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The Sun - Our Star• Sun’s diameter 100 times

the Earth’s• Sun’s mass is 700 times the

mass of all the planets put together

• The energy source of the Sun is the conversion of hydrogen atoms into helium atoms through nuclear fusion in the core of the Sun. Without this energy source the Sun would collapse.

• Composition of the Sun is 71% Hydrogen, 27% Helium, 2% heavier elements

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How do we know what we know?• Our understanding of the

Sun is a combination of measurements of observed quantities (diameter, surface temperature, spectra) and computer models based on physical laws (internal temperature and density).

• Models are correct so far as they can predict the observed properties of the Sun.

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Solar Structure• The Sun is completely gaseous.• The atoms in this gas strongly absorb light. • Because of this absorption energy travels

very slowly through the Sun. It takes about 100,000 years for energy to travel from the core to the solar surface.

• Above a certain height however the gas is too thin to absorb light effectively. The point at which the gas becomes transparent is called the photosphere.

• The photosphere is considered the “surface” of the Sun. Surface temperature about 6000 Kelvin.

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Solar Structure• At the core of the Sun, where nuclear

fusion occurs, temperatures are about 15 million degrees Kelvin.

• As this energy propagates outwards it initially is carried by photons of light. This region of the Sun is called the radiative zone.

• Near the photosphere this process of energy transfer becomes less efficient and energy begins moving outward through convective motions of the gas. As gas heats it rises to the surface. Once it reaches the surface it releases heat and sinks. This region is called the convective zone.

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Hydrostatic Equilibrium

• Requires that the pressure generated by the fusion reactions in the core of the Sun must be in exact balance with the weight of material falling inwards due to the Sun’s gravity.

• Without this balance the Sun would either collapse (gravity wins) or explode (pressure wins).

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• The Sun is not burning. If it were burning fuel like coal it would have exhausted its fuel long ago.

• The slow collapse of the Sun was once thought to be the energy source but that wouldn’t have lasted more than a few million years.

• It wasn’t until the 20th century that physicists understood the process behind what powers the Sun.

The Sun’s Energy Source

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Einstein’s Mass-Energy Relation

Albert Einstein (1879-1955)

• In 1905 Albert Einstein recognized that mass and energy were related through the formula:

E=mc2

(m =mass, E=energy, c=speed of light)

• What this means is that a small amount of mass could be converted into an enormous amount of energy.

• The means by which the Sun generates this energy is through nuclear fusion.

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• Nuclear fusion involves combining two atomic nuclei together to form one, larger nucleus.

• This can not occur under normal conditions because the electrical repulsion between protons prevents the nuclei from fusing.

• At extremely high temperatures however this electrical repulsion is overcome and the nuclei fuse.

Nuclear Fusion

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Nuclear Fusion• The specific steps of nuclear

fusion follow a process called the proton-proton chain

• Through this process 2 neutrinos, 2 positrons, 2 1H and a 4He is created by the fusion of 6 1H.

• The mass of all the particles created is less than the sum of the masses of colliding particles. This difference in mass was converted into energy through E=mc2.

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Studying the Solar Interior• The neutrinos generated in

the fusion process escape rapidly through the Sun.

• These neutrinos are very difficult to detect but they can be detected.

• This type of neutrino “telescope” can check whether our understanding of the solar interior is correct.

• It seems that the number of neutrinos measured match computer models but only if neutrinos have mass.

A neutrino detector in Japan.This is a giant tank of water buried deep underground. Tiny bursts of light due to neutrinos from the Sun are detected by light sensitive detectors on the walls of the tank.

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Solar Magnetic Activity• Like several of the

planets the Sun generates a magnetic field.

• Unlike the planets the strength of the field strongly varies with time and with location on the Sun

• The most easily detectable evidence for solar magnetic activity are the Sunspots

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Sunspots

This sunspot is twice the size of the Earth!

• Sunspots are dark splotches on the photosphere of the Sun.

• Sunspots are dark because they are cooler than their surroundings.

• The reason they are cooler is because intense magnetic fields prevent hot, ionized gases from entering so the region cools forming a spot

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Prominences & Flares

• Prominences are associated with pairs of sunspots are are arcs of magnetic field with glowing gas trapped inside.

• Flares are explosive events where strong bursts of X-rays are emitted. These are due to rapid changes in the magnetic field.

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The Corona and Solar Wind • The outer atmosphere of the Sun is

called the corona. The temperature of the corona can be several million degrees.

• Magnetic fields are thought to be responsible for these very high temperatures.

• The corona is so hot it expands outward along with the Sun’s magnetic field and streams through the Solar System. This is known as the Solar Wind.

• Depending on the Sun’s magnetic activity level, the Solar Wind may change in velocity and density.

The corona is only visible from Earth duringa Solar Eclipse.

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The Solar Cycle• Sunspots, prominences,

flares and the strength of the solar wind all vary over an approximately 11-year cycle

• The magnetic field of the Sun will strengthen and weaken over 11 years.

• The polarity of the Sun’s field will also flip. The North pole becomes the South pole and vice versa. It takes 22 years to return to its original polarity.

A plot of the number of sunspotswith time from 1860 - 2000