance of Eclipses

• lunar eclipse red interaction w/air molecules remove blue light as it pass thorugh atmosphere and bent (what happens when sen setting).

• annular eclipse: leave sun surface still visible. moon doesn’t completely cover sun even if it in line with sun. cause moon far away.

• no eclipse every month cause of moon orbit tipped with respect to Earth’s orbit.

  • moon shadow may pass above or below earth (new moon)

  • Earth shadow pass above or below moon (full moon)

Chapter 2: The Rise of Astronomy

2.1 The Early Ideas of Heavens: Classical Astronomy

The Shape of the Earth

• Most ancients believed earth is round.

  • Pythagoras teaching Earth is sphere cause it perfect shape and gods would use it for earth.

  • Aristotle more logical: earth shadow on moon curved.

  • move south see stars previoulsy hidden below horizon. This can’t happen on flat earth

The Size of the Earth

• Eratosthenes, head of Library at Alexandria. made first measurement of Earth’s size.

  • His measurements are amazing using logic and math, and observation

• Aristarchus estimated realtive size of earth, moon, and sun, and relative distance to moon and sun. Used geometry.

  • aris realized could estimate moon’s size by comparing to size of earth shadow during lunar eclipse.

  • 1st to show Earth is not largest

• Angular size: apparent size that an object covers in the sky.

Arguments for an Earth-Centered Universe

• Is earth move around Sun, position of star should chnage during course of year. Shift due to Earth’s changing persepctive.

• Parallax: shift in position of foreground star relative to background.

• Didn’t believe helio cause they didn’t see parallax.

2.2 The Planets

• Planets, Planetai = wanderers

• Planets aways remain close to ecliptic, within zodiacs

  • cause motion of planets lies in same zone cause of orbits, like earth’s, lie in nearly same plane.

    • their path tilted by 23.5 degree to celestial equator

• Planet very slow, planet raise and set each day (reflecting rotation of earth)

  • move eastward through stars result of orbital motion around Sun (apparent motion west to east, but doesn’t mean planet rise in west and set in east.)

• Retrograde motion: star path backward, it changes direction cause of earth’s motion

Explaining the Motion of the Planets

• Geocentric Models: earth at the center of the universe.

  • doesn’t explain retrograde motion

  • tried to explain it with an epicycle: small circle on a larger circle.

  • motion of planet from east to west across sky caused by large circle, small circle causes retrograde.

2.3 Astronomy in the Renaissance

Nicolaus Copernicus

• he began the demolition of geocentric model

• Heliocentric Models: Sun center of system. Proposed 2000 years ago by Aristarchus rejected cause can’t see parallax. Helio explain retrograde way better

• distance of the planet from sun (AU) (insisted motion were circles instead of ellipses)

Tycho Brahe

• Built instruments of high accuracy greater than all in Europe.

• Believed that the earth went around the sun, offered explanations for it.

• The rich guy with the fake nose

Johannes Kepler

• Kepler was Brahe’s assistant, dervicecd from Tycho’s large set of data precise information detailing the paths of orbits.

• Shape of orbits = elliptical, long/short dimensions, major and minor axes.

• Measured relative size of orbits

  • used semimajor axis, half of major axes. cause can’t be described as single radius cause orbits are ellipses.

• Kepler’s 3 laws: Describe essential feature of planetary motion

  • One: Planets move in elliptical orbits with Sun at 1 focus of ellipse

  • two: Orbital speed of the planet varies so that a line joining Sun and planet will sweep over equal areas in equal time interval (planets don’t move with constant speed in orbit, closer to Sun travels faster)

  • three: amount of time planet takes to orbit Sun related to orbit’s size. The equation: P² = a³

    • further from sun (larger a) has longer orbital period. planet close to Sun move along orbit faster

2.4 The Birth of Astrophysics

Galileo Galilei

• Satellites: moon, object orbiting another larger body.

• he studied the motions of objects. Credited for a lot of scientific experiment process