The Sun Living in the solar system, we have the chance to study, at close range, perhaps the most common type of cosmic object—a star. Our Sun is a star, and a fairly average star at that, but with one unique feature: it is very close to us—some 300,000 times closer than our next nearest neighbor, Alpha Centauri. Whereas Alpha Centauri is 4.3 light years distant, the Sun is only 8 light minutes away from us. Consequently, astronomers know far more about the properties of the Sun than about any of the other distant points of light in the universe. A good fraction of all our astronomical knowledge is based on modern studies of the Sun. Just as we studied our parent planet, Earth, to set the stage for our exploration of the solar system, we now study our parent star, the Sun, as the next step in our exploration of the universe.
Physical Properties • The Sun's mass, 2.0x1030 kg, follows from application of Newton's laws of motion and gravity to the observed orbits of the planets. • The Sun's radius, roughly 700,000 km, is most directly determined by measuring its angular size (0.5° ) and then employing elementary geometry. • The average solar density derived from these quantities, is ~ 1400 kg/m3, quite similar to that of the jovian planets and about one-quarter the average density of Earth.
Size • Having a radius of more than 100 Earth radii, a mass of more than 300,000 Earth masses, and a surface temperature well above the melting point of any known material, the Sun is clearly a body very different from any other we have encountered so far. • In Perspective: A sand grain is to a beach ball, as a beach ball is to the earth, as the earth is to the sun.
Photosphere • The Sun has a surface of sorts—not a solid surface (the Sun contains no solid material) but rather that part of the brilliant gas ball we perceive with our eyes or view through a heavily filtered telescope. This "surface"—the part of the Sun that emits the radiation we see—is called the photosphere.
Different Wavelength Views of Our Sun What you see depends upon what range of wavelengths you look at.
Solar Observations • Soho http://sohowww.nascom.nasa.gov/ • Trace http://vestige.lmsal.com/TRACE/ • Yohkoh http://www.lmsal.com/SXT/homepage.html • Observatories • NSO Sun Spot http://www.sunspot.noao.edu/latest_solar_images.html • Big Bear http://www.bbso.njit.edu/ • Mona Loa Solar http://mlso.hao.ucar.edu/index.html#CURRENT
Rotation • The solar rotation period is found by timing sunspots and other surface features as they traverse the solar disk. These observations indicate that the Sun rotates in about a month, but it does not do so as a solid body. Instead, it spins differentially—faster at the equator and slower at the poles, like Jupiter and Saturn.
Solar Rotation • Using Sun Spots
Sunspot Magnetic Field Sunspots are areas with large magnetic fields.
Sunspot Pairs Like poles of magnets.
Kinks • Differential rotation kinks up magnetic field lines.
Prominence • During a Solar Eclipse
Changes in Latitude Latitude changes in spot locations during a solar cycle.
Temperature • The Sun's surface temperature is measured by applying the radiation laws to the observed solar spectrum. The distribution of solar radiation has the approximate shape of a blackbody curve for an object at about 5800 K. The average solar temperature obtained in this way is known as the Sun's effective temperature.
Heat Transfer Radiation, Convection, Conduction
Radiation Energy transport via electromagnetic waves
Convection Energy transport by mass motion
Conduction Energy transport by vibrational translation The jostling of atoms and molecules in close proximity in a solid, especially one with high conductivity.