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
~¼ 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
- 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
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
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)
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.
Relatively dark regions - intense magnetic fields
Large range of sizes
Measure sun's rotation rate
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
Atmospheric layer above the
2000 - 10,000 km thick
Hot, low density gas
Produces 'flash spectrum' during eclipse
Reddish-pink because of hydrogen
Temp. increases to 106 oK at 10,000 km altitude, transition
from chromosphere to corona.
Spicules: narrow spears of cool gas reaching into 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
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
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