Sun

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Sun

  1. 3. Basic Information <ul><li>The Sun is the star at the center of the Solar System . The Sun has a diameter of about 1,392,000 kilometres (865,000 mi) (about 109 Earths ), and by itself accounts for about 99.86% of the Solar System's mass ; the remainder consists of the planets (including Earth ), asteroids , meteoroids , comets , and dust in orbit . [10] About three-fourths of the Sun's mass consists of hydrogen , most of the rest is helium . Less than 2% consists of other elements, including iron , oxygen , carbon , neon , and others. [11] </li></ul><ul><li>The Sun's color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering . [12] It has a spectral class of G2V , informally designated a &quot;yellow star&quot; because the majority of its radiation is in the yellow-green portion of the visible spectrum. [13] The G2 indicates its surface temperature of approximately 5,780 K (5,510 °C.) The V ( Roman five ) in the spectral class indicates that the Sun, like most stars, is a main sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. Once regarded as a small and relatively insignificant star, the Sun is now presumed to be brighter than 85% of the stars in the galaxy , most of which are red dwarfs . [ </li></ul>
  2. 4. <ul><li>The Sun's hot corona continuously expands in space creating the solar wind , a hypersonic stream of charged particles that extends to the heliopause at roughly 100 AU . The bubble in the interstellar medium formed by the solar wind, the heliosphere , is the largest continuous structure in the Solar System. [18] [19] </li></ul><ul><li>The Sun is currently traveling through the Local Interstellar Cloud in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way Galaxy. Of the 50 nearest stellar systems within 17 light-years from the Earth, the Sun ranks 4th [20] in mass. The Sun orbits the center of the Milky Way galaxy at a distance of approximately 24,000–26,000 light years from the galactic center , completing one clockwise orbit, as viewed from the galactic north pole , in about 225–250 million years. </li></ul><ul><li>The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though this varies as the Earth moves from perihelion in January to aphelion in July. [21] At this average distance, light travels from the Sun to the earth in about 8 minutes 19 seconds. The energy from this sunlight supports almost all life on Earth via photosynthesis , [22] and drives the Earth's climate and weather . The enormous impact of the Sun on the Earth has been recognized since pre-historic times , and the Sun has been regarded by some cultures as a deity . An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little notion of the Sun's physical composition and source of energy. This understanding is still developing; there are a number of present-day anomalies in the Sun's behavior that remain unexplained. </li></ul>
  3. 5. Characteristics <ul><li>The Sun is a G-type main sequence star comprising about 99.86% of the total mass of the Solar System . It is a near-perfect sphere, with an oblateness estimated at about 9 millionths, [23] which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles . This behavior is known as differential rotation , and is caused by convection in the Sun and the movement of mass, due to steep temperature gradients from the core outwards. This mass carries a portion of the Sun’s counter-clockwise angular momentum, as viewed from the ecliptic north pole, thus redistributing the angular velocity. The period of this actual rotation is approximately 25.6 days at the equator and 33.5 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days. [24] The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. The tidal effect of the planets is even weaker, and does not significantly affect the shape of the Sun. </li></ul>
  4. 6. <ul><li>The Sun is a Population I , or heavy element-rich, [note 1] star. [26] The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae . [27] This is suggested by a high abundance of heavy elements in the Solar System , such as gold and uranium , relative to the abundances of these elements in so-called Population II (heavy element-poor) stars. These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star. [26] </li></ul><ul><li>The Sun does not have a definite boundary as rocky planets do, and in its outer parts the density of its gases drops approximately exponentially with increasing distance from its center. [28] Nevertheless, it has a well-defined interior structure, described below. The Sun's radius is measured from its center to the edge of the photosphere . This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light, and is therefore the surface most readily visible to the naked eye . [29] </li></ul><ul><li>The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation . However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves ( infrasound ) traversing the Sun's interior to measure and visualize the star's inner structure. [30] Computer modeling of the Sun is also used as a theoretical tool to investigate its deeper layers. </li></ul>
  5. 7. Trivia <ul><li>Comets' tails point away from the Sun at all times. So, when a comet is moving away from the Sun, its tail is actually leading. Comet tails are caused by dust and gas being lost from the comet and then pushed away from the Sun by the solar wind (charged particles moving out from the Sun) and by radiation pressure from the Sun. Lightning is 5 times hotter than the Sun. The Earth is actually closer to the Sun during the Northern hemisphere's winter (when the weather is colder), and further away during the warm summer months! Our sun is expected to last about 5 billion more years. It has already been in existence about 4.5 billion years. The sun is 93 million miles from earth, yet it's 270,000 times closer than the next nearest star. The sun contains 99% of all the mass in our solar system. During a total solar eclipse, local temperatures can drop up to 20F degrees. A cosmic year is the amount of time it takes the Sun to revolve around the center of the Milky Way, about 225 million years. </li></ul>
  6. 8. The Sun's Outer Layers <ul><li>Only the Sun's outer layers, collectively referred to as the solar 'atmosphere', can be observed directly. There are distinct regions to the solar atmosphere: the photosphere, the chromosphere, and the corona . These three regions have substantially different properties from each other, with regions of gradual transition between them. </li></ul>
  7. 9. Photosphere <ul><li>The Sun has basically the same chemical elements as found on Earth. However, the Sun is so hot that all of these elements exist in the gaseous state.There is not really a &quot;surface&quot; to the Sun. Think of it this way: the Sun is a bunch of gas which gets denser and denser as you move from space toward the solar core. The photosphere would then represent the depth at which we can see no deeper toward the core. Think of what a thick cloud looks like when you look down on it from an airplane - it looks solid, but it isn't. </li></ul><ul><li>The Sun's atmosphere changes from being transparent to being opaque over a distance of only a few hundred kilometers. This is remarkable given the size of the Sun, and represents such a huge change that we often think of it as a true boundary. When we speak of the size of the Sun, we usually mean the size of the region surrounded by the photosphere. The photosphere is slightly different from one place on the Sun to another, but in general is has a pressure about a few hundredths of the sea-level pressure on Earth, a density of about a ten-thousandth of the Earth's sea-level atmospheric density, and a temperature in the range 4500-6000 Kelvin . </li></ul>
  8. 10. The Chromosphere <ul><li>The gases which extend away from the photosphere make up the chromosphere. These gases are transparent to most visible radiation. The chromosphere is about 2500 km thick. The density of the gases decreases as you move away from the photosphere into the chromosphere, but the temperature increases! From the bottom to the top of the chromosphere, the average temperature goes from 4500 to 10,000 Kelvin! Needless to say, this rise was not anticipated by scientists when they first measured it. Throughout the rest of the Sun, temperature decreases as you move further away from the core. </li></ul>
  9. 11. The Corona <ul><li>The chromosphere merges into the outermost region of the Sun's atmosphere, the corona. The corona extends for millions of miles into space above the photosphere. Usually, we cannot see the corona because of the brightness of the photosphere. However, during a total solar eclipse , the corona shines beautifully against the dark sky. The corona has a density about 0.0000000001 times that of the Earth's sea-level atmosphere. It is very hot - millions of Kelvin. Because of this high temperature, the bulk of the radiation from the corona is emitted at ultraviolet and X-ray wavelengths. Magnetic fields on the Sun seem to play an important part in heating the gas to such a high temperature. However, the exact way that this happens is not well understood. The image you see to the left was taken during a solar eclipse in 1980; light from the photosphere is blocked out by the Moon (the dark disk). </li></ul>
  10. 12. The Corona <ul><li>The chromosphere merges into the outermost region of the Sun's atmosphere, the corona. The corona extends for millions of miles into space above the photosphere. Usually, we cannot see the corona because of the brightness of the photosphere. However, during a total solar eclipse , the corona shines beautifully against the dark sky. The corona has a density about 0.0000000001 times that of the Earth's sea-level atmosphere. It is very hot - millions of Kelvin. Because of this high temperature, the bulk of the radiation from the corona is emitted at ultraviolet and X-ray wavelengths. Magnetic fields on the Sun seem to play an important part in heating the gas to such a high temperature. However, the exact way that this happens is not well understood. The image you see to the left was taken during a solar eclipse in 1980; light from the photosphere is blocked out by the Moon (the dark disk). </li></ul>

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