The Sun, the center of our Solar System, is a million times larger in volume than our Earth and contains 99.9 % of the mass of the Solar System. It provides the light and other energy that we need to live.

The layer of the Sun that we see on a normal day is called the photosphere, which simply means the sphere from which the light comes (from the Greek "photos", meaning "light"). As is typical from many stars, about 94 % of the atoms and nuclei in the outer parts are hydrogen, about 5.9 % are helium, and a mixture of all the other elements makes up the remaining one-tenth of one per cent. The overall composition of the interior is not very different, though deep in the core helium is being formed out of hydrogen nuclei. The core, which is about 10 % of the solar diameter at this stage of the Sun's life, is the place where all the energy is generated. The temperature there is about 15 million ⁰C. The energy is generated in the Sun by nuclear fusion in the Sun's core. In the deeper parts of the Sun, energy is carried out by radiation. Above that region, in the upper part of the of the solar interior, energy is carried by convection, the way that matter moves when is heated below and carries energy as it moves upward against gravity.

Just above the photosphere is a jagged, spiky layer about 10,000 km thick, only about 1.5 % of the solar radius. This layer glows colorfully pinkish when seen at an eclipse and is thus called the cromosphere (from the Greek "chromos", meaning "color". Above the chromosphere, a ghostly white halo called the corona (from the latin, menaing "crown") extends tens of millions of kilometers into space. The corona is continually expanding into interplanetary space. What we call the solar wind is based on this expansion.

When we study the Sun in white light - all the visible radiation taken together - with 1 arc second resolution, we see a salt-and pepper texture, called granulation, and sunspots. The granulation effect is similar to that seen in boiling liquids on Earth, which are undergoing convection. Each granule is only about 1000 km across and represents a volume of gas that is raising from and falling to a shell of convection, called the convection zone, located below the photosphere. The sunspots are the most obious sign of solar activity. They are areas of the Sun that appear relatively dark because thay are giving of less radiation than the phtosphere that surrons them. This implies that they are cooler areas of the the solar surface, since cooler gas radiates less than hotter gas. Actually, if we could somehow remove a sunspot from the solar surface and put it off in space, it would appear bright against the dark sky; a large one would give of as much light as the full moon. Suspots were discovered in 1610, independently by Galileo in Italy, Fabricius and Christopher Scheiner in Germany, and Thomas Harriot in England. The number of sunspots varies with an 11-year cycle, called the sunspot cycle. Sunspots are thought to be a result of differential rotation winding up the Sun's magnetic field. Every 11-year cycle, the north magnetic pole and south magnetic pole on the Sun reverse polarity; what had been a north magnetic pole is then a south magnetic pole and vice versa. This changeover occurs a year or two after the number of sunspots has reached its maximum.

Observer´s Log