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- Catalogue
- since there are so many astronomical objects in the sky, the scientists who discover them organize them into catalogues. In many cases, the only name an astronomical object has is its catalogue number. Some objects appear in more than one catalogue and have more than one name.
- Cathode
- an electrode from which positive electric charge leaves.
- CCD
- an electronic detector that is far more sensitive to light than the human eye.
- Cen A
- Centaurus A, also known as NGC 5128, is a lenticular galaxy in the constellation Centaurus. The “A” in Cen A means it is the brightest radio source within that constellation. Cen A is a radio galaxy, and at only 11 million light years from Earth it is the closest active galaxy to us.
- Chandrasekhar Mass Limit
- the upper mass limit of a white dwarf, approximately 1.4 solar masses. If a white dwarf exceeds this limit it will explode in a Type 1A supernova and may collapse to a neutron star.
- Chronometer
- a watch that has been specifically designed to keep very accurate time.
- Climate Change
- the change in a planet’s weather patterns over long periods of time ranging from decades to hundreds of thousands of years.
- Collimated
- all energy moves in a confined column of light, with the rays of energy traveling nearly parallel to one another. This means they behave less like the spherically dispersed light from a light bulb, which fades quickly as you get further away, and more like the directed light generated by a laser.
- Coma Cluster
- a galaxy cluster located in the Coma Berenices constellation. The cluster contains over 1,000 galaxies and is 320 million light years from Earth.
- Comets
- dusty bodies of ice that orbit a star. We typically imagine comets with their characteristic tails; but the tails only form when their orbits bring them close to a star. Comets have three distinct tails: one caused by dust pushed out by radiation pressure from the star, the ion tail caused by solar winds melting the frozen ice and gas and pushing it back, and a tail of sodium escaping from the dust. The sodium tail is not visible to the naked eye. These tails point in slightly different directions but always away from the star.
- Compile
- to translate source code written by a computer programmer into code that computer hardware can understand.
- Compiler
- a software program that translates source code into machine code.
- Computerized Tomography
- a form of X-ray imaging where a computer collects slices of two-dimensional X-ray images to produce a three-dimensional view.
- Conditional Statement
- a conditional statement checks to see if a particular condition is true or false. It then executes a certain block of computer code if the condition is true, and a different block if it is false.
- Conductive Plasma
- plasma is a gas in which the electrons have been stripped off the atoms. The resulting positively charged particles are called ions, and they are just atoms that have lost some electrons. Plasma also contains negatively charged particles, which are the electrons that have been stripped away. The process of stripping electrons from atoms is called ionization. Ionization happens very easily in stars because they are so hot. The temperature variations within stars, along with their rotation, causes the plasma to move. Since the plasma contains charged particles, under the right conditions this motion can create an electric current. The current, in turn, gives the star a magnetic field.
- Contrast
- the difference between light and dark areas of an image.
- Cosmic Microwave Background (CMB)
- this is the radiation left over from the big bang. It was produced early in the age of the universe, when the average density and temperature were much higher than today. The expansion of the universe has cooled the radiation to its current temperature of about 2.7 kelvin (The SI unit of temperature is the kelvin, symbol K. The name is to honor the 19th Century Scottish physicist William Thomson, who is more commonly known as Lord Kelvin).
- Countdown
- a series of time-sensitive procedures that are carried out leading up to the launch of a mission. While the last few seconds of the countdown are the most iconic, countdowns for spaceflight missions often range between 72 and 96 hours. T minus indicates the time remaining until launch, after launch, the clock starts counting up and mission time is measured as T plus.
- Crab Nebula
- a supernova remnant (expanding cloud left after a supernova explosion) in the constellation Taurus. The supernova itself was recorded by Chinese and Arab astronomers in 1054 A.D.
- Critical Angle
- the angle at which incident light rays reflect rather than refract as they interact with a material.
