144 PART 2^ |^ THE STARS
Granuleb Sinking gas Rising gasaa Visual-wavelength imageVisual-wavelength image■ Figure 8-2
(a) This ultra-high-resolution image of the photosphere shows granulation.
The largest granules here are about the size of Texas. (b) This model explains
granulation as the tops of rising convection currents just below the photo-
sphere. Heat fl ows upward as rising currents of hot gas and downward as
sinking currents of cool gas. The rising currents heat the solar surface in
small regions seen from Earth as granules.
about 10 percent of the way to the sun’s center. With fantastically
effi cient insulation, you could fl y a spaceship right through the
photosphere.
Th e spectrum of the sun is an absorption spectrum, and that
can tell you a great deal about the photosphere. You know from
Kirchhoff ’s third law that an absorption spectrum is produced
when the source of a continuous spectrum is viewed through a
gas. Th e deeper layers of the photosphere are dense enough to
produce a continuous spectrum, but atoms in the photosphere
absorb photons of specifi c wavelengths, producing absorption
lines of hydrogen, helium, and other elements.
In good photographs, the photosphere has a mottled appear-
ance because it is made up of dark-edged regions called granules.
Th e overall pattern is called granulation (■ Figure 8-2a). Each
granule is about the size of Texas and lasts for only 10 to 20 min-
utes before fading, shrinking, and being replaced by a new
This visible image of the sun shows a few sunspots and is cut
away to show the location of energy generation at the sun’s
center. The Earth–moon system is shown for scale. (Daniel Good)Celestial Profi le 1: Th e Sun
From Earth:
Average distance from Earth 1.000 AU ( 1.496 108 km)
Maximum distance from Earth 1.017 AU ( 1.521 108 km)
Minimum distance from Earth 0.983 AU ( 1.471 108 km)
Average angular diameter 0.53° (32 arc minutes)
Period of rotation 25.4 days at equator
Apparent visual magnitude −26.74Characteristics:
Radius 6.96 105 km
Mass 1.99 1030 kg
Average density 1.41 g/cm^3
Escape velocity at surface 618 km/s
Luminosity 3.83 1026 J/s
Surface temperature 5800 K
Central temperature 15 106 K
Spectral type G2 V
Absolute visual magnitude 4.83Personality Profi le:
In Greek mythology, the sun was carried across the sky in a golden
chariot pulled by powerful horses and guided by the sun god Helios.
When Phaeton, the son of Helios, drove the chariot one day, he lost
control of the horses, and Earth was nearly set ablaze before Zeus smote
Phaeton from the sky. Even in classical times, people understood that life
on Earth depends critically on the sun.