Celestial equator;Projection of the Earth's equator onto the imaginary celestial sphere, dividing it into two hemispheres.
Celestial poles;Intersections of the axis of rotation of the Earth with the celestial sphere, thus producing a celestial north pole and a south pole.
Celestial sphere;Imaginary sphere enveloping the Earth, which is at its center. One could imagine that the stars are projected onto this sphere.
Centrifugal force;Force applied to bodies in a rotating system.
Cepheids;Variable stars with short periods. Their brightness changes according to a regular pattern.
Chandrasekhar limit;Limit for the mass of a star. Stars with a mass higher than the Chandrasekhar limit cannot develop directly into white dwarfs.
Charge-coupled-device (CCD);Electronic plate in the HST, storing images in the form of electric charge.
Chromosphere;Layer of the solar atmosphere directly above the visible surface of the Sun.
Circumpolar stars;Stars and constellations that, when viewed from a specific position, do not disappear behind the horizon.
Classification of galaxies;Catalogue comprising more than 100 nebulae and star clusters, first published by Messier.
Comet;Small body in the solar system, consisting of frozen gases and dust.
Comet nucleus;Solid matter at the center of the comet's head.
Compass;Instrument that determines directions.
Composite image;Image generated from a number of individually taken pictures.
Compound;Substance that consists of two or more chemically bound elements. Hydrogen and oxygen form the compound substance of water.
Concave;Curved inwards, similar to a bowl. Concave mirrors focus light, while concave lenses disperse light.
Conjunction;Constellation: another celestial object is located in direct line between the Earth and a planet.
Constellation;Group of stars in the celestial sphere.
Convection;Vertical transfer of energy or mass as a result of circulation.
Convection zone;Energy exchange zone: the kinetic energy of particles located near a source of heat is increased.
Convex;Curved outwards. Convex mirrors disperse light, while convex lenses focus light.
Coordinates;Numbers indicating the location of a point on a plane or within a volume.
Copernicus;Copernicus Nicolaus, 1473-1543, Polish astronomer.
Core shadow;Central dark area of the shadow cone during an eclipse.
Corona;Outermost, extremely thin and hot layer of the atmosphere of the Sun.
Cosmology;Study of the origin, development and general structure and composition of the universe.
Cosmos;General term describing the world as an entity, synonym to universe and space.
COSTAR;Corrective Optics Space Telescope Axial Replacement: correcting device for the incorrectly cut main mirror of the HST.
Crab nebula;Gaseous nebula located in the constellation of Taurus (object M1 in the Messier catalogue): example of an object consisting of remains of a supernova.
Crust;Solid surface layer of a planet or other solid body.
Culmination;Highest position on the celestial sphere that stars can reach relative to the observer.
Cycle;Sequence of events that occur at regular intervals.
Dark matter;Any type of matter that is not visible, whether directly or indirectly, is referred to as dark matter. 
Dark nebula;Cold cloud made of dust particles and gas, not emitting or reflecting any light.
Declination;Angular distance of a celestial object from the celestial equator.
Deformation;Deviation of an (rotating) object from spherical shape (flattening).
Density;Measure for the mass within a given volume.
Diffraction;Refraction of light, for example at the edges of concave mirrors.
Distance module;Difference between apparent magnitude (m) and absolute magnitude (M) of a star.
Doppler effect;Apparent variation in frequency of sound and electromagnetic waves resulting from the relative movement of the source and the observers to each other.