The Sun Closest star to Earth - only star that we can see details on surface - easily studied Assumption: The Sun is a typical star

The Sun Closest star to Earth - only star that we can see details on surface - easily studied Assumption: The Sun is a typical star

Why is the Sun hot and bright? Surface Temperature of the Sun: T = 5800 K (9980 o F) Energy Output (Luminosity) of the Sun: L = 3.86 x 10 26 Watts Estimated Age of Sun: t = 4.6 x 10 9 years Burning coal (chemical reaction)? - But, cannot provide enough energy - But, sun is composed of mostly H, He - Not coal, wood, oxygen, etc. Kelvin - Helmholtz Contraction? - Gravitational Potential Energy - contraction of gas due to gravity converts: gravitational energy HEAT - But, Sun is not contracting - But, can only supply energy for less than 25 million years

Radioactive Heating? - 1908, radioactive materials produce heat as they decay - But, Sun composed of mostly H, He - not heavy elements (U, Pu, Ra, etc.) Nuclear Reactions? Special Theory of Relativity (Einstein,1905) E = m c 2 where E = energy, m = mass c = speed of light Matter contains enormous amount of energy Nuclear Reactions: - can change mass to energy Chemical Reactions: - only affect electrons orbiting nucleus - no affect on nuclei of atoms Burning propane: C 3 H 8 + 5 O 2 3 CO 2 + 4 H 2 O carbon, oxygen, hydrogen get rearranged

Nuclear Reactions: - change the nucleus of an element - can change one element into another Nuclear Fission: Heavy nucleus splits into lighter nuclei Nuclear Fusion: Light nuclei combine to form heavier Nucleus

Conditions for Nuclear Fusion Properties of the Electromagnetic (EM) Force - attractive or repulsive - only affects particles with an electric charge - infinite range Properties of the Strong (Nuclear) Force - attractive - affects protons and neutrons but not electrons - short range - strongest force in nature b

High Temperature: - nuclei naturally repel each other - at high T, nuclei have faster speeds - need nuclei come close enough for Strong Force to fuse them High Density: - larger number of nuclei in a volume - more collisions - higher chance of fusion http://www.astro.ubc.ca/~scharein/a311/sim/fusion/fusion.html

Center of Sun has the right conditions Energy source of Sun: H fusion 4 H 1 He + neutrinos + energy 4 H (protons) more massive than 1 He 1000 gm H fused into 993 gm He 7 gm converted into energy Estimate mass of central region of Sun - can produce energy at current rate 10 billion years

Structure of the Sun "Typical" stars produce energy by H fusion Core: - central region where fusion occurs - Temp greater than 10 7 million K Envelope: - region surrounding core - no fusion in this region - energy from core flows through envelope to surface CREDIT: University of Montreal Solar Physics Research Group

Basic Observations of the Sun - not changing size - not changing temperature - not changing luminosity Which means: - must have stable internal structure - must have stable energy production - must have stable energy transfer Stable Size of Sun: Gravity (caused by mass) pushes inward Gas Pressure (caused by energy source) pushes outward

Hydrostatic Equilibrium Gravity = Gas Pressure - at every point inside sun - uniquely determines the size of a star Typical Stars are very stable If we force star to expand: expanding gas cools less gas pressure to counter gravity so, it contracts returns to original size If we force star to contract: contracting gas heats up more gas pressure to counter gravity so, it expands returns to original size

Stable Temperature of Sun: Energy leaves the surface Luminosity Energy is produced in the core H fusion Thermal Equilibrium (T remains constant) Energy Produced = Luminosity

Heat Transport: Must be a stable way to move energy from core to the surface Methods of Heat Transport: Conduction Convection Radiation Conduction - heat moved by molecular vibration - most efficient in solids

Convection - heat moved by the motion of the material itself Radiation - heat moved by photons - not same as radioactivity

Internal Structure of the Sun: Core: Energy produced by H fusion Radiative Zone: inner envelope, energy transported by radiation (photons) Convective Zone: outer envelope, energy transported by convection

http://outreach.atnf.csiro.au/ Photons created in the core: - take 10 5 to 10 6 yrs to travel from core to surface

Photon Random Walk Simulation http://www.siena.edu/science Surface of sun (Photosphere) - Top of Convective Zone - granulation, hot gas bubbling up to surface http://www.noao.edu/

Solar Granulation: http://wn.com/solar_granulation; http://www.bbso.njit.edu/ Simulations: http://www.aip.de/groups/sternphysik/stp/convect.html

Testing the Model Solar Neutrinos - produced in each fusion reaction - weakly interacting particle - come straight out of sun - only direct probe into core of the sun - very difficult to detect

But, 10 10 neutrinos from the Sun pass through Earth per cm 2 per sec!! Detecting Neutrinos: - Ray Davis experiment (1960 s) - if neutrino hits Cl nucleus, can change it to Argon - 100,000 gallons of cleaning fluid Theory: detect 1 per day

Experiment: detect 1 every 3 days More recent experiments: - Italy, Russia, Japan, Canada Sudbury, Canada Results were still less than prediction Super Kamiokande, Japan

Solar Neutrino Problem: Problem with detectors - not sensitive enough? Problem with Solar Model - Wrong estimate of core temp? - different H fusion reaction? Problem with Neutrino Model - could change properties while traveling to Earth? - neutrino oscillations April 2002: Evidence for neutrino oscillations! - neutrinos change from one type to another! - in 8 minutes, about 2/3 of neutrinos change - exactly matches prediction - confirms theoretical solar model Also, this means they must have mass! - Important since neutrinos are the most numerous particle in the universe.