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Astronomy: The Sun
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The Sun is a typical star like many other stars in the universe. It consits of 71% hydrogen and 27% helium and some other materials like carbon, nitrogen and oxygen. Inside its core a temparature of 15 million oC (27 million oF) exists with a density of 9400 lb/ cubic feet (150.000 kg/cubic metre). On the visible surface (the photosphere) a temperature exists of only 5.500 oC. In the core the hydrongen nuclei combine to form helium-4, a little of the mass is lost in this process and this sets free a great amount of energy. This energy is transported from the core for 85% by photons. This process is very slow and can take up to hunderd of thousands of years. The remaining 15% tranport of this energy to the Sun's surface is by convection. Hot gas rises to the surface and cooler gas descends down. The hot gas bubbles in bubbles of thousands of kilometers in size on the surface, which is called granulation. Sunspots and the magnetosphereOn the surface sunspots can be observed. These are cooler regions of the Sun with a temperature as low as 3.700 oC. Because they are relatively cool they appair as dark spots on the surface.The sunspots mostly come in pairs that are joint together by inverse magnetic fields (like two magnets stuck together) and can have sizes from a few hunderd kilometres across to ten times the size of Earth. The larger sunspots can last months. Twentytwo year cycleAt the end of the 19th century an astronomer named Richard Carrington discovered that sunspots at the equator move faster than sunspots at the both poles. The Sun rotates in 25.5 days, but like gas planets it is clear that the Sun does not rotate as a whole.Carrington registered sunspots for years and discovered an eleven year pattern in frequency and behaviour of the sunspots. The Sun has a strong magnetoshpere with magnetic lines running from pole to pole. Because of the faster rotation of the Sun at the equator the magnetic lines get distorted and sunspots are visible from time to time. The magnetosphere gets more and more distorted during this eleven years until the point that all magnetic fields collapse and polarity is reversed. This also lasts for eleven years, so the Sun in fact has a twentytwo year cycle. The sunspots, solar flames and coronal mass ejections are distortions and can be seen as symptons of this twentytwo year cycle. Corona and Solar windThe solar wind is a flow of sun material at a few hundred kilometres per second into space. The solar wind follows patterns, leaves the sun through so-called coronal holes and follows complex lines from the sun's magnetosphere. |
Do you want to see live Sun images?
Watch this live green filtered image from SOHO from the solar activity in a popup window. Or watch sunspots in this orange SOHO image in a popup window.  In May 2003 Mercury (dark dot on the right) made a rare crossing in front of the Sun. Also visible are sunspots, granulation and solar flares.  Solar activity in closeup taken in June 2003 in enhanced colors.  14 September 1999, a huge coronal mass ejection recorded by Soho  The corona is visible during the few minutes of totality of a solar eclipse |
PlanetsThe Sun has nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus and Pluto.The Sun is so massive compared to the planets; it accounts for 99 % of all the mass in our solar system. Of these planets Jupiter is the biggest. When our solar system only consisted of protoplanetary disks from which the planets were formed, Jupiter was to small to start fusion and become a (brown)star. 
In that scenario Jupiter would have collected enough mass to be a star itself and had an even bigger gravity and thus bigger influence on the other planets, asteriods and meteors. And last but not least the temperature in the solar system would have been substantial warmer; it is thinkable that (human) life on Earth would be impossible. Solar eclipsesA solar eclipse occures on average six times in ten years around the globe. By coincidence the Moon has roughly the size of the Sun seen from Earth.When the Moon passes in front of the Sun, a solar eclipse occures and the Sun's corona is visible. Because of the tides on Earth the rotation of our planet slows down. And our Moon therefore slowly moves away: 3,8 centimeters every year. This means that in thousand of years from now a solar eclipse will be far less intense; the Sun will only be partially masked by the Moon. And partial solar eclipses are hardly noticable without the help of especially sun designed astronomer tools. Related subjects>> How are a star and planets formed?>> Different types of stars >> Solar observatory Skylab >> Solar observatory SOHO >> Spacecraft Genesis >> Other stars near the Sun >> Our Milky Way Galaxy |
 This partial solar eclipse was photographed in 1992 from Hawaii |
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