Sun on the Boil
Heat energy can generally be transferred by three different processes—conduction, convection
and radiation. Conduction is usually associated with the transfer of heat through solid matter:
molecules vibrate but are unable to move about. An example is the conduction of heat energy
from the bottom of a hot pan to the metal in its handle. (Be careful in your instructions to students
to ensure they don’t demonstrate this phenomenon!) Convection is the transfer of heat energy
through a liquid or gas by its bulk movement. Meteorologists use convection to refer to the up
and down movements of air, for example the formation of cumulus clouds in summer. When the
Sun goes down, and the heating of the Earth's surface ceases, the convective cumulus clouds
rapidly disappear. Radiation is the transfer of heat energy by electromagnetic wave motion. This
can occur through air or the near vacuum of space, and is the means by which the Sun heats the
Earth. This radiation is simply light with wavelengths slightly longer than our unaided eyes can
detect. Night vision devices show the heat radiation so an infrared picture of a parking lot reveals
which cars have been recently driven. Because there is no solid matter in the Sun, conduction is
not a mechanism by which heat is transferred. However, convection and radiation play important
Beneath the visible disk of the Sun lie three separate regions, each defined by the processes that
occur there: the energy-generating core, the radiative zone, and the convection zone. Energy is
generated in the immense nuclear furnace of the core and diffuses outward by radiation through—
you guessed it—the radiative zone. An interface layer marks the boundary between the radiative
and the convection zones and is now believed to be the source of the Sun’s magnetic field, though
there’s still uncertainty about the exact process. The convection zone is the outermost layer and
extends up to the visible surface or photosphere. Heat energy trapped below the convectiion zone
causes its plasma to "boil" and convert up to the surface. Ultimately energy reaches the surface
much as heat reaches the top of a pot of boiling water.
These two simple Activities invite students to observe the characteristic motions associated with
convection, using liquid in a transparent glass container to provide a cross-sectional view
(something we can’t yet do with the Sun!), and by boiling a dense liquid providing a top-down
view of the shapes and motions of convection cells similar to those provided by spacecraft and
telescopes. These Activities may be done as demonstrations or by teams of 3-4 students as
equipment and classroom protocol permits.
Students will observe convection currents moving through a liquid
Students will describe and draw the characteristic circulation patterns which result
Students will compare the patterns they see with those occurring in the Sun’s photosphere and
recognize convection as one of the processes that shape the Sun’s visible disk
radiation, convection, conduction, circulation, cell
per student team for the beaker-food dye activity:
beaker, 100 mL or larger
candle or other heat source
for teacher demo or per student team:
medium sauce pan
hot plate or stove top
close-up white light images of the Sun’s surface from reference sources (print or online)
or a transparency made from the Blackline master provided.
Ask students to describe the processes which take place inside the Sun. How does energy
generated in the core move out to the Sun’s surface? How is energy from the Sun able to pass
through the near vacuum of space to heat the Earth? Are these processes different from those we
see on Earth? Show them images of the Sun’s surface: brainstorm about processes which can
cause such shapes and patterns. Hint that students who like to cook—boys and girls—may have a
slight advantage in coming up with answers!
Fill the beaker with cold water. Use a dropper to carefully add one drop of food coloring (try not
to stir the water). Observe what happens to the colored dye. Place the beaker above a heat source.
A single flame like a candle may provide the clearest example. The water nearest the heat source
should rise to the surface, spread out, cool, and then flow back down to the bottom carrying a
stream of the food coloring with it, thereby demonstrating a convection current in cross-section.
Have students time and draw the details of what happens.
Add oatmeal and water to a medium saucepan and slowly bring the mixture to a boil. For this
particular Activity a hot plate or burner providing multiple hot spots would be best. Have students
time and draw the details of what happens. They should be able to observe multiple places where
the hot mixture rises to the surface, cools and descends to be heated and repeat the journey once
again, in much the same way that convective cells of plasma rise to the solar surface.
After students have observed and recorded results, have them present the patterns on chalkboard,
posters, transparencies or computer projector. Ensure they recognize the distinctive mushroom
shape over a point source of heat, and material spreading out and cooling at the surface. Ask them
where they can see such shapes in Earth’s atmosphere (in the spreading tops of cumulus or storm
Discuss how in this Activity you can, in fact, see all 4 states of matter at work! A candle or burner
flame is a kind of low-energy plasma. The flame’s heat energy radiates across a short space to
warm the solid beaker or saucepan. Conduction through this solid matter transfers heat to the
liquid, producing the distinctive convection patterns they’ve recorded. And if you’ve done the
boiling oatmeal Activity, whiffs of steam will show liquid water becoming gaseous vapor.
Use print, online or video reference materials to show cross-sections of the Sun.
Technology savvy teachers and/or older students can attempt to videotape the boiling oatmeal
mixture. (Keep the camera a safe distance from the heat source!) Use an image processing
program such as NIH Image to create a short animation or Quicktime movie of the boiling
mixture, add false color, and the result will bear a striking resemblance to the actual turbulent
For examples of such images from Tim McCollum’s classes go to:
For alternate demonstrations of convection currents using larger and more complex apparatus (an
aquarium, and/or smoking tapers, see the LIVE FROM THE HUBBLE SPACE TELESCOPE
Teacher’s Guide, pp 35-36), also available online at http://passporttoknowledge.com/hst
The Solar Interior: excellent information and the latest on the problem of the missing neutrinos,
one of the great mysteries of solar astronomy, from NASA’s Marshall Space Flight Center Solar
SOHO Lesson Plans: to take this Activity a step further this is a good high school level activity,
developed by SOHO scientist Terry Kucera, who appears in the LIVE FROM THE SUN videos.
Stanford Solar Center: These movies of solar oscillations were taken by SOHO on September 1,
1996. The first link is definitely worth the 20 minute download time!!! (You have been warned.)