Our Star

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

It is powered by NUCLEAR ENERGY!

Luminosity~ 10 billion years

Nuclear Potential Energy (core)

E = mc2

—Einstein, 1905

Why does the Sun shine?

– Chemical & Gravitational energy can’t explain how Sun could shine for > 25 million years.

– Sun must be stable, balancing gravity pulling in with forces pushing out from light and heat.

ChemicalEnergy?

Gravitational Energy?

Why does the Sun shine?

Gravitational equilibrium = balance of forces in & out

Sun shines because gravitational equilibrium keeps core hot & dense & generate energy through nuclear fusion.

Weight of upper layers compresses lower layers

Gravitational equilibrium:

Energy provided by fusion maintains the pressure.

What is the Sun’s structure?

What is the Sun’s structure?—From what we can see going outwards…

• Photosphere

• Chromosphere

• CoronaThese we CAN see – & measure with Visible-light, UV, X-ray telescopes in orbit & on Earth

Radius:

6.9 108 m

(109 times Earth)

Mass:

2 1030 kg

(300,000 Earths)

Luminosity:

3.8 1026 watts

Photosphere

Photosphere:

Visible surface ~ 6,000 K

Seen in Visible Light

Shows

•Dark “limb”

•Granules

•Sunspots!

•Absorption lines

Granules seen in the

Photosphere

Granules seen in the Photosphere are the visible “top” of a current of hot gas

Sunspots seen in the Photosphere

Credit: Chromosphere image courtesy Luc Viatour and Windows to the Universe, http://www.windows.ucar.edu

The Chromosphere – seen during a

total solar eclipse

Chromosphere:

Middle layer of solar atmosphere

~ 104–105 K

Seen in UV Light

Shows emission lines

Features: Prominences, Flares, Spicules

Spicules seen in the Chromosphere (UV)

Prominences seen in the

Chromosphere

Prominences seen in the

Chromosphere

Look carefully – you can see

Chabot!

Corona:

Outermost layer of solar atmosphere

~1 million K

Seen in X-rays

Shows emission lines + faint continuous spectrum

The Corona – Visible Light

The Corona – X-ray Light

Solar wind:

A flow of charged particles (electrons, protons, some helium nuclei) from the surface of the Sun

Creates Aurora

How do we know what is happening inside the Sun?

What is the Sun’s structure?—Going *inwards* to layers we cannot see…

• Convection zone

• Radiation zone• Core

These we can’t see – we model with computer simulations, observations of surface features to suggest structure, and observations of neutrinos to hint at fusion reactions taking place in the core.

Convection zone:

Beneath Photosphere

Creates granules we see

Energy transported upward by rising hot gas

Bright blobs on photosphere where hot gas reaches the surface

Convection (rising hot gas) takes energy to the surface.

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

Radiation zone:

Energy transported upward by Gamma Ray & X-ray photons

Determine extent based on computer models

Core:

Energy generated by nuclear fusion

~ 15 million K

Generates gamma rays

Inner 10% of sun

Data on solar vibrations agree with mathematical models of solar interior.

How does nuclear fusion occur in the Sun?

Fusion is the UNITING of light atomic nuclei into heavier nuclei,

releasing binding energy in the form of gamma ray radiation and other particles

Fission

Big nucleus splits into smaller pieces

(Nuclear power plants)

Fusion

Small nuclei stick together to make a bigger one

(Sun, stars)

High temperatures enable 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

IN4 protons

OUT4He nucleus

2 gamma rays2 positrons2 neutrinos

Total mass is 0.7% lower.

Neutrinos created during fusion fly directly through the Sun.

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

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 slightly.B. The core would expand and cool.C. 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 slightly.B. The core would expand and cool.C. The Sun would blow up like a hydrogen bomb.

Solar thermostat keeps burning rate steady

Solar Thermostat

Decline in core temperature causes fusion rate to drop, so core contracts and heats up

Rise in core temperature causes fusion rate to rise, so core expands and cools down

Structure of the Sun

What causes solar activity?

Solar activity is like “weather”

• Sunspots

• Solar flares

• Solar prominences

All are related to magnetic fields.

Sunspots…

Are cooler than other parts of the Sun’s surface (4,000 K)

Are regions with strong magnetic fields

Zeeman Effect

We can measure magnetic fields in sunspots by observing the splitting of spectral lines

Charged particles spiral along magnetic field lines.

Loops of bright gas often connect sunspot pairs.

Magnetic activity causes solar flares that send bursts of X-rays and charged particles into space.

Magnetic activity also causes solar prominences that erupt high above the Sun’s surface.

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

How does solar activity affect humans?

Coronal mass ejections send bursts of energetic charged particles out through the solar system.

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

How does solar activity vary with time?

The number of sunspots rises and falls in 11-year cycles.

The sunspot cycle has something to do with the winding and twisting of the Sun’s magnetic field.

Hans Bethe Video Clips

• http://www.youtube.com/watch?v=apgB_NR59ss

• http://www.youtube.com/watch?v=c7iJtT41yy8

• http://www.webofstories.com/play/4536?o=MS