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active region  A region of the Sun’s surface layers that has a large magnetic field and in which sunspots, flares, and prominences preferentially occur.
chromosphere  The part of the Sun’s atmosphere between the photosphere and the corona.
convection zone  The outer part of the Sun’s interior in which convection occurs.
coronal hole  A low-density, dim region in the Sun’s corona. Coronal holes occur in regions of open magnetic field lines where gases can flow freely away from the Sun to form the solar wind.
coronal mass ejection  A blast of gas moving outward through the Sun’s corona and into interplanetary space following the eruption of a prominence.
deuterium  An isotope of hydrogen. The nucleus of a deuterium atom is a deuteron.
differential rotation  Rotation in which the rotation period of a body varies with latitude. Differential rotation occurs for gaseous bodieslike the Sun or for planets with thick atmospheres.
filament  A dark line on the Sun’s surface when a prominence is seen projected against the solar disk.
granule  A bright convective cell or current of gas in the Sun’s photosphere.
heliopause  The boundary of the heliosphere, where the solar wind merges into the interstellar gas.
helioseismology  A technique used to study the internal structure of the Sun by measuring and analyzing oscillations of the Sun’s surface layers.
heliosphere  The region of space dominated by the solar wind and the Sun’s magnetic field.
Kelvin-Helmholtz time  The time it would take a star to contract from infinite diameter down to the main sequence while radiating away the gravitational energy released during contraction.
limb darkening  The relative faintness of the edge of the Sun’s disk (limb) compared with the center of the Sun’s disk.
Maunder minimum  A period of few sunspots and low solar activity that occurred between 1640 and 1700.
mode of oscillation  A particular pattern of vibration of the Sun.
neutrino  A particle with no charge and probably no mass that is produced in nuclear reactions. Neutrinos pass freely through matter and travel at or near the speed of light.
opacity  The ability of a substance to absorb radiation. The higher the opacity, the less transparent the substance is.
penumbra  The outer part of the shadow of a body where sunlight is partially blocked by the body.
perihelion  The point in the orbit of a body when it is closest to the Sun.
photosphere  The visible region of the atmosphere of the Sun or another star.
polarity  The property of a magnet that causes it to have north and south magnetic regions.
prominence  A region of cool gas embedded in the corona. Prominences are bright when seen above the Sun’s limb, but appear as dark filaments when seen against the Sun’s disk.
proton-proton chain  A series of nuclear reactions through which stars like the Sun produce energy by converting hydrogen to helium. Named because the first reaction in the series is the reaction of one proton with another.
solar flare  An explosive release of solar magnetic energy.
spicule  A hot jet of gas moving outward through the Sun’s chromosphere.
sunspot  A region of the Sun’s photosphere that appears darker than its surroundings because it is cooler.
sunspot cycle  The regular waxing and waning of the number ofspots on the Sun. The amount of time between one sunspot maximum and the next is about 11 years.
sunspot group  A cluster of sunspots.
supergranulation  The pattern of very large (15,000 to 30,000 kmin diameter) convective cells in the Sun’s photosphere.
umbra  (A) The inner portion of the shadow of a body, within which sunlight is completely blocked. (B) The dark central portionof a sunspot.
Zeeman effect  The splitting of a spectral line into two or more components when the atoms or molecules emitting the line are located in a magnetic field.

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