Chapter 10Our Star

Explanation: Plumes of hot gas shoot across the surface on even an average day on the Sun. Such volatile activity was captured in dramatic detail recently by the new Hinode satellite launched by Japan in late September. Bright regions are hotter and more active. The bubbling granularity and continuous activity of the Sun's photosphere is visible in the foreground.

10.1 A Closer Look at the Sun

• Our Goals for Learning

• Why does the Sun shine?

• What is the Sun’s structure?

Why does the Sun shine?

Is it on FIRE?

Is it on FIRE?

Luminosity~ 10,000 years

Chemical Energy Content

Is it on FIRE? … NO!

Luminosity~ 10,000 years

Chemical Energy Content

Is it CONTRACTING?

Luminosity

Gravitational Potential Energy

Is it CONTRACTING?

~ 25 million years

Luminosity

Gravitational Potential Energy

Is it CONTRACTING? … NO!

~ 25 million years

Luminosity~ 25 million years

Gravitational Potential Energy

Is it CONTRACTING? … NO!

E = mc2

- Einstein, 1905

Is it powered by NUCLEAR ENERGY?

Luminosity~ 10 billion years

Nuclear Potential Energy (core)

Is it powered by NUCLEAR ENERGY? … YES!

Luminosity~ 10 billion years

Nuclear Potential Energy (core)

How Sun started to shine?

Gravitational contraction provided energy which heated core as Sun was forming.

Contraction stopped when high enough density and temperature were reached in the core and fusion began.

Sun converts mass into energy in the process of nuclear fusion. Nuclear fusion requires extremely high temperatures and densities.

Sun is now in gravitational equilibrium:

The energy released by fusion heats the gas and thus generates pressure.

The outward push of pressure balances the inward pull of gravity.

Weight of upper layers compresses lower layersgravity

pressure

What is the Sun’s structure?

Radius:

6.9 x 108 m

(109 times Earth)

Mass:

2 x 1030 kg

(300,000 Earths)

Luminosity:

3.8 x 1026 watts (J/s)

Sun is a ball of plasma! Plasma - charged particles they interact with magnetic fields.

The differing temperatures and densities of plasma at different depths give the Sun the layered structure:

Solar wind:

A flow of charged particles from the surface of the Sun

Corona:

Outermost layer of solar atmosphere. Low density gas

~1 million K

Chromosphere:

Middle layer of solar atmosphere. It emits most of the solar UV radiation.

~ temperature drops to 10,000 K

Photosphere:

Lowest layer of atmosphere. What we see as as the visible, boiling surface of the Sun. It also contains sunspots.

~ 6,000 K

The Suns interior:

Convection Zone:

Energy transported upward by rising hot gas. Energy is released in the photosphere, and that is why the Sun looks like ‘boiling’.

Radiation Zone:

Calmer plasma. Energy transported upward by photons. Temperature almost 10 million K.

Core:

Energy generated by nuclear fusion

~ 15 million K

And pressure 200 billion times that on the surface of the Earth.

It takes a million of years for energy released here to reach the Sun’s surface.

Why does the Sun shine?

The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.

What have we learned?

What have we learned?

• What is the Sun’s structure?

10.2 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)

Why we do not use fusion in power plants?

Fission

Big nucleus splits into smaller pieces

(Nuclear power plants)

Fusion

Small nuclei stick together to make a bigger one

(Sun, stars)

Why we do not use fusion in power plants? Fusion gives more energy and it is cleaner (we would get energy from water), but we do not know yet how to produce such high temeratures and pressures in a controlled way!

High temperature enables nuclear fusion to happen in the core

Sun releases energy by fusing four hydrogen nuclei into one helium nucleusQuestion: which nuclei is more massive, He or 4 H nuclei?

Proton-proton chain is how hydrogen fuses into helium in Sun. It proceeds through several steps involving just two nuclei at a time.

IN4 protons

OUT4He nucleus

2 gamma rays2 positrons2 neutrinos

Total mass is 0.7% lower

Thought Question

What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy?

A. The core would expand and heat up slightlyB. The core would expand and coolC. The Sun would blow up like a hydrogen bomb

Thought Question

What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy?

A. The core would expand and heat up slightlyB. The core would expand and coolC. The Sun would blow up like a hydrogen bomb

Solar thermostat keeps burning rate steady

Solar Thermostat

Temperature Restored

Temperature Decreases

Fusion Rate Decreases

Core compresses

Solar Thermostat

Temperature Restored

Temperature Increases

Fusion Rate Increases

Core expands

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

Energy gradually leaks out of radiation zone in form of randomly bouncing photons. They bounce of the charged particles in plasma, moving just few millimeters between changing direction. It takes millions of years for photons to finally exit this zone.

Convection zone is cooler (2 million K) and it contains gaseous neutral atoms and ions. Also, the energy of photons is degraded to the visible light. So atoms and ions in this zone are able to absorb photons and hold on to them. Energy absorbed by atoms makes them extremely hot. Convection (rising hot gas) takes energy to surface.

Bright blobs on photosphere are where hot gas is reaching surface

How do we know what is happening inside the Sun?

We learn about inside of Sun by …

1. Making mathematical models

2. Observing sun quakes

3. Observing solar neutrinos

1. Mathematical models:We observe: composition and mass

Based on our understanding of fusion and gravity, we calculate: temperature, pressure and density at any depth

Computer models also predict radius, luminosity, age…and luckily they agree very well.

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

Results agree very well with mathematical models of solar interior

3) Solar neutrinos:

Neutrinos created during fusion fly directly through the Sun.

Neutrinos interact very rarely with matter. I.e. every second there is about trillion of solar neutrinos passing through our bodies!

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

Solar neutrino problem:

Early searches for solar neutrinos failed to find the predicted number

Solar neutrino problem:

Early searches for solar neutrinos failed to find the predicted number

More recent observations find the right number of neutrinos, but some have changed form

The main tank of the Sudbury neutrino observatory in Canada.

What have we learned?• How does nuclear fusion

occur in the Sun?• Fusion of hydrogen into helium,

which occurs via the proton–proton chain. Gravitational equilibrium acts as a thermostat that keeps the fusion rate steady.

What have we learned?

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

• Energy moves through the deepest layers of the Sun—the core and the radiation zone—in the form of randomly bouncing photons. After energy emerges from the radiation zone, convection carries it the rest of the way to the photosphere, where it is radiated into space as sunlight.

What have we learned?• How do we

know what is happening inside the Sun?

• theoretical models

• use known laws of physics and then check the models against observations and studies

10.3 The Sun-Earth Connection

• Our Goals for Learning

• What causes solar activity?

• How does solar activity affect humans?

• How does solar activity vary with time?

What is solar activity?

Solar activity is like “weather”

• Sunspots

• Solar Flares: huge explosions

• Solar Prominences: gigantic loops of hot gas extending into Sun’s corona.

All related to magnetic fields!

Sunspots

Are cooler than other parts of the Sun’s surface (4000 K as oppose to 5800K in the surrounding)

They are regions with strong magnetic fields, and that explain why they stay cooler than the surroundings…

The size of the Earth

Charged particles spiral along magnetic field lines

We can represent magnetic

fields with magnetic field lines:

Loops trace magnetic field lines. Such magnetic structures form thought the Suns surface.

With hot plasma unable to enter the region, the region cools down a little. Sunspots last for about a week, before magnetic field weakens.

Solar prominences: sun’s chromosphere and corona become trapped in these magnetic loops, making giant prominences -> loops of bright gas often connect sunspot pairs

Solar storms:

The magnetic fields winding through sunspots sometimes undergo sudden change: they get to twisted and knotted that they reorganize themselves, producing short lived, but intense storms in the Sun.Solar flares send bursts of X-rays and charged particles into space.

Why chromosphere and corona are much warmer than the Sun’s surface? the bright (hot) spots in the corona tend to be directly above sunspots in photosphere -> we think that the magnetic field carries energy upward from the solar surface to the chromosphere and corona. Or to outer space (the coronal holes.

How does solar activity affect humans?

Coronal mass ejections send bursts of energetic charged particles traveling outward from the Sun in huge bubbles.

Charged particles streaming from Sun can disrupt electrical power grid and can disable communications satellites.For example, in 1989 during particularly strong Sun storm US Air Force temporarly lost track of 2,000 satellites.

Energetic particles high in Earth’s atmosphere cause auroras (Northern Lights)

These storms, on the positive side, lead to unusually strong auroras.

How does solar activity vary with time?

Number of sunspots rises and falls in 11-year (in average) cycle.

They form at mid latitudes. As cycle progresses new sunspots form closer and closer to the equator.

Sunspot cycle has something to do with winding and twisting of Sun’s magnetic field.During one cycle magnetic fields connecting two sun spots all point in one, lets say east west direction. In the next cycle they point in the opposite-> the entire Sun’s magnetic field flip-flops with each cycle.The Sun rotates faster at it equator (every 25 days) than its poles (every 30 days). When magnetic field lines get very wind up, that causes increase in number of sun spots and solar activity.

What have we learned?• What causes solar

activity?• Convection combined

with the rotation pattern of the Sun—faster at the equator than at the poles—causes solar activity because these gas motions stretch and twist the Sun’s magnetic field.

What have we learned?• How does solar activity

affect humans?• Bursts of charged particles

ejected from the Sun during periods of high solar activity can hamper radio communications,disrupt electrical power generation,and damage orbiting satellites.

What have we learned?

• How does solar activity vary with time?• The sunspot cycle, or the variation in the

number of sunspots on the Sun’s surface,has an average period of 11 years.The magnetic field flip-flops every 11 years or so, resulting in a 22-year magnetic cycle.