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Linné on line arrow Physics and the Cosmos arrow Macrocosmos arrow The sun

The Sun

Energy is produced in the sun by fusion of lighter atomic nuclei into heavier ones. This proceeds through a series of nuclear reactions which convert the original material hydrogen into heavier elements in the so called solar cycle. In these processes energy is released in the form of kinetic energy of the produced particles and heat radiation in the form of photons.

The nuclear reactions are mainly taking place deep inside the sun and the energy must then be transported out through the sun. It has been estimated that it takes about one million years for a photon to get from the centre of the sun to the surface. This should be compared with the time it takes for a photon to travel from the sun to the earth, which is no more then 8.3 minutes. The photons that reach the earth therefore do not reflect what happens in the centre of the sun but rather what happens close to the surface today. However, there are particles that are only very weakly interacting with the sun, namely neutrinos which can travel through the sun as it was almost not there. The flux of the produced neutrinos has been measured on earth and one has found that they are only about half of what one would expect for massless neutrinos. The explanation is that the produced neutrinos oscillate into other types of neutrinos which one did not look for until recently. This used to be called the solar neutrino problem, but it is now completely understood. The oscillations also mean that neutrinos have mass, although the masses are very small.

Other particles can also be emitted from the sun. These are emitted from the surface of the sun when the solar activity is high. They are mainly electrons, protons and alpha-particles, but also some heavier nuclei. Together they form the so called solar wind. Particles in the solar wind can be captured in the earth's magnetic field and give rise to northern lights close to the north pole and southern lights close to the south pole.