Astro Glossary

Cassiopeia, the Queen

A constellation best seen in autumn in the northeastern sky.

Castor

A crowded six-star system about 50 light-years from Earth. It forms one of the “heads” of the twins of Gemini, and is Gemini’s second-brightest star, after nearby Pollux.

Celestial Coordinates

Astronomers measure the positions of astronomical objects using coordinates called right ascension and declination — the equivalent of longitude and latitude here on Earth. If you’re standing on Earth’s equator, the celestial equator stretches from east to west directly across the top of the sky, dividing the sky into northern and southern hemispheres.

Centaurus, the Centaur

Cepheid Variable

An unstable star that pulses in and out, causing a periodic change in brightness. In 1912, Henrietta Leavitt discovered what is known as the period-luminosity relationship for Cepheid variables, by which the period of the brightness change is related to the luminosity, and therefore the distance, of the star. An astronomer can plot a Cepheid’s changing brightness to create a light curve for the star. The distance to the Cepheid variable is then obtained by measuring the period of the light curve, which shows the length of each pulsation.

Cepheus, the King

A northern constellation that is best viewed in the evening sky during autumn.

Ceres

The largest known asteroid, Ceres is less than 600 miles (975 km) in diameter, and orbits roughly 260 million miles (420 million km) from the Sun. Italian astronomer Giuseppe Piazzi discovered Ceres in 1801. Since it was the first asteroid ever discovered, it also carries the designation “1 Ceres.” In 2006, astronomers also classified Ceres as a dwarf planet.

Cetus, the Whale or Sea Monster

A constellation named for a mythological sea monster. He was sent by Poseidon, god of the sea, to punish Ethiopia for the vanity of its queen, Cassiopeia, who boasted that she was more beautiful than the sea nymphs. The king, Cepheus, ordered his daughter Andromeda chained beside the sea shore as a sacrifice. At the last moment, however, she was rescued by Perseus, the hero, who either beheaded Cetus or turned him to stone by showing him the snake-covered head of Medusa, one of the Gorgons.

Chandra X-Ray Observatory

A space-based observatory that observes the universe in X-ray wavelengths. Launched in 1999, it is one of NASA’s four Great Observatories.

Chemistry of the Universe