• The Sun is a star. Compared to Earth, it's huge (Diameter = 100 times greater, mass 300,000 times greater). It is fluid, composed mostly of hydrogen and helium. Its surface temperature is about 6000 K.
• The Sun radiates an enormous amount of energy. Only about 10^-9 (one part in a billion) falls on the Earth. Yet, the power of sunlight that illuminates a patch of desert 100 km x 100 km is equal to the total power consumption of the US.
• Scientists can infer the properties of the Sun's interior from a theoretical principle called hydrostatic equilibrium, which says that heat pressure must balance gravity. From this principle, we can tell that the Sun's core has a temperature of about 16 million K.

• The heat of the Sun's interior takes thousands of years to reach its surface (photosphere). It leaks out by a combination of radiative diffusion and convection.
• In the early 1900s, scientists believed that the Sun's energy was derived from the energy of gravity. According to the "contraction hypothesis" by Kelvin and Helmholtz, the Sun was constantly shrinking and heating up, and could only last about 20 million years.

• Sir Arthur S. Eddington realized that the Kelvin-Helmholtz contraction hypothesis must be wrong, because evidence from other branches of science showed that the Sun must last for billions of years. The conflict of the theory with the evidence is an example of a scientific crisis.
• In 1920, Eddington put together two new clues: Einstein's theory of relativity, E = mc², and very accurate measurements of the masses of atoms. From these clues, he reasoned that the Sun could last for billions of years if hydrogen was being converted to helium in its core.
• It took another 30 years before Professor Hans Bethe of Cornell University figured out the exact sequence of reactions by which the Sun burns.

• About 5% of the Sun's power escapes as neutrinos, which can pass through the Sun and the Earth without reacting and are very difficult to detect. But now scientists can detect neutrinos from the Sun, and they find that the flux of neutrinos is about 1/3 of the value originally predicted by theory. At first, they thought that there must be something wrong with the theory of the Sun -- another scientific crisis?

• But, by measuring oscillations on the Sun's photosphere, scientists can now infer the temperature of the Sun's interior to a precision of a few parts in 1000. Doing so, they verified that the theory of the Sun's interior was OK.
• We now have evidence that about 2/3 of the neutrinos from the Sun are converted into different kinds of neutrinos that are not detected by the present experiments. So the problem wasn't with our theory of the Sun, it was with our ideas about how neutrinos behave.
• The convection in the Sun's outer layers generates magnetic fields that are especially strong in sunspots.

• The number of sunspots varies throughout the 11-year solar cycle. We don't know why.

• The convection and magnetic fields heats the gas above the Sun's photosphere to temperatures of millions of degrees. The hot tenuous gas is called the Sun's corona.

• Occasionally, the magnetic fields above the Sun's surface will erupt, releasing energy in huge explosions called coronal mass ejections.
• Coronal mass ejections can disturb the Space Weather near the Earth, sometimes damaging communication satellites. They also cause the Aurora Borealis and Aurora Australis (Northern and Southern Lights).

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Last modified January 18, 2002
Copyright by Richard McCray