Our Star

A Closer Look at the Sun

• Our goals for learning

• Why was the Sun’s energy source a major mystery?

• Why does the Sun shine?

• What is the Sun’s structure?

Why was the Sun’s energy source a major mystery?

• Scientists knew the Sun had existed and shone for millions of years.

• Could calculate the Sun’s daily energy output

• Could not work out what fuel source could provide so much power for so long

Is it on FIRE? … NO!

Luminosity~ 10,000 years

Chemical Energy Content

Luminosity

Gravitational Potential Energy

Is it CONTRACTING? … NO!

~ 25 million years

It can be powered by NUCLEAR ENERGY! E=mc2

Luminosity~ 10 billion years

Nuclear Potential Energy (core)

Why does the Sun shine?

Composition of the Sun• The Sun is a plasma - an extremely hot gas• The atoms’ electrons are ionized - stripped

away from the nucleus

• The Sun is made mostly of: Hydrogen, the smallest element (~70%) Helium, the 2nd smallest element (~30%)

Atom IonAtom Ion

Fusion occurs in the innermost part of the Sun because the weight of upper layers compresses then crushes lower layers

Gravitational contraction:

Provided energy that heated core as Sun was forming. Intense heat eventually forced fusion.

Contraction stopped when fusion began

Stable Star• Sun’s core fuses hydrogen into helium, releasing

energy into the outer layers of the Sun.

• The energy causes gas expansion, pushing outward against the gravity pulling inward. This expansion is called thermal pressure.

• When the pressures balance, a star is in gravitational equilibrium.

What have we learned?

• Why was the Sun’s energy source a major mystery?– Chemical and gravitational energy sources

could not explain how the Sun could sustain its luminosity for more than about 25 million years

• Why does the Sun shine?– The Sun shines because gravitational

equilibrium keeps its hydrogen core hot and dense enough to release energy through nuclear fusion.

Nuclear Fusion in the Sun

• Our goals for learning

• How does nuclear fusion occur in the Sun?

• How does the energy from fusion get out of the Sun?

• How do we know what is happening inside the Sun?

How does nuclear fusion occur in the Sun?

Fission

Big nucleus splits into smaller pieces

(Nuclear power plants)

Fusion

Small nuclei stick together to make a bigger one

(Sun, stars)

Fusion occurs in the innermost part of a star because the weight of upper layers compresses then crushes lower layers

High temperatures cause particles to smash together. Enables nuclear fusion to happen in the core

Sun releases energy by fusing four hydrogen nuclei into one helium nucleus

Proton-proton chain is how hydrogen fuses into helium in Sun

The Complete Fusion Reaction

IN4 protons

OUT4He nucleus

2 gamma rays2 positrons2 neutrinosENERGY

Total mass is 0.7% lower

Solar Thermostat• The energy released from fusion causes the sun’s gases to

expand (thermal expansion).• The balancing act between thermal expansion pushing out

and gravity contracting inward, forces the temperature to stay constant.

Solar Thermostat

Thermal pressure drops, gravity gains =>core contracts and heats back up

Thermal pressure increases over gravity.=>core expands, which causes core to cool down

If temperature falls because fusion rate drops :

If temperature rises because fusion rate increases:

Solar ThermostatTherefore, gravitational equilibrium, causes thermal equilibrium, as long as a star’s core continues to fuse.

Sun is estimated to do this for another 5 billion years!

© 2014 Pearson Education, Inc.

What is the net fusion reaction that produces energy in the core of the

Sun?

a) 4 hydrogen nuclei form 1 helium nucleus plus energy.

b) 2 hydrogen nuclei form 1 helium nucleus plus energy.

c) 6 hydrogen nuclei form 1 helium nucleus, 1 carbon nucleus plus energy.

d) 3 hydrogen nuclei form 1 helium nucleus plus energy.

e) 4 hydrogen nuclei form 1 helium nucleus, 1 carbon nucleus, plus energy.

© 2014 Pearson Education, Inc.

What balances the inward push of gravity inside the Sun?

a) the rigidity of the solid central core

b) electron degeneracy pressure

c) thermal pressure of the gas

d) neutron degeneracy pressure

What have we learned?

• How does nuclear fusion occur in the Sun?– The core’s extreme temperature and density are

just right for nuclear fusion of hydrogen to helium through the proton-proton chain

– Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature

What is the Sun’s structure?

Sun

Contains•Core

•Radiation Zone

•Convection Zone

•Photosphere

•Chromosphere

•Corona

•Solar wind

interior

Atm.

Core:

Energy generated by nuclear fusion

~ 15 million K

Radiation Zone:

Energy transported upward by photons

Convection Zone:

Energy transported upward by rising hot gas

Photosphere:

Visible surface of Sun

~ 6,000 K

Chromosphere:

Middle layer of solar atmosphere

~ 104 - 105 K

Absorption lines due to the Sun’s atmosphere

Corona:

Outermost layer of solar atmosphere

~1 million K

Emits x-rays and streamers that follow the magnetic lines

Solar wind:

The corona dissipates, becoming a flow of charged particles from the surface of the Sun

How does the energy from fusion get out of the Sun?

Energy moves randomly around the radiation zone in form of randomly bouncing photons. Eventually it makes it to the edge of the radiative zone and into the convective zone

Convection (rising hot gas) takes energy to surface

Bright blobs on photosphere are where hot gas is reaching the surface (effect is called granules, or granulation)

How we know what is happening inside the Sun?

We learn about inside of Sun by …

• Making mathematical models

• Observing solar vibrations

• Observing solar neutrinos

Patterns of vibration on surface tell us about what Sun is like inside

The vibrations reflect, reinforce or cancel each other which changes the patterns and frequencies seen at the Sun’s surface.

Data on solar vibrations agree very well with mathematical models of solar interior

Neutrinos created during fusion fly directly through the Sun

Observations of these solar neutrinos can tell us what’s happening in core

© 2014 Pearson Education, Inc.

Which of the following layers of the Sun is farthest from the core?

a) convection zone

b) photosphere

c) chromosphere

d) corona

What have we learned?

• What is the Sun’s structure?– From inside out, the layers are:

• Core

• Radiation Zone

• Convection Zone

• Photosphere

• Chromosphere

• Corona

• Solar wind

What have we learned?

• How does the energy from fusion get out of the Sun?– Randomly bouncing photons carry it through

the radiation zone– Rising of hot plasma carries energy through the

convection zone to photosphere

• How do we know what is happening inside the Sun?– Mathematical models agree with observations

of solar vibrations and solar neutrinos

The Sun-Earth Connection

• Our goals for learning

• What causes solar activity?

• How does solar activity vary with time?

• How does solar activity affect humans?

What causes solar activity?

Solar activity is like “weather”

• Sunspots

• Solar Flares

• Coronal Mass Ejections (CMEs)

• Solar Prominences

• All are related to magnetic fields

Sunspots

Sunspots

Are cooler than other parts of the Sun’s surface (4000 K)

Are regions with strong magnetic fields

Charged particles spiral along magnetic field lines, emitting light as they go: x-ray light.

Sun at Multiple Wavelengths

Microwave Visible Ultra-violet X-Ray

Magnetic field lines pierce the surface

Loops of bright gas often connect sunspot pairs

The Corona appears bright in X-ray photos in places where magnetic fields trap hot gas

Magnetic activity can cause solar prominences that erupt high above the Sun’s surface

The particles associated with the prominance can be flung out into space making a Coronal Mass Ejection

Magnetic activity also causes solar flares that send bursts of X-rays into space

How does solar activity vary with time?

Number of sunspots rises and falls in 11-year cycle

Sun in X-Rays

over Solar Cycle

Sunspot cycle has something to do with winding and twisting of Sun’s magnetic field

Bursts of charged particles are Coronal Mass Ejections

Bursts of x-rays are Solar Flares

The magnetic lines erupt out and can snap.

The snapping lines fling particles or energy (or both) out through the solar system

Coronal Mass

Ejection or Solar Flare

How does solar activity affect humans?

Solar Flare Headed toward Earth

Charged particles streaming from Sun can disrupt electrical power grids and can disable communications satellites

Alaska Aurora

What have we learned?

• What causes solar activity?– Stretching and twisting of magnetic field lines

near the Sun’s surface causes solar activity.– Activity detectable as sunspots

• How does solar activity vary with time?– Activity rises and falls with an 11-year period

• How does solar activity affect humans?– Bursts of charged particles from the Sun can

disrupt communications, satellites, and electrical power generation

Live Solar Observation at the Big Bear Solar Observatory

http://www.bbso.njit.edu/

Daily Space weather and geostorm warning

http://www.spaceweather.com