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Sun
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.
Physical Appearance
Solar Surface
Sunspots
The Sun in X-ray
The Sun in Infra-red (H-alpha)
The Sun in UltraViolet (Helium)
The Sun in Radio
Different Wavelength Views of Our Sun
What you
see depends
upon what
range of
wavelengths
you look at.
Solar Observations
• Soho
• Trace
• Yohkoh
http://sohowww.nascom.nasa.gov/
http://vestige.lmsal.com/TRACE/
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.
Rotation
Solar Rotation
• Using Sun Spots
Sunspot Magnetic Field
Sunspots are areas with
large magnetic fields.
Sunspot Pairs
Like poles of
magnets.
Differential Rotation
Kinks
• Differential rotation kinks up magnetic field lines.
Flares
Prominence
• During a Solar Eclipse
Sunspot Counts
Maunder Minimum
Spot Cycle
Changes in Latitude
Latitude changes
in spot locations
during a solar cycle.
Sunspot Latitude
Aurora
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.
Convection
Interior Structure
Radiative Interior
Random Walk
Interior Structure