Sun Physics

Basic Fusion Reactions in the Sun

Assembled by

Ken Mitchell

Livermore TOPScience

Paste the URL into your browser before starting:

http://www.astro.ubc.ca/~scharein/a311/Sim/fusion/Fusion.html

                

From Core to CoronaLayers of the Sun

Core Temperature and Pressure are key to fusing Hydrogen into Helium

p + p d + + +0.42 MeV

When temperatures exceed 10 Million Degrees Kelvin the kinetic energy is large enough overcome the coulomb barrier to start the proton-proton reaction.

When enough deuterium (d) is present, the proton-deuteron (deuterium) reaction begins.

p + d He3 + + 5.5 MeV

See the cartoon on the next slide

The p – p Reaction

+ (Positron)

(Neutrino)Deuteron

Two Hydrogen

Nuclei (Protons)

+0.42 MeV

One Deuteron Plus One Hydrogen

Nuclei Helium 3 Nuclei

Gamma Ray

+ 5.5 MeV

The p – d Reaction

This next leads to:

He3 + He3 He4 + 2p + 12.8 MeV

The overall result of the above series of reactions is

4p He4 + 2+ + 2 + 2 + 26.7 MeV

Where: p = proton, + = positron,

=gamma, and =neutrino

See the cartoon on the next slide.

The He3 – He3 Reaction into He4

Proton

Proton

The energy is in particle velocity.

Helium 3 Nuclei

Helium 3 Nuclei

Helium 4 Nuclei

12.8 MeV

Alpha Particle

Copy the URL into your browser: http://www.astro.ubc.ca/~scharein/a311/Sim/fusion/Fusion.html

for an active p -- p fusion animation. Click on instructions.

The Reaction SummaryThe reaction process depicted above is the dominant fusion mechanism in light stars, including our sun.

In the P-P chain, two pairs of protons fuse, forming two deuterons.

Each deuteron fuses with an additional proton to form helium-3.

The two helium-3 nuclei which then fuse to create beryllium-

6, which is unstable (5.9 x10-21 seconds) and disintegrates into two protons plus a helium-4 (alpha particle).

In addition, the process releases two neutrinos, two positrons, and gamma rays. The positrons annihilate quickly with electrons in the plasma, releasing additional energy in the form of gamma rays.

The neutrinos interact so weakly that they fly right out of the sun immediately.

"P-P": Solar Fusion Chain

Summary Diagram

Hot Stuff

Sun’s core is about 27 million degrees F and is about ten times the density of Lead.

Each second it fuses 600 Megatons of Hydrogen into 595 Megatons of Helium.

The mass difference is 5 Million tons of energy, which is the equivalent of about 1 Billion 1 MT H-Bombs..

Solar Envelope

Outside of the core is the radiative envelope, which is surrounded by the convective envelope. Radiative Zone is about 185,000 miles thick. Convection Zone is about 130,000 miles thick. Protons do a “Random Walk” delaying their exit for thousands to millions of years. The temperature is 4 million degrees Kelvin (7 million degrees F).

The density of the solar envelope is much less than that of the core. The core contains 40 percent of the sun's mass in 10 percent of the volume, while the solar envelope has 60 percent of the mass in 90 percent of the volume.

The solar envelope puts pressure on the core and maintains the core's temperature.

Convection Zone

"Convective cells are arranged in tiers containing cells of progressively smaller size

as the surface is neared.

Photosphere

The photosphere is the zone from which the sunlight we see is emitted.

The photosphere is a comparatively thin layer of low pressure gasses surrounding the envelope.

It is only a few hundred kilometers thick, with a temperature of 6000 K.

The composition, temperature, and pressure of the photosphere are revealed by the spectrum of sunlight. In fact, helium was discovered in 1896 by William Ramsey, when in analyzing the solar spectrum he found features that did not belong to any gas known on earth.

The newly-discovered gas was named helium in honor of Helios, the mythological Greek god of the sun.

                

From Core to CoronaLayers of the Sun

Chromosphere

In an eclipse, a red circle around the outside of the sun can sometimes can be seen.

This is the chromosphere. Its red coloring is caused by the abundance of hydrogen.

From the center of the sun to the chromosphere, the temperature decreases proportionally as the distance from the core increases.

The chromosphere's temperature, however, is 7000 K, hotter than that of the photosphere.

Temperatures continue to increase through the corona.

Annular Solar Eclipse over Spain

Sunspots

Sunspots are dark spots on the photosphere, typically with the same diameter as the Earth.

They have cooler temperatures than the photosphere. The center of a spot, the umbra, looks dark gray if heavily filtered and is only 4500 K (as compared to the photosphere at 6000K). Around it is the penumbra, which looks lighter gray (if filtered).

Sunspots come in cycles, increasing sharply (in numbers) and then decreasing sharply. The period of this solar cycle is about 11 years. (See PPT on ‘The Sun’ for more details.)

The sun has enormous organized magnetic fields that reach from pole to pole. Loops of the magnetic field oppose convection in the convective envelope and stop the flow of energy to the surface. This results in cool spots at the surface which produce less light than the warmer areas. These cool, dark spots are the sunspots.

This picture of the sun was taken with heavily filtered visible light.

Corona

The outermost layer of the sun is the corona. Only visible during eclipses, it is a low density cloud of plasma with higher transparency than the inner layers.

The corona is hotter than some of the inner layers. Its average temperature is 1 million K (2 million degrees F) but in some places it can reach 3 million K (5 million degrees F).

Temperatures steadily decrease as we move farther away from the core, but after the photosphere they begin to rise again. There are several theories that explain this, but none have been proven.

This picture, showing more turbulence, was taken with x rays. The heat and energy of the corona cause the emission of x rays

Solar Flares

In the corona, above sunspots and areas of complex magnetic field patterns, are solar flares.

These sparks of energy sometimes reach the size of the Earth and can last for up to several hours.

Their temperature has been recorded at 11 million K (20 million degrees F).

The extreme heat produces x rays that create light when they hit the gasses of the corona.

Small prominences extend from the chromosphere up into the lower corona.

Prominences

Prominences are generally less violent than solar flares.

They are "cool sheets” of gas that condense out of the corona above the active regions.

Some are quiet and hang there for weeks, others rain matter down on the photosphere, still others literally explode into space, pushing the corona out in front of them in a great burst that carries the gas off the sun altogether."

A Solar Prominence (from SOHO)

Coronal Mass Ejections

Large flares are often associated with huge ejections of mass from the Sun.

Solar plasma is heated to tens of millions of degrees, and electrons, protons, and heavy nuclei are accelerated to near the speed of light. The super-heated electrons from CMEs move along the magnetic field lines faster than the solar wind can flow.

Each CME releases up to 100 billion kg (220 billion lb) of this material, and the speed of the ejection can reach 1000 km/second (2 million mph) in some flares.

Solar flares and CMEs are currently the biggest "explosions" in our solar system, roughly approaching the power in ONE BILLION hydrogen bombs! (See ‘The Sun’s Magnetic Personality’)

CMEs from SOHO

CMEs with UV Filter over the Sun

Solar Wind

The solar corona is constantly losing particles.

Protons and electrons evaporate off the sun, and reach the earth at velocities of 500 km/s.

Most of the mass of the sun is held in by magnetic fields in the corona, but particles slip through occasional holes in the fields.

Solar wind affects the magnetic fields of all the planets in the solar system.

When the solar wind hits the Earth's magnetic field, the wind compresses the field and creates a shock wave called the Bow shock.

Solar Wind

Solar Wind - - continued

Closer to the Earth are the Van Allen radiation belts where solar particles are trapped due to magnetic forces.

Still closer are huge rings of electric current around the poles, formed by the influence of the solar wind on the magnetic field. Earth, Jupiter, Saturn, Uranus, and Neptune have magnetotails where the wind extends their magnetic field.

The heliopause is the boundary where the sun's solar wind hits the gasses of interstellar space.

The sun's particles flow at least to Neptune, and probably farther. That means that we're inside the sun!