The Sun



Ordinary star



Powered by nuclear reactions - H bombs (Fusion)



Critical to life on Earth



Structure of Sun



Ball of hot gas



Sun's mass 2 x 1030 kg



Radius = 7 x 105 km



~ H



~ He (helium)



1-2 % heavier elements



Chemical composition different from earth, typical for a star.





Surface Temperature 5800 K



Emits electromagnetic radiation from radio to gamma ray



Most of energy emitted in ultraviolet, visible, near infrared



Sun also emits streams of particles (mostly protons and electrons)



Source of Energy

(What makes the Sun shine?)



Interior(core) very hot and dense, pressure is high -

gas of bare nuclei and electrons



4 H nuclei -> 1 He nucleus



Small amount energy released



Conditions for fusion:



1) Temp. must be high

10 - 15 x 106 oK

2) Gas must be dense



Proton-Proton Reaction (Cycle)



2 x (1H + 1H -> 2H + e+ + ν)



2 x (2H + 1H -> 3He + γ )



3He + 3He -> 4He + 1H + 1H



γ : gamma ray



e+: positron, like electron but positive charge



ν: neutrino - very small mass, travels close to c



E = mc2 where m is difference in mass of 4 1H and 4He



Sun converting H -> He



Converts 4.4 x 106 metric tons

of matter to energy every second [1 metric ton= 1000 kg]



Sun ~ 5 x 109 years old



Has enough hydrogen to last another 5 x 109 years



Photons carry energy from core towards surface - may take

7 x 105 years

Transfer of energy by radiation (photons) - radiative zone



Closer to surface (~70% from center), convection is principle method of energy transfer - convective zone -

hot gas rises

cool (denser) gas sinks





Solar Neutrino Problem

Neutrinos pass right through sun



A clue to activity at the core,

if can be detected



Theory predicts rate of production of neutrinos



Observations



- Homestake Gold mine - cleaning fluid. Observe 0.5v /day. Prediction 1.8v /day



- Kamiokande, Japanese expt. (1987) Observe half predicted rate of neutrinos



These experiments sensitive to 'rare', high-energy neutrinos



SAGE - Soviet American Gallium Expt.



- more common, lower energy neutrinos



- saw 60% expected number



Suggests problem:



1) Solar model wrong



2) Ideas about neutrinos wrong



Most model adjustments ruled out



Current theory - 3 types of neutrinos-



type we can observe converts to type we can't between solar interior and earth!!



Observations support this theory



Helioseismology



Study of sun's vibrations

Most common period 2 - 11 minutes



Clue to interior of sun - compare models & observations



Oscillations probably due to pressure waves triggered by solar convection



Differential rotation at sun's

surface - 25 days at equator

36 days near poles



Helioseismology suggests differential rotation extends only to bottom of convection zone, radiative zone exhibits rigid rotation (~ 27 days)



Solar Atmosphere



Photosphere - lowest layer (500 km thick)



Layer we 'see' - origin of most of observed photons



Granulation: convection cells (granules), 1000 - 2000 km wide. Some cells much larger.

Sunspots



Relatively dark regions - intense magnetic fields



Large range of sizes



Measure sun's rotation rate

(Differential rotation)

Darker because cooler



About 1500 oK cooler than rest of photosphere



Strong magnetic fields inhibit energy flow



Zeeman effect: spectral lines formed in magnetic fields split into components.



Sunspots indicate bipolar magnetic regions - active regions - magnetic field lines exit and re-enter sun's surface



In each solar hemisphere, spots have polarity pattern.

Pattern reverses 9 - 11 years



1995: Water found in sunspot. Infrared observation. T = 3200o



Stars have similar spots



Chromosphere:

Atmospheric layer above the

photosphere



2000 - 10,000 km thick



Hot, low density gas



Produces 'flash spectrum' during eclipse



Reddish-pink because of hydrogen

emission

Temp. increases to 106 oK at 10,000 km altitude, transition

from chromosphere to corona.



Spicules: narrow spears of cool gas reaching into corona



Corona:



Outer atmosphere of sun.



Plasma - mixture of positively and negatively charged particles. Mostly protons and electrons (components of H)



Extends up to 30 x solar radius.



Very hot, density very low.



Temp. increases as get farther from sun.

5 x 105 - 3.5 x 106 oK



Radiates in extreme ultraviolet

and x-rays, some radio emission



Not obvious why temp. increases

as get farther from sun in chromosphere and corona. Magnetic field activity believed to be important.



Prominences - clouds of denser, cooler gas suspended by magnetic fields in corona



Quiescent: suspended for weeks

to months without changing



Eruptive: material surges between surface and corona along magnetic field lines.



Flares: violent energy release and ejection of matter



Photons from gamma rays to radio



Particles mostly protons and

electrons



Caused by 'magnetic reconnection' - joining of

magnetic field lines



Usually associated with sunspots



Particles produce aurora, may damage satellites.



X-rays interfere with communications.



Coronal Mass Ejections: huge bubbles of plasma escaping from corona



Seem to be associated with flares



Produce magnetic storms on earth



Coronal holes: magnetic field lines extend into solar system. Particles escape from sun - produce 'solar wind'.



The Solar Cycle



All forms of activity have ~11 year cycle



Overall magnetic cycle ~ 22 years





Magnetic field originates in

convection zone



Cause of cycle not well understood



Sun slightly more luminous at

times of maximum activity than

times of minimum activity.



MAY impact earth's climate