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


- 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


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.


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


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

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