active Sun
The group of solar phenomena that vary with time, such as active regions and their phenomena.
chromosphere
The part of the atmosphere of the Sun (or another star) between the photosphere and the corona. It is probably entirely composed of spicules and probably roughly corresponds to the region in which mechanical energy is deposited.
convection
The method of energy transport in which the rising motion of masses from below carries energy upward in a gravitational field. Boiling is an example.
core
The central region of a star or planet.
corona
The outermost region of the Sun (or of other stars), characterized by temperatures of millions of kelvins.
coronal holes
Relatively dark regions of the corona having low density; they result from open field lines.
filaments
A feature of the solar surface seen in H-alpha as a dark wavy line; a prominence projected on the solar disk.
flare
An extremely rapid brightening of a small area of the surface of the Sun, usually observed in H-alpha and other strong spectral lines and accompanied by x-ray and radio emission.
general theory of relativity
Einstein's 1916 theory of gravity.
granulation
Convection cells on the Sun about 1 arc sec across.
magnetic-field lines
Directions mapping out the direction of the force between magnetic poles; the packing of the lines shows the strength of the force.
Maunder minimum
The period 1645-1715, when there were very few sunspots, and no periodicity, visible.
penumbra
(a) For an eclipse, the part of the shadow from which the Sun is only partially occulted; (b) of a sunspot, the outer region, not as dark as the umbra.
photosphere
The region of a star from which most of its light is radiated.
prominence
Solar gas protruding over the limb, visible to the naked eye only at eclipses but also observed outside the eclipses by its emission-line spectrum.
quiet Sun
The collection of solar phenomena that do not vary with the solar activity cycle.
solar atmosphere
The photosphere, chromosphere, and corona of the Sun.
solar wind
An outflow of particles from the Sun representing the expansion of the corona.
solar-activity cycle
The 11-year cycle with which solar activity like sunspots, flares, and prominences varies.
streamers
Coronal structures at low solar latitudes.
sunspot
A region of the solar surface that is dark and relatively cool; it has an extremely high magnetic field.
sunspot cycle
The 11-year cycle of variation of the number of sunspots visible on the Sun.
umbra (umbrae)
(a) Of a sunspot, the dark central region; (b) of an eclipse shadow, the part from which the Sun cannot be seen at all.
white light
All the light of the visible spectrum together.