The Sun Chapter 10

The Sun - Our Star “Average” star Made entirely of gas too hot for liquids or solids Surface temp = 5000 K Center temp = 10 million K Composition (by mass): 70% H 28% He 2% everything else (C, N, O, etc.)

Major Regions of the Sun Interior Core Radiative Zone Convective Zone Atmosphere Photosphere Chromosphere Corona

Photosphere Effective “surface” of sun Thin atmospheric layer not solid region we see by eye Thin atmospheric layer Few hundred km Low density gas 0.01% Earth’s atmosphere Close-up shows granulation convection cells Sunspots Cooler than surrounding gas appear dark

Atmospheric Features Sunspots Larger than Earth Occur in groups Associated with magnetic fields Galileo used to measure solar rotation 25 days at equator longer at poles Number of sunspots Cyclical 11 yr period Correlated with solar activity greatest activity at sunspot maximum least at minimum

Most Detailed Sunspot Image Ever! Swedish Vacuum Telescope

Solar Rotation

Atmospheric Features Sunspots Larger than Earth Occur in groups Associated with magnetic fields Galileo used to measure solar rotation 25 days at equator longer at poles Number of sunspots Cyclical 11 yr period Correlated with solar activity greatest activity at sunspot maximum least at minimum

Atmospheric Features Prominences Loops of hot gas Base near sunspots Trace magnetic fields

Chromosphere Region just above photosphere Emission-line spectrum indicates higher temperature reddish color from hydrogen line Temperature increases outward

Transition Region Between chromosphere and corona Temperature increases from 10,000 K to 106 K Spicules (“spikes”) gas jets poking up from photosphere duration ~10 minutes heights 5,000-20,000 km

Corona Outermost region of atmosphere Seen during eclipse Extends millions of km Seen during eclipse Very high temperature millions of degrees K heated by solar magnetic field

Solar Wind Gas flows away from sun 10 million tons/yr protons & electrons speed 400-800 km/s Material goes outward into solar system Hits Earth’s atmosphere guided by magnetic field makes atmosphere glow (aurorae)

Solar Flare

Solar Flares Eruptions caused by magnetic fields may last 5-10 min release huge amounts of energy gas heated to 107 K produces X-rays and UV radiation Coronal Mass Ejections very large flares large mass of gas ejected from corona

Solar Flares & Effects on Earth CME hits Earth’s Magnetic Field Aurorae intensified affects magnetic field produces power surges ’89 Quebec/Montreal power outage electrical interference Disrupts radio communications Cell phone outage Danger to satellites/astronauts Space Weather at: http://www.sel.noaa.gov/today.html

Solar Variability & Earth’s Climate Solar Luminosity Varies ~1% Highest at sunspot maximum Lowest at minimum activity Maunder minimum 1650-1700 Very few sunspots “Little Ice Age” in Europe extreme cold temps shorter growing season Still learning how Sun affects Earth’s climate

Solar Interior Interior is gaseous Constant battle: Star shrinks Gravity pulls in Pressure pushes out Star shrinks becomes hotter, until Pressure balances gravity Hydrostatic Equilibrium

Why Does Sun Shine? Sun in Hydrostatic Equilibrium Hot objects Pressure requires high temperature Sun is hot Hot objects Radiate (emit light) lose energy Solar Luminosity 1026 Watts Age of Sun greater than 4.5 billion years (age of earth) What is energy source? Sun shines because it is hot

Sun’s Energy Source Thermal energy (chemical reactions)? wood or fuel burning Entire Sun used in a few thousand years NO Gravitational contraction? energy released as Sun contracts would suffice for 100 million years NO Nuclear Energy? Einstein: E = mc2 Can convert mass into energy 1 gram of matter = energy of 15,000 barrels of oil Need 4 million tons/sec Will suffice for 10 billion years YES

Nuclear Reactions Fusion: Fission: two light nuclei joined into one Powers Sun Fission: massive nucleus splits in two Powers nuclear reactors (Davis-Besse)

Converting Mass to Energy Nuclear reactions change mass Mass increase consumes energy Mass decrease releases energy Mass decreases in Fission of heavy nuclei Can occur spontaneously natural radioactivity Fusion of light nuclei Like charges repel Fusion requires high speed Temperature > 10 million K

Fusion: Sun’s Energy Source Sun’s core: Temperature = 15 million K Large enough to fuse hydrogen p-p chain (proton-proton) Series of fusion reactions Converts: 4 hydrogen to 1 helium nucleus mass of 4 H > mass of 1 He mass decrease is energy source

p-p Chain in the Sun