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Absorption Line
A dark  line at a particular  wavelength of a  spectrum, formed when a cool, tenuous gas between a hot radiating source and the observer absorbs  electromagnetic radiation of that wavelength.
Advanced Composition Explorer. A spacecraft studying the  heliosphere and  cosmic rays.
Acoustic Wave
A wave for which pressure is the restoring force. Also known as a sound wave.
Active Region
An area of the Sun where the  magnetic fields are very strong. At  ultraviolet and  X-ray wavelengths they appear bright. In visible light they exhibit  sunspots.
Abbreviated . A unit of length equal to 10-8 cm (one-hundredth of a millionth of a centimeter). An Angstrom is on the order of the size of an atom.
Arc Degree 
A unit of angular measure in which there are 360 arc degrees in a full circle.
Arc Minute 
Abbreviated arcmin. A unit of angular measure in which there are 60 arc minutes in 1  arc degree
Arc Second
Abbreviated arcsec. A unit of angular measure in which there are 60 arc seconds in 1 arc minute and therefore 3600 arc seconds in 1  arc degree. One arc second is equal to about 725  km on the Sun.
Astronomical Unit
The average distance between the Earth and Sun, about 150 million kilometers (93 million miles).
A gaseous envelope surrounding a planet, or the visible layers of a star; also a unit of pressure (abbreviated atm) equal to the pressure of air at sea level on the Earth's surface.
Atmospheric reddening
Preferential  scattering of blue light over red by air particles so that an object appears redder than it actually is.
The smallest particle of an  element that exhibits the chemical properties of the element.
A display of colored light given off by collisions between charged particles trapped in a planet's  magnetic fields and atoms of atmospheric gases near the planet's magnetic poles. Aurora are visible on Earth as the aurora borealis or northern lights and the aurora australis or southern lights.

Big Bang model
A theory of the evolution of the  universe that postulates its origin, in an event called the Big Bang, from a hot,  dense state that rapidly expanded to cooler, less-dense states.
Blackbody spectrum
The continuous  spectrum emitted by a blackbody; the flux at each wavelength is given by Planck's law.
Black Hole
mass that has collapsed to such a degree that the escape velocity from its surface is greater than the speed of light, so that light is trapped by the intense gravitational field.
Blue shift
A decrease in the  wavelength of the  radiation emitted by an approaching celestial body as a consequence of the  Doppler effect; a shift toward the short-wavelength (blue) end of the  spectrum.

Celestial equator
An imaginary projection of the earth's pole onto the  celestial sphere; a point about which the apparent daily rotation of the stars takes place.
Celestial sphere
An imaginary sphere of very large radius centered on the earth, on which the celestial bodies appear fastened and against which their motions are charted.
Abbreviated C. A unit of temperature. Zero degrees Celsius is equal to 273  Kelvin. Also known as centigrade. Water freezes at 0°C and boils at 100°C. 
Degrees  Fahrenheit = Degrees Celsius *(9/5) +32.
Charge, Element, and Isotope Analysis System.
Instrument aboard  SOHO which analyzes the constituents of the  solar wind.
Centimeter-Gram-Second (abbreviated cm-gm-sec or cm-g-s). The system of measurement that uses these units for distance, mass, and time. 
Charge Coupled Device.
A semiconductor light detector which converts light to electrical impulses. Such detectors are used by a number of the instruments on  SOHO, including  EITLASCO, and  MDI.
A process by which one atom gives up electrons to a proton or another atom. For example, if a positively-charged proton encounters a neutral atom, the neutral atom may give up one of its electrons to the proton. The proton-electron pair would then be neutral, and the formerly neutral atom would be positively-charged.
The layer of the  solar atmosphere that is located above the  photosphere and beneath the transition region and the  corona. The chromosphere is hotter than the photosphere but not as hot as the corona.
Chromospheric Network
A large scale cellular pattern visible in  hydrogen-alpha and other parts of the spectrum associated with the  chromosphere. The network appears at the boundaries of  super-granulation cells and contains a magnetic field which has been swept to the edges of the cells by the flow of material in the cell.
Short for  Coronal Mass Ejection.
The transfer of energy via collisions of randomly moving atoms and  electrons.
Continuous spectrum
spectrum showing emission at all  wavelengths, unbroken by either absorption lines or  emission lines.
The physical up-welling of hot matter, thus transporting energy from a lower, hotter region to a higher, cooler region. A bubble of gas that is hotter than its surroundings expands and rises. When it has cooled by passing on its extra heat to its surroundings, the bubble sinks again. Convection can occur when there is a substantial decrease in temperature with height, such as in the Sun's  convection zone.
Convection Zone
A layer in a star in which  convection currents are the main mechanism by which energy is transported outward. In the Sun, a convection zone extends from just below the  photosphere to about seventy percent of the solar radius.
In solar astronomy, the innermost part of the Sun, where energy is generated by  nuclear reactions.
The outermost region of the Sun's  atmosphere, consisting of thin,  ionized gases at a  temperature of about 1,000,000  K. It is visible to the naked eye during a solar eclipse.
Telescope for observing the  corona. Often contains an occulting disk which covers the  disk of the Sun so that the corona may be more easily observed.
Coronal Diagnostic Spectrometer.
An  ultraviolet  spectrometer aboard  SOHO.
Coronal Hole
An area of the corona which appears dark in X-rays and ultraviolet light. They are usually located at the poles of the Sun, but can occur other places as well. The  magnetic field lines in a coronal hole extend out into the solar wind rather than coming back down to the Sun's surface as they do in other parts of the Sun.
Coronal interstellar gas
High-temperature interstellar  plasma made visible by its  X-ray emission.
Coronal Mass Ejection
A huge magnetic bubble of  plasma that erupts from the Sun's  corona and travels through space at high speed.
Coronal Streamer
Large scale magnetic structures observed in the Sun's  corona.
Cosmic blackbody microwave radiation
Radiation with a  blackbody spectrum at a  temperature of about 2.7  K permeating the  universe; believed to be the remains of the primeval fireball in which the Universe was created.
Cosmic Ray
High energy charged particles traveling through interstellar space at nearly the velocity of light.
Cosmic rays
Charged atomic particles moving in space with very high energies (the particles travel close to the speed of light); most originate beyond the solar system, but some of low energy are produced in  solar flares.
The study of the nature and evolution of the physical universe.
Crab nebula
supernova remnant, located in the constellation Taurus, produced by the supernova explosion visible from earth in 1054 CE; a pulsar in the  nebula marks the neutron-star corpse of the exploded star.
Critical density
In  cosmology, the  density that marks the transition from an open to a closed  universe; the density that provides enough gravity to just bring the expansion to a stop after infinite time.

D Region
The lowest layer of the Earth's  ionosphere. It is between about 50 and 95 kilometers above Earth's surface. This is the layer which reflects radio waves. Also called the D Layer.
Dark cloud
An interstellar cloud of gas and dust that contains enough dust to blot out the light of stars behind it (as seen from the Earth). 
Deep Space Network
A NASA radio navigation network used to communicate with spacecraft beyond Earth's orbit.
Degenerate electron gas
An  ionized gas in which  nuclei and  electrons are packed together as much as possible, filling all possible low energy states, so that the perfect gas law relating pressure, temperature, and density no longer applies.
Degenerate neutron gas
Matter made up of  neutrons packed together as tightly as possible.
The amount of mass or number of particles per unit volume. In  cgs units mass density has units of gm cm-3. Number density has units cm-3 (particles per cubic centimeter).
Differential Rotation
The change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees per day) than do high latitudes (approx. 12 degrees per day).
The spreading of light as it passes a sharp edge of an opaque object.
Diffuse (bright) nebula
A cloud of  ionized gas, mostly hydrogen, with an  emission-line spectrum.
The visible surface of the Sun (or any heavenly body) projected against the sky.
Doppler Shift
A change in the  wavelength of  radiation received from a source because of its motion along the line of sight. A Doppler shift in the spectrum of an astronomical object is commonly known as a  redshift when the shift is towards longer wavelengths (the object is moving away) and as a  blueshift when the shift is towards shorter wavelengths (the object is approaching).
Something which converts energy of motion into an electric current.

E Region
The lowest layer of the Earth's  ionosphere. It is between about 95 and 130 kilometers above Earth's surface. Also called the E Layer.
From the Earth, the apparent yearly path on the  celestial sphere of the Sun with respect to the stars; also, the plane of the earth's orbit. 
Electromagnetic force
Electromagnetic force is one of the four forces of nature; particles with electromagnetic charge either attract or repel each other depending upon whether the two charges are opposite or identical.
Electromagnetic radiation
A self-propagating electric and magnetic wave, such as light, radio, ultraviolet, or infrared radiation; all types travel at the same speed and can be differentiated by  wavelength or  frequency.
Electromagnetic Spectrum
The entire range of all the various kinds or  wavelengths of  electromagnetic radiation, including (from short to long wavelengths)  gamma raysx-raysultraviolet, optical (visible), infrared, and radio waves.
A negatively charged elementary particle that normally resides outside (but is bound to) the  nucleus of an atom.
Electron Volt
Abbreviated eV. A unit of energy used to describe the total energy carried by a particle or  photon. The energy acquired by an  electron when it accelerates through a potential difference of 1 volt in a vacuum. 1 eV = 1.6 × 10-12  erg.
A substance that is made of  atoms with the same chemical properties, and which cannot be decomposed chemically into simpler substances.
Emission Line
A bright  line at a particular  wavelength of a  spectrum emitted directly by a hot gas.
Emission (bright) lines
Light of specific  wavelengths or colors emitted by  atoms; sharp energy peaks in a  spectrum caused by downward  electron transitions from a discrete quantum state to another discrete state.
Emission-line spectrum
spectrum containing only  emission lines.
Derived from experiment or observation.
Energetic Neutral Atoms (ENAs)
Atoms with no charge that move very quickly. These atoms have equal numbers of positively-charged protons and negatively-charged electrons. ENAs form when charged particles from the  solar wind travel outward and encounter atoms from the  interstellar medium. Because the ENAs are neutral, they do not react to any  magnetic fields. Some of these ENAs travel toward the inner solar system and are captured by the IBEX spacecraft.
cgs unit of energy equal to work done by a force of 1 dyne acting over a distance of 1 cm. 107 (ten million) erg s-1 (ergs per second) = 1 watt. Also, 1 Calorie = 4.2 × 1010 (42 billion) ergs.
The process of raising an  atom to a higher energy level.
The dimming of light when it passes through some medium, such as the earth's  atmosphere, or  interstellar material.

F Region
The lowest layer of the Earth's  ionosphere. It is between about 160 and 400 kilometers above Earth's surface. Also called the F Layer.
A wave mode generated by a surface gravity wave.
Bright regions of the photosphere visible in  white light near the limb of the Sun. They are brighter than their surroundings because they are higher in temperature and density.
Abbreviated F. A unit of temperature. In the Fahrenheit scale, water freezes at 32 °F and boils at 212 °F.
Degrees  Celsius = (Degrees Fahrenheit - 32)*5/9.
A structure in the  corona consisting of cool  plasma supported by  magnetic fields. Filaments are dark structures when seen against the bright solar  disk, but appear bright when seen over the solar  limb, Filaments seen over the limb are also known as  prominences.
Flare (Solar)
Rapid release of energy from a localized region on the Sun in the form of  electromagnetic radiation, energetic particles, and mass motions.
Flare Star
A member of a class of stars that show occasional, sudden, unpredicted increases in light. The total energy released in a flare on a flare star can be much greater that the energy released in a  solar flare.
Flash spectrum
The spectrum that appears immediately before the totality of a solar eclipse as the normal absorption spectrum is replaced briefly by the  corona's own  emission spectrum
The amount of energy flowing through a given area in a given amount of time, usually expressed as counts per area per second.
Frame of reference
A set of axes with respect to which the position or motion of something can be described, or physical laws can be formulated.
Free Electron
An  electron that has broken free of it's atomic bond and is therefore not bound to an atom.
The number of repetitions per unit time of the oscillations of an  electromagnetic wave (or other wave). The higher the frequency, the greater the energy of the radiation and the smaller the  wavelength. Frequency is measured in  Hertz
A nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy

A wave mode generated by a  gravity wave.
Galactic equator
The great circle along the line of the  Milky Way, marking the central plane of the  Galaxy
Galactic latitude
The angular distance north of south of the  galactic equator.
Galactic longitude
The angular distance along the  galactic equator from a zero point in the direction of the galactic center.
A huge assembly of stars (between one hundred and one million), plus gas and dust, which is held together by  gravity; the Galaxy, our own galaxy, containing the Sun.
Gamma Ray
The highest energy (shortest  wavelengthphotons in the  electromagnetic spectrum. Gamma rays are often defined to begin at 10  keV, although radiation from around 10 keV to several hundred keV is also referred to as hard  x-rays
A unit of magnetic field strength.
Geomagnetic Storm
A worldwide disturbance of the Earth's  magnetic field, associated with solar activity.
Geosynchronous Orbit 
The orbit of a satellite that travels above the Earth's equator from west to east so that it has a speed matching that of the Earth's rotation and remains stationary in relation to the Earth (also called geostationary). Such an orbit has an altitude of about 35,900  km (22,300 miles).
Giant molecular clouds
Large interstellar clouds, with sizes up to tens of  parsecs and containing 100,000 solar masses of material; found in the spiral arms of the  Galaxy, giant molecular clouds are the sites of massive star formation.
A roughly circular region on the Sun whose bright center indicates hot gases rising to the surface, and whose dark edges indicate cooled gases that are descending towards the interior. Individual granules appear and disappear on time scales of about 5 minutes and are typically about 1000 km.
In Newtonian terms, a force between masses that is characterized by their acceleration toward each other; the magnitude of the force depends directly on the product of the masses and inversely on the square of the distance between them; in Einstein's terms, the curvature of space-time.
Gravitational focusing
The directing of the paths of small masses by a larger one so that their paths cross, which enhances their accretion onto the larger mass.
Gravitational lens
The bending effect of a large mass on light rays so that they form an image of the source of light.
Gravitational redshift
The change to longer  wavelengths that marks the loss of energy by a  photon that moves from a stronger to a weaker gravitational field.
Gravity Wave
In fluid mechanics (and thus  helioseismology) this refers to a wave for which buoyancy is the restoring force.

Halo CME
CME pointed either towards or away from the Earth so that it looks roughly like a halo or ring around the Sun in images from a  coronagraph.
The boundary between the Sun's solar wind and the interstellar medium.
The study of the interior of the Sun by the analysis of its natural modes of oscillation.
The place where the solar wind slows down and begins to interact with the interstellar medium. The heliosheath has a few parts: the termination shock (the innermost part of the boundary), the heliopause (the outermost part of the boundary) and the part in between the inner and outer boundary.
The region around the Sun where the  solar wind dominates over the interstellar medium.
Abbreviated Hz. A unit of frequency equal to one cycle per second. One kHz = 1000 Hz. One MHz = 106 (one million) Hz. One GHz = 109 Hz. 
Hertzsprung-Russell diagram 
A diagram which plots temperature (or color) vs. luminosity for a population of stars.
High-velocity clouds
Clouds of gas associated with the  Galaxy, moving at speeds of hundreds of kilometers per second.
High-velocity stars
Stars in the Galaxy with velocities greater than 60 km/s relative to the Sun; they have orbits with high eccentricities, often at large angles with respect to the Galactic plane.
Hubble's Law
A description of the expansion of the  Universe, such that the more distant a  galaxy lies from us, the faster it is moving away; the relation, v = Hd, between the expansion velocity (v),and the distance (d) of a galaxy, where H is the Hubble constant.
Hubble constant
The proportionality constant relating velocity and distance in the  Hubble law; the value, now around 75 km/s/Mpc, changes with time as the Universe expands.
Hubble time
Numerically the inverse of the  Hubble constant; it represents, in order of magnitude, the age of the  universe.
Hydrogen Alpha
Also called H-alpha. Light emitted at a wavelength of 6563   from an atomic transition in hydrogen. This wavelength is in the red portion of the visible  spectrum and is emitted by plasma at about 10,000 K, mainly in the solar  chromosphere.

Infrared telescope
A telescope, optimized for use in the infrared part of the  spectrum, fitted with an infrared detector.
Intergalactic medium
The gas and dust found between the  galaxies.
Interstellar dust
Small (micrometers in diameter) solid particles in the  interstellar medium
Interstellar extinction curve
The amount of  extinction from  interstellar dust as a function of  wavelength.
Interstellar gas
Atomsmolecules, and  ions in the  interstellar medium.
Interstellar hydrogen
Hydrogen whose origin is from between the stars. This hydrogen is generally neutral, consisting of a single proton-electron pair.
Interstellar medium
All the gas and dust found between stars.
An  atom that has become electrically charged by the gain or loss of one or more  electrons.
The process by which  ions are produced, typically occurring by collisions with atoms or  electrons ("collisional ionization"), or by interaction with  electromagnetic radiation("photoionization"). 
Ionized gas
A gas that has been  ionized so that it contains free  electrons and charged particles; a  plasma if it is electrically neutral overall.
The region of the Earth's upper atmosphere (100 to 700 kilometers above the surface) containing a small percentage of  free electrons and  ions produced by  photoionization of the constituents of the atmosphere (oxygen and nitrogen) by solar ultraviolet radiation. The ionosphere significantly influences  radiowave propagation of frequencies less than about 30 MHz.
One of two or more atoms having the same number of  protons in its  nucleus, but a different number of  neutrons and, therefore, a different mass.
Having no preferred direction in space.
International Solar-Terrestrial Physics Science Initiative.
Collaborative effort by US, European, and Japanese space agencies to obtain coordinated, simultaneous investigations of the Sun-Earth space environment over an extended period of time. SOHO is a part of this program.

Abbreviated K. A unit of temperature with its zero point located at "absolute zero," the point at which the random motion of atoms and molecules ceases. Each degree has the same change as in Celsius, and zero degrees Celsius is at 273.16 K
Abbreviated km. 1 km = 1000 meters = 105cm = 0.62 mile.

Lagrange Points
The five gravitational balance points between two orbiting masses. The first Lagrange Point (L1) is in between the two bodies.
Large Magellanic Cloud (LMC)
A small  galaxy, irregular in shape, about 50  kpc from the Milky Way. 
A north-south coordinate measured on the surface of a sphere. It is the angular distance from the equator in the direction of one of the rotational poles.
A curved piece of glass designed to bringlight rays to a focus.
The distance light travels in one earth year, about 30,900,000,000,000 kilometers.
See  Solar Limb.
Local Group
gravitationally bound group of about 20 galaxies to which our  Milky Way Galaxy belongs.
Local Supercluster
The supercluster of galaxies in which the Local Group is located; spread over 10,000,000  pc, it contains the Virgo and Coma clusters.
An east-west coordinate measured on the surface of a sphere. 
The total rate at which radiative energy is given off by a celestial body, over all  wavelengths; the Sun's luminosity is about 4x1026 watts.

Magnetic Field
A field of force that is generated by electric currents. The Sun's average large-scale magnetic field, like that of the Earth, exhibits a north and a south pole linked by lines of magnetic force.
Magnetic Field Lines
Imaginary lines that indicate the strength and direction of a  magnetic field. The orientation of the line and an arrow show the direction of the field. The lines are drawn closer together where the field is stronger. Charged particles move freely along magnetic field lines, but are inhibited by the magnetic force from moving across field lines.
Magnetic flux
The number of magnetic field lines passing through an area.
Magnetic reconnection
The process by which two separate  magnetic field lines bend toward each other and "fuse" to create a new field line, allowing charged particles that were previously travelling in two different directions to now travel toward the same point.
A map showing the strength of the  magnetic field in different locations.
A device to measure the strength of a  magnetic field.
The region around a planet where particles from the  solar wind are trapped by the planet's  magnetic field.
An astronomical measurement of an object's brightness; larger magnitudes represent fainter objects.
Main Sequence
An area on the  Hertzsprung-Russel Diagram containing "middle aged" stars like the Sun.
A measure of an object's resistance to change in its motion (inertial mass); a measure of the strength of  gravitational force an object can produce (gravitational mass).
Michelson Doppler Imager/Solar Oscillations Investigation. 
Helioseismology instrument aboard  SOHO which analyzes the vibrational modes of the Sun. Also measures the Sun's  magnetic field in the  photosphere.
One million  parsecs (Mpc).
An explosive force equal to one million metric tons of TNT. The energy released in the explosion of one megaton of TNT is equal to 4.2 × 1022  ergs.
Region of the Earth's  atmosphere between 50 and 100 km where the temperature falls rapidly.
Metallic hydrogen
A state of hydrogen, reached at high pressures, where it is able to conduct electricity.
Microwave background radiation
A universal bath of low-energy  photons having a  blackbody spectrum with a  temperatureof 2.7 K.
Milky Way
The band of light that encircles the sky, caused by the bending of light from many stars lying near the plane of the  Galaxy; also sometimes used to refer to the Galaxy in which the Sun belongs.
Molecular cloud
Large, dense, massive clouds in the plane of a spiral galaxy; they contain dust and a large fraction of gas in molecular form.
A combination of two or more atoms bound together electrically; the smallest part of a compound that has the properties of that substances.
The product of an object's mass and velocity. 

0.000000001 of a meter; common unit of  wavelength measurement for light. 
The National Aeronautics and Space Administration.  NASA web site.
Nebula (Latin for "cloud") 
A cloud of  interstellar gas and  dust.
See  Chromospheric Network.
An elementary particle with no charge and almost no mass which interacts very weakly with other matter.
A subatomic particle with about the mass of a  proton and no electric charge; one of the main constituents of an atomic nucleus; the union of a proton and an  electron. A neutron is 1839 times heavier than an  electron.
North magnetic pole
One of two points on a star or planet from which magnetic lines of force emanate and to which the north pole of a compass points.
Nova (Latin, "new")
A star that has a sudden outburst of energy, temporarily increasing its brightness by hundreds to thousands of times; now believed to be the outburst of a degenerate star in a binary system; also used in the past to refer to some stellar outbursts that modern astronomers now call  supernovas.
The chain of thermonuclear fusion processes by which hydrogen is converted to helium, helium to carbon, and so on through all the  elements of the periodic table.
The massive and positively charged core of an atom, consisting of  protons and  neutrons(except for hydrogen), around which  electrons orbit. 

Observable universe
The parts of the  Universe that can be detected by the light they emit.
The property of a substance that hinders (by absorption or  scattering) light passing through it; opposite of transparency.
Optical Radiation
Electromagnetic radiation (light) that is visible to the human eye. 
The curved path, usually elliptical, described by a planet, satellite, spaceship, etc., around a celestial body, such as the Sun.
Orbital Period
The amount of time it takes a spacecraft or other object to travel once around it's orbit.
Orbiting Solar Observatories
A series of eight Sun observing satellites launched from 1962 to 1975.
Orion Nebula
A hot cloud of  ionized gas that is a nearby region of recent star formation, located in the sword of the constellation of Orion; also catalogued as Messier 42 (M42).

A wave mode generated by an  acoustic wave (or "sound wave").
The change in an object's apparent position when viewed from two different locations; specifically, half the angular shift of a star's apparent position as seen from opposite ends of the Earth's orbit. 
Parsec (pc)
The distance an object would haveto be from the earth so that its heliocentric  parallax would be 1 second of arc; equal to 3.26 light years; a kiloparsec is 1000 parsecs.
The time interval for some regular event to take place; for example, the time required for one complete revolution of a body around another.
The breakup of a  molecule by the absorption of light with enough energy to break the molecular bonds.
A light-sensitive detector placed at the focus of a telescope; it is used to make accurate measurements of small  photon  fluxes.
A discrete quantity of electromagnetic energy. Short  wavelength (high frequency) photons carry more energy than long wavelength (low  frequency) photons. See  Electromagnetic Radiation
The visible surface of the Sun. It consists of a zone in which the gaseous layers change from being completely opaque to radiation to being transparent. It is the layer from which the light we actually see (with the human eye) is emitted.
Pickup ion
An  atom from the  interstellar medium which has been  ionized by the Sun's  radiation, and is then swept along with the  solar wind.
Bright areas seen near  sunspots in the  chromosphere. Seen in  H-alpha light. From the French for beach.
Planck curve
The continuous spectrum of a  blackbody radiator.
Planck's constant
The number that relates the energy and  frequency of light; it has a value of 6.63 x 10-34joule seconds.
Planetary nebula
A thick shell of gas ejected from and moving out from an extremely hot star; thought to be the outer layers of a red giant star thrown out into space, the core of which eventually becomes a white dwarf.
Plasma consists of a gas heated to sufficiently high temperatures that the atoms  ionize. The properties of the gas are controlled by electromagnetic forces among constituent  ions and  electrons, which results in a different type of behavior. Plasma is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the plasma state.
Polar Plume
Bright structure of out-flowing gas which occur along  magnetic field lines in  coronal holes. These field lines extend into the solar system. Although plumes usually occur at the poles, they can appear anywhere there is a coronal hole.
A structure in the  corona consisting of cool  plasma supported by  magnetic fields. Prominences are bright structures when seen over the solar  limb, but appear dark when seen against the bright solar  disk. Prominences seen on the disk are also known as  filaments.
A massive, positively charged elementary particle; one of the main constituents of the  nucleus of an  atom. A proton is 1836 times heavier than an  electron.
A neutron star (burnt-out star) that emits radio waves which pulse on and off.

Usually refers to  electromagnetic waves, such as light, radio, infrared, X-rays, ultraviolet; also sometimes used to refer to atomic particles of high energy, such as  electrons (beta-radiation), helium  nuclei (alpha-radiation), and so on.
Radiation Belt
A ring-shaped region around a planet in which electrically charged particles (usually  electrons and  protons) are trapped. The particles follow spiral trajectories around the direction of the  magnetic field of the planet. The radiation belts surrounding Earth are known as the  Van Allen belts
Radiative Zone
An interior layer of the Sun, lying between the  core and the  convection zone, where energy travels outward by radiation.
Red Giant
A large, bright, cool star. Red giants are formed when a star runs out of nuclear fuel in its core. The star starts to contract, which in turn leads to heating and nuclear reactions in layers outside the core and the expansion of the star's outer layers. These outer layers become cooler and redder as they expand.
The preferential  scattering or absorption of blue light by small particles, allowing more red light to pass directly through.
Red shift
An increase in the  wavelength of the radiation received from a receding celestial body as a consequence of the  Doppler effect; a shift towards the long-wavelength (red) end of the  spectrum.
Reflection nebula
A bright cloud of gas and dust that is visible because of the reflection of starlight by the dust.
Relativistic Doppler shift
Wavelength shift from the radial velocity of a source as calculated in  special relativity, so that very large red shifts do not imply that the source moves faster than light.
Two theories proposed by A. Einstein; the special theory describes the motion of nonaccelerated objects, and general relativity is a theory of  gravitation.

Scattering (of light)
The change in the paths of  photons without absorption or change in  wavelength
Scientific model
A mental image ofhow the natural world works, based on physical, mathematical, and aestheticideas.
Second of arc
1/3600 of a degree, or 1/60 of a minute of arc.
A space station which orbited Earth in the 1970s. Performed many important observations of the Sun.
The Solar and  Heliospheric Observatory.
Solar Atmosphere
The atmosphere of the Sun. An atmosphere is generally the outermost gaseous layers of a planet, natural satellite, or star. Only bodies with a strong gravitational pull can retain an atmosphere. Atmosphere is used to describe the outer layer of the Sun because it is relatively transparent at visible  wavelengths. Parts of the solar atmosphere include the  photospherechromosphere, and the  corona.
Solar-B. Japanese spacecraft to observe the Sun
Solar core
Region of the Sun's interior where  temperatures and  densities are high enough for fusion reactions to take place.
Solar cosmic rays
Low energy  cosmic rays generated in  solar flares.
Solar Cycle
The approximately 11-year quasi-periodic variation in frequency or number of  sunspotscoronal mass ejections, solar  flares, and other solar activity.
Solar flare
Sudden burst of  electromagnetic energy and particles from a magnetic loop in an active region.
Solar Limb
The apparent edge of the Sun as it is seen in the sky.
Solar Maximum
The month(s) during the  solar cycle when the number of  sunspots reaches a maximum. The most recent solar maximum occurred in 2014.
Solar Maximum Mission
A satellite dedicated to observing the Sun, especially solar flares. It was in orbit throughout the 1980s.
Solar Minimum
The month(s) during the  solar cycle when the number  sunspots is lowest. The most recent minimum occurred in 2009.
Solar wind
A stream of charged particles, mostly  protons and  electrons, that escapes into the Sun's outer  atmosphere at high speeds and streams out into the solar system.
South Atlantic Anomaly
The region over the South Atlantic Ocean where the lower  Van Allen belt of energetic, electrically charged particles is particularly close to the Earth's surface. The excess energy in the particles presents a problem for satellites in orbit around the Earth.
Spectral Continuum
A broad, continuous band of light in a spectrum.
A 4-dimensional  universe with space and time unified; a continuous system of one time coordinate and three space coordinates by which events can be located and described.
Spectral Line
A line in a  spectrum due to the emission or absorption of  electromagnetic radiation at a discrete  wavelength. Spectral lines result from discrete changes in the energy of an atom or molecule. Different atoms or molecules can be identified by the unique sequence of spectral lines associated with them.
An instrument that spreads light or other  electromagnetic radiation into it's component  wavelengths (spectrum), recording the results photographically or electronically.
An instrument for measuring the intensity of radiation as a function of  wavelength. See  Spectrograph.
An instrument for examining  spectra; also a spectrometer or spectrograph if the spectrum is recorded and measured.
The analysis of light by separating it by  wavelengths (colors, in visible light).
Electromagnetic radiation arranged in order of  wavelength. A rainbow is a natural spectrum of visible light from the Sun. Spectra are often punctuated with emission or absorption lines, which can be examined to reveal the composition and motion of the radiating source.
A predominantly vertical structure extending from the solar  chromosphere into the  corona, observed in  Hydrogen-Alpha. Spicules are concentrated on the boundaries of  super-granulation cells, have lifetimes of 5 to 10 minutes, and are about 1000  km across and 10,000 km long.
Spiral galaxy
galaxy with spiral arms; the presumed shape of our  Milky Way galaxy.
Stellar nucleosynthesis
A process in which nuclear fusion builds up heavier  nuclei while supplying the energy by which stars shine.
See  Coronal Streamer
Solar Ultraviolet Measurements of Emitted Radiation.
An  ultraviolet  spectrometer aboard  SOHO.
A temporary disturbed area in the  solar photosphere that appears dark because it is cooler than the surrounding areas. Sunspots consist of concentrations of strong magnetic flux. They usually occur in pairs or groups of opposite polarity that move in unison across the face of the Sun as it rotates.
Super-Granulation Cells
Convective cells about 30,000 km in diameter in the solar  photosphere. The boundaries between super-granule cells can be seen by emission in the  chromosphere known as the  chromospheric network and contain concentrations of magnetic flux tubes.
A stupendous explosion of a massive star, which increases its brightness hundreds of millions of times in a few days.
Supernova remnant
Expanding gas cloud from the outer layers of a star blown off in a  supernova explosion; detectable at radio  wavelengths; moves through the  interstellar medium at high speeds.

A measure of the average random speeds of the microscopic particles in a substance.
Termination Shock
The boundary marking one of the outer limits of the Sun's influence. At the termination shock,  solar wind particles slow down as they begin to press into the particles forming the  interstellar medium. The solar wind particles then continue to travel outward at a slower rate of speed. This is similar to cars speeding along a highway which then slow down as they encounter many more cars involved in a traffic jam. The jammed cars continue to move outward, although much more slowly.
In the SI system, a unit of measure of  magnetic flux
An idea in science which is supported by numerous pieces of evidence, and which has thus far withstood the rigors of testing by other scientists. It is different from a hypothesis, which is an idea based on observations but which has not yet been tested, or does not have further evidence supporting it. Theory in the scientific sense is very different from the colloquial definition, where a theory is essentially a guess. Examples include the  Big Bang theory, the theory of  gravitation, and the theory of evolution.
Thermal radiation
Electromagnetic radiation due to the fact that a body is hot; often characterized by a  blackbody spectrum.
Thermonuclear Fusion
The combination of atomic  nuclei at high temperatures to form more massive nuclei with the simultaneous release of energy. Thermonuclear fusion is the power source at the core of the Sun. Controlled thermonuclear fusion reactors, when successfully implemented, could become an attractive source of power on the Earth. 
A layer in the earth's  atmosphere, above the mesosphere, heated by  X-rays and ultraviolet radiation from the Sun.
A measure of the flow of events.
Transition Region and Coronal Explorer
A NASA satellite launched in 1998. TRACE observes the Sun's atmosphere in  ultraviolet wavelengths.
Transition Region
Area of the Sun's atmosphere with temperatures between those of the  chromosphere and the  corona, 20,000 - 1,000,000°C. 

The part of the  electromagnetic spectrum whose radiation has somewhat smaller  wavelengths than  optical radiation, but longer wavelengths than  X-rays . Because ultraviolet light is absorbed by the Earth's atmosphere, ultraviolet astronomy is performed in space.
Universal Time
Abbreviated UT. The same as Greenwich Mean Time (GMT) in England. US Eastern Standard Time (EST) is five hours earlier than Universal Time. 
The totality of all space and  time; all that is, has been, and will be.

Van Allen Belts
Two ring-shaped regions that girdle the Earth's equator in which electrically charged particles are trapped by the Earth's  magnetic field. See  South Atlantic Anomaly and  radiation belts.

The distance from crest to crest or trough to trough of an electromagnetic wave (see  electromagnetic radiation) or other wave.
White Dwarf
A star which has used up its nuclear fuel and collapsed to a very small size.
White Light
Visible light that includes all colors and, therefore, all visible  wavelengths.

The part of the  electromagnetic spectrum whose radiation has somewhat greater  frequencies and smaller  wavelengths than those of ultraviolet radiation. Because x-rays are absorbed by the Earth's atmosphere, x-ray astronomy is performed in space.
X-rays High-energy
electromagnetic radiation with a  wavelength of about 10-10 meter. 

A satellite which observes X-rays from the Sun. Launched in 1991.

Zeeman Effect
A splitting of  spectral lines in response to a  magnetic field. It is used to measure magnetic field strengths on the Sun and on other astronomical objects.
Zodiacal Light
A pale glow sometimes visible in the night sky in the path of the Earth's orbit around the Sun. (The constellations in this path make up the zodiac). It is caused by the scattering of sunlight off of dust in the plane of Earth's orbit.


Introductory Astronomy & Astrophysics, Third Edition
On-Line Glossary of Solar-Terrestrial Terms and Kenneth R. Lang's book: Sun, Earth, and Sky.
Solar Flare Theory web site by Gordon Holman and Sarah Benedict.