ESO Astronomical Glossary - C


Cataclysmic variable
Cataclysmic variables are close binary systems which include a white dwarf accreting matter from a less massive companion. The white dwarf is often seen to have an accretion disc. As matter falls onto the white dwarf from the companion star, the dwarf can become unstable, and violent nova or even supernova explosions can occur.

Charge-coupled device (CCD)
CCDs are silicon-based light detectors, widely used in astronomy for imaging of visible, ultraviolet and infrared radiation. They are also used in commercial digital cameras. The introduction of CCDs for astronomical imaging in the late 1970s, as replacements for the photographic plate, made possible many of the breakthroughs that have shaped modern astronomy.

Cepheid variable star
Cepheid variables, named after the first star discovered of this type, Delta Cephei, are stars that are seen to vary in brightness on a very regular timescale. The variation is believed to be caused by a regular expansion and contraction ('pulsation') of the star's outer gas layers. Because of the regularity of their variation in luminosity they can be used as distance indicators to clusters and other galaxies.

Colour–Magnitude Diagram
A scatter graph of stars showing the relationship between their colour (represented by their B-V colour index, on the horizontal axis) and their absolute magnitude (on the vertical axis). Rather than being randomly distributed, the stars typically fall into specific areas on the diagram, such as the Main Sequence. Such a diagram is often referred to as a Hertzsprung–Russell diagram.

Comets are known as the 'voyagers' of the solar system, passing through the system at high speeds. They are icy rocks originating from the very outer regions of the solar system, where the gravity of the massive planets can cause them to 'fall' into the inner solar system. The nucleus starts to emit gases and dust as it interacts with the Sun's radiation on its path, giving rise to the characteristic 'tails'. Comets are classified according to the period of their orbits - some return at regular intervals, while others are seen just once.

Compact star
The term 'compact star' is used when observations suggest a star to be small and very massive, often when its exact nature is unknown. This description can include black holes, white dwarfs, and neutron stars.

A constellation is a group of stars that, when seen from Earth, form a pattern. Their names date back to the ancient Greeks. The entire sky is divided up into 88 constellations, including the constellations of the Zodiac and many others.

Cosmic microwave background radiation (CMBR, CMB or CBR)
The cosmic microwave background radiation is electromagnetic radiation that is strongest in the microwave range, first detected in 1965, and thought to be the relic of the Big Bang explosion in which the Universe was created. The Big Bang theory predicts that leftover radiation fills the entire Universe with virtually no detectable variations in intensity. The discovery of the CMBR and the confirmation of this homogeneity with subsequent studies is one of the strongest supporting pieces of evidence for the theory.

Cosmology is the branch of science concerned with the large-scale structure and evolution of the Universe.