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The Solar Atmosphere Click on image for full size (65K JPEG)Image of the solar corona in white light (outer circle, blue and white) and X-Rays (inner circle, red, yellow, and black) on April 22, 1994, courtesy of the High Altitude Observatory and the Yohkoh Science team. The dashed circle is the solar radius. The visible solar atmosphere consists of three regions: the photosphere, the chromosphere, and the solar corona. Most of the visible (white) light comes from the photosphere, this is the part of the Sun we actually see. The chromosphere and corona also emit white light, and can be seen when the light from the photosphere is blocked out, as occurs in a solar eclipse. The sun emits electromagnetic radiation at many other wavelengths as well. Different types of radiation (such as radio, ultraviolet, X-rays, and gamma rays) originate from different parts of the sun. Scientists use special instruments to detect this radiation and study different parts of the solar atmosphere. The solar atmosphere is so hot that the gas is primarily in a plasma state: electrons are no longer bound to atomic nuclei, and the gas is made up of charged particles (mostly protons and electrons). In this charged state, the solar atmosphere is greatly influenced by the strong solar magnetic fields that thread through it. These magnetic fields, and the outer solar atmosphere (the corona) extend out into interplanetary space as part of the solar wind. Solar Activity Click on image for full size (125K JPEG)A coronal mass ejection and prominence eruption observed in white light from the SMM (Solar Maximum Mission) spacecraft, courtesy of the High Altitude Observatory. The time of each panel increases from left to right. The dashed inner circle in each panel is the solar radius, the occulting radius is at 1.6 solar radii. The Sun is not a quiet place, but one that exhibits sudden releases of energy. One of the most frequently observed events are solar flares: sudden, localized, transient increases in brightness that occur in active regions near sunspots. They are usually most easily seen in H-alpha and X-rays, but may have effects in the entire elecromagnetic spectrum. The X-ray brightness from a large flare often exceeds the X-ray output from the rest of the Sun. Another type of event, the coronal mass ejection, typically disrupt helmet streamers in the solar corona. As much as 1e13 (10,000,000,000,000) kilograms of material can be ejected into the solar wind. Coronal mass ejections propagate out in the solar wind, where they may encounter the Earth and influence geomagnetic activity. Coronal mass ejections are often (but not always) accompanied by prominence eruptions, where the cool, dense prominence material also erupts outward. All of these forms of solar activity are believed to be driven by energy release from the solar magnetic field. How this energy release occurs, and the relationship between different types of solar activity, is one of the many puzzles facing solar physicists today. The amount of solar activity on the Sun is not constant, and is closely related to the typical number of sunspots that are visible. The number of sunspots and the levels of solar activity vary with an 11 year period known as the solar cycleThe Fate of the Sun Click on image to watch animation. Image from White Dwarf Research Corp.In about 5 billion years, the hydrogen in the center of the Sun will start to run out. The helium will get squeezed. This will speed up the hydrogen burning. Our star will slowly puff into a red giant. It will eat all of the inner planets, even the Earth. As the helium gets squeezed, it will soon get hot enough to burn into carbon. At the same time, the carbon can also join helium to form oxygen. The Sun is not very big compared to some stars. It will never get hot enough in the center to burn carbon and oxygen. These elements will collect in the center of the star. Later it will shed most of its outer layers, creating a planetary nebula, and reveal a hot white dwarf star. Nearly 99 percent of all stars in the galaxy will end their lives as white dwarfs. By studying the stars that have already changed, we can learn about the fate of our own Sun. Solar Eclipses This is a diagram of a typical solar eclipse. During a total solar eclipse, the umbra reaches the Earth. During an annular eclipse, it does not. An eclipse occurs when the Moon passes in the path of the Sun and Earth.Click on image for full size (221K JPG)Windows Original An eclipse of the Sun occurs when the Earth passes through the Moon's shadow. A total eclipse of the Sun takes place only during a new moon, when the Moon is directly between the Sun and the Earth. When a total eclipse does occur, the Moon's shadow covers only a small portion of the Earth, where the eclipse is visible. As the Moon moves in its orbit, the position of the shadow changes, so total solar eclipses usually only last a minute or two in a given location. In ancient times, people were frightened by solar eclipses (even back then people realized that the Sun was essential to life on Earth). Now eclipses are of great interest to the public and to astronomers. Eclipses provide an opportunity to view the Sun's outer atmosphere, the solar corona. If you ever get to view a solar eclipse, make sure to never look at the Sun directly! Always use one of these safe techniques. Sun Reference Data Diameter: 1.4 million km (870,000 miles) Age: 4.5 billion years Mass: 330,000 x Earth Distance from Earth: 149.6 million km (93 million miles) Density: 1.41 (water=1) Distance to Nearest Star: 4.3 light years Solar Wind Speed: 3 million km/hr. Luminosity: 390 billion billion megawatts Solar Cycle: 8 - 11 years Temperature at surface: 5,500o C (9,932o F) Temperature at Core: 14 milliono C (22.5 milliono F) Temperature of Sunspots: 4,000o C (7,232o F) Rotation Period at Equator: 25 Earth days Rotation Period at Poles: 35 Earth days Solar Interior     This is an image of the sun's core. (Courtesy of the Sacramento Peak Observatory)(73K GIF)   The main regions of the solar interior. [110K JPEG]   A schematic view of the interior of the Sun. [239K GIF] Sun's Surface     These images show two Jupiter-sized sunspot groups on the face of the Sun (left) and an extreme close-up of a different, smaller sunspot group (right). [76K JPEG]   A closeup view of a typical pair of sunspots, with Earth superimposed to show scale. [72K JPEG] This is a region around a sunspot.(217K GIF)   The Sun at solar max and solar min, showing variation in sunspot counts. [62K JPEG] Ultraviolet image of a solar flare in Nov. 2003. [63K JPEG]   This is an image of a solar flare. (Courtesy of the Sacramento Peak Observatory)(163K GIF) This is an X-ray image of the sun taken with the Soft X-Ray Telescope (SXT) on the orbiting Yohkoh satellite. (Courtesy of Lockheed Palo Alto Research Laboratory)(101K GIF)   This is an X-ray image from the Yohkoh satellite.(172K GIF) This is a full disk H-alpha image of the sun. (From Learmonth, Australia)(139K GIF)   This is a small image of the Sun in visible light.(8K GIF) Rotating Sun with Sunspots [234K QuickTime movie]     Sun's Atmosphere     An ultraviolet image of two large prominences erupting from the Sun in March 2003. [120K JPEG]   A Coronal Mass Ejection (CME) blasts Comet NEAT as it passes the Sun in Feb. 2003. [91K JPEG] This is an image of Red Corona. (Courtesy of Sacramento Peak Observatory)(142K GIF)   This is an image of the corona. (Courtesy of the Sacramento Peak Observatory)(162K GIF) This is a white-light corona-meter image from the High Altitude Observatory Mauna Loa Solar Observatory. (Courtesy, High Altitude Observatory, National Center for Atmospheric Research (NCAR), Boulder, Colorado, USA. NCAR is sponsored by the National Science Foundation.)(123K GIF)     This is an X-ray image of the sun taken with the Soft X-Ray Telescope (SXT) on the orbiting Yohkoh satellite. (Courtesy of Lockheed Palo Alto Research Laboratory)(101K GIF)   This is an X-ray image from the Yohkoh satellite.(172K GIF) Coronal Mass Ejection (CME) erupting from the Sun in January 2002 [1M animated GIF]     Solar Eclipses     This is an image of a solar eclipse in 1994. The image was taken by the Newkirk White Light Coronal camera. (Image courtesy NCAR's High Altitude Observatory.)(36K JPG)   Another eclipse image. (Courtesy of Jay Keller)(204K GIF) Another image of an eclipse (Courtesy of Aris Multimedia Entertainment, Inc. 1994)(23K GIF)   Another image of an eclipse (Courtesy of Aris Multimedia Entertainment, Inc. 1994)(18K GIF) This is an image of an eclipse taken in Mexico showing little corona but a nice view of prominences. (Courtesy of Jay Keller)(135K GIF)   Another image of an eclipse. (Courtesy of NASA)(75K GIF) This is an eclipse of the sun in Mexico on 11 July 1991 behind light clouds. (Courtesy of Northern Lights Planetarium)(7K GIF)