 |
|
The Earth-Sun distance is defined as the astronomical unit, and is equal to  km (IAU 1976). The
Sun's luminosity is
 J s-1, the solar mass is
 kg, and the
solar radius is
 m. The sun is approximately 4.6 billion years old, and will remain on the
main sequence for another 4.4 billion years.
The sun's atmosphere consists of the chromosphere, corona, and photosphere. The photosphere is the bright visible
surface of the sun, with a surface temperature of 5,800 K. The chromosphere consists of the bright gases just above the
photosphere of the sun and is 10,000-15,000 km thick. The corona is the sun's faint (and hot) outer atmosphere
from  (about 20,000 km above the photosphere) to at least several solar radii. The temperature is about 1.5
MK. The surface of the sun consists of convection cells termed granules. The granules have diameters of 1000 km and lifetimes
of 5-10 min and are observable in visible light. Superimposed upon the granules are supergranules (observed by Doppler shift
measurements) which have diameters of 30,000 km and lifetimes of 20-25 h. The Doppler measurements show that the motion is
entirely horizontal, with negligible vertical velocity. Why the sun should have only two convective length scales is not well
understood. Magnetically heated filamentary regions which appear near the limb of the sun are called faculae. The
continuum opacity of the sun is provided by bound-free and free-free transitions of the hydrogen ion  . This was
proposed by R. Wildt in 1940, and shown to be quantum mechanically consistent with observations by Chandrasekhar  in
1945.
The sun's magnetic field has a surface strength of 2 gauss. It reverses with a period of 22 y (twice the period of the
sunspot cycle). The sun rotates with a period of 24 days at the equator (corresponding to 27.2753 days as seen
from the Earth) and 28 days near the poles. The number of rotations of the sun is counted using the
Carrington rotation number. The sun's rotation appears to be constant on radial cones. Helioseismology,
the study of oscillations of the sun indicates that the rotation persists at least 200,000 km down.
Dark, cool sunspots appear on the sun's surface often in pairs. They have magnetic field strengths
of 100-1000 gauss and lifetimes of 2 months. The number of sunspots varies with a period of 11 y.
During each cycle, sunspots migrate from high latitudes to the equator. Sunspots
may affect Earth's climate, as deduced from the low number present during the little ice age of 1640-1710. During a
solar cycle, all sunspot pairs in the northern hemisphere have the leading spot north and the trailing spot south
(it is reversed in the southern hemisphere). Polarities are then reversed during the next solar cycle. An apparent
80-90 year cycle of long-term variations in solar activity is called the Gleisberg cycle.
The sun's output changes over the period of several days, as measured by the Solar Max satellite. The variation over
the solar cycle was observed to be 0.04% between 1980 and 1986, when the flux at the Earth (known as the solar
constant declined from 1368.5 to 1367 W m-2. The flux may be influenced by sunspots crossing
the disk. Large sunspots groups moving across the sun's equator temporarily decrease the flux. This
occurred April 3-10, 1980 when the Solar Maximum Mission on Nimbus 7 revealed a decrease of about 1.5 W m-2
during passage of a large sunspot group across the sun. However, faculae surrounding sunspots
cause a general small solar flux increase!
An interplanetary dust ring was observed around the sun in the mid-1960s.
 Analemma, Annular Eclipse, Anticrepuscular Ray, Aphelion, Astronomical Twilight, Astronomical Unit, Atmospheric Phenomena, Babcock's Dynamo Hypothesis, Carrington
Rotation Number, Chromosphere, Circumzenithal Arc, Civil Twilight, Conjunction,
Corona, Crepuscular Ray, Day, Eclipse, Elongation, Equinox, Faculae,
Faint Early Sun Paradox, First Point in Aries, Green Flash, Heliographic Coordinates,
Helioseismology, Inferior Conjunction, Lunar Eclipse, Mean Solar Time, Midnight,
Mock Sun, Neap Tide, Noon, Opposition, Parsec, Partial Eclipse,
Perihelion, Photosphere, Saros Cycle, Season, Solar Constant, Solar Day,
Solar Eclipse, Solar System, Solar Wind, Solstice, Spring Tide, Star,
Sun Pillar, Sundog, Sunrise, Sunset, Sunspot, Superior Conjunction,
Syzygy, Tide, Total Eclipse, Tropic of Cancer, Tropic of Capricorn, Twilight,
Year, Zero Point of Longitude, Zodiac, Zodiacal Light
Arnett, W. "The Nine Planets: The Sun." Nine Planets.
Bray, R. J.; Loughhead, R. E.; and Durrant, C. J. The Solar Granulation, 2nd ed. Cambridge, England: Cambridge University Press, 1984.
Cox, A. N.; Livingston, W. C.; and Matthews, M. S. Solar Interior and Atmosphere. Tucson, AZ: University of Arizona Press, 1991.
Duffett-Smith, P. "The Sun." In Practical Astronomy with Your Calculator, 3rd ed. Cambridge, England: Cambridge University Press, pp. 83-100, 1992.
Foukal, P. Solar Astrophysics. New York: Wiley, 1990.
Golub, L. and Pasachoff, J. M. Nearest Star: The Exciting Science of Our Sun. Cambridge, MA: Harvard University Press, 2001.
Gribbin, J. R. Blinded by the Light: The Secret Life of the Sun. New York: Harmony Books, 1991.
Hufbauer, K. Exploring the Sun: Solar Science Since Galileo. Baltimore, MD: Johns Hopkins University Press, 1991.
Kippenhahn, R. Discovering the Secrets of the Sun. New York: John Wiley and Sons, 1994.
Kuiper, G. P. (Ed.). The Solar System, Vol. 1: The Sun. Chicago: University of Chicago Press, 1953.
Lang, K. R. Sun, Earth, Sky. New York: Springer-Verlag, 1995.
McLean, D. J. and Labrum, N. R. (Ed.). Solar Radiophysics: Studies of Emission from the Sun at Metre Wavelengths.
Cambridge, England: Cambridge University Press, 1985.
Menzel, D. H. Our Sun, rev. ed. Cambridge, MA: Harvard University Press, 1959.
National Oceanographic and Atmospheric Administration. "Daily Total Solar Irradiance." http://www.ngdc.noaa.gov/stp/SOLAR/IRRADIANCE/irrad.html.
Noyes, R. W. The Sun, Our Star. Cambridge, MA: Harvard University Press, 1982.
Pasachoff, J. M. The Complete Idiot's Guide to the Sun. Alpha Books, 2003.
Phillips, K. Guide to the Sun. Cambridge, England: Cambridge University Press, 1992.
Priest, E. R. (Ed.). Solar Flare Magnetohydrodynamics. New York: Gordon and Breach, 1981.
Ryan, J. M. et al. . High-Energy Solar Phenomena.
Sonett, C. P.; Giampapa, M. S.; and Matthews, M. S. The Sun in Time. Tucson, AZ: University of Arizona Press, 1991.
Stenflo, J. O. Solar Magnetic Fields: Polarized Radiation Diagnostics. Norwell, MA: Kluwer, 1994.
Stix, M. The Sun: An Introduction, corrected 2nd printing. Berlin: Springer-Verlag, 1991.
Strong, K. T.; Saba, J. L. R; Haisch, B. M.; and Schmelz, J. T. (Eds.).
The Many Faces of the Sun: A Summary of Results from NASA's Solar Maximum Mission. New York: Springer-Verlag, 1999.
Sturrock, P. A. (Ed.). Physics of the Sun, Vol. 1: The Solar Interior. Dordrecht, Netherlands: Reidel, 1986.
Sturrock, P. A. (Ed.). Physics of the Sun, Vol. 2. Dordrecht, Netherlands: Reidel, 1986.
Sturrock, P. A. (Ed.). Physics of the Sun, Vol. 3. Dordrecht, Netherlands: Reidel, 1986.
 Weisstein, E. W. "Books about Solar Astronomy."
http://www.ericweisstein.com/encyclopedias/books/SolarAstronomy.html.
Wilson, P. R. Solar and Stellar Activity Cycles. Cambridge, England: Cambridge University Press, 1994.
Zirin, H. Astrophysics of the Sun. Cambridge, England: Cambridge University Press, 1988.
© 1996-2007 Eric W. Weisstein
|
