• 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 1

• 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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• 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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• 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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• 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.
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• In 1905 Albert Einstein recognized that mass and energy were related through the formula:
E=mc 2
(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 .
Albert Einstein (1879-1955)
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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.
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• The specific steps of nuclear fusion follow a process called the proton-proton chain
• Through this process 2 neutrinos, 2 positrons, 2 1 H and a 4 He is created by the fusion of 6 1 H.
• 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=mc 2 .
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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.
• 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.
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• 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 are dark splotches on the photosphere of the Sun.
• Sunspots are dark because they are cooler than their surroundings.
This sunspot is twice the size of the Earth!
• 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 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 outer atmosphere of the Sun is called the corona . The temperature of the corona can be several million degrees.
The corona is only visible from Earth during a Solar Eclipse.
• 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.
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A plot of the number of sunspots with time from 1860 - 2000
• Sunspots, prominences, flares and the strength of the solar wind all vary over an approximately 11year 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.
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