Cassiopeia

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy

ABSTRACT: Cassiopeia is a constellation with a zigzag shape that resembles the letter M or W, depending on the constellation’s position in the sky. Cassiopeia is a Northern Hemisphere constellation with a long and rich mythological history. It is named after a boastful queen from Greek mythology. Astronomers and amateur stargazers look to Cassiopeia for examples of double and triple stars, variable stars, nebulae (clouds of dust and gas), and star clusters.

A Queen of Constellations

Cassiopeia is a constellation, or a region of space defined by a group of stars that appear to form a pattern or picture. Since ancient times, astronomers have studied the constellations. Cassiopeia was first identified in antiquity and was included in star charts as early as the second century CE. It is one of the eighty-eight constellations officially recognized by the International Astronomical Union. Although many cultures have observed and told tales about this constellation, the name Cassiopeia is derived from an arrogant queen from Greek mythology.

A star formation of the Northern Hemisphere, Cassiopeia is notable for being circumpolar, which means it appears to move around the pole star. Due to this type of motion, Cassiopeia often appears reversed in the night sky. Cassiopeia has a declination of approximately 60 degrees north relative to the celestial equator and a right ascension of one hour.

Attributes of Cassiopeia

Cassiopeia is most visible in the Northern Hemisphere between the latitudes of 90 degrees north and 20 degrees south. Evenings in late autumn, particularly November, are the best times to view the constellation. During those times, the constellation appears in the northeastern sky. Cassiopeia looks like a flattened zigzag form or the letter W or M (depending on the direction the constellation is facing). It is set against the backdrop of the Milky Way, the galaxy that includes Earth.

The constellation of Cassiopeia contains many diverse space objects. Among these objects are five primary stars as well as fifty-three other stars designated by letters of the Greek alphabet, roughly in descending order of brightness. Astronomers have discovered a great variety of star types in the constellation. For example, some of the stars in Cassiopeia are actually pairs or groups of stars. Eta Cassiopeiae is a double star, and Iota Cassiopeiae is a triple star. The stars forming the W of Cassiopeia, which are easily visible, are Alpha, Beta, Gamma, Delta, and Epsilon Cassiopeiae (also known by other traditional names, mostly Arabic: Schedar, Caph, Cih (Tsih), Ruchbah, and Segin, respectively).

The constellation’s leading star, Alpha Cassiopeiae (also known as Schedar), is an irregularly variable star. That means the star occasionally changes in brightness, although astronomers have not observed any major changes on Schedar since the nineteenth century. Gamma and Rho Cassiopeiae are also types of variable stars. The Gamma star, in the center of the constellation, sometimes dramatically increases in brightness due to unpredictable emissions of gas. The Rho star, which is about a half-million times brighter than the sun, has shown signs of increasing instability and astronomers suspect it will eventually explode into a supernova. Two known supernovas have occurred in Cassiopeia in the past several centuries.

Despite the great brightness of the variable stars, Beta Cassiopeiae (also known as Caph) is the consistently brightest star in the constellation. People can often see Caph without optical aids. The name Caph is one of several star names in Cassiopeia that refer to parts of the body. Caph comes from an Arabic word meaning "palm" or "hand." Shedir means "breast," and Ruchbah (Delta Cassiopeiae) means "knee."

Cassiopeia also contains several other deep-sky objects. There are several nebulae, vast clouds of dust and gas in space in which new stars may form, in or near the constellation. There are also clusters of stars, many of which are relatively new, having formed in the nearby nebulae. Unfortunately, many of these new stars are obscured from sight by the thick clouds of dust in that region of space.

A Tragic Boast

As with many other constellations, Cassiopeia has inspired myths and tales from cultures around the world. In China, people identified the star pattern as a representation of a bold charioteer. Among the Welsh, the constellation may have been considered the court of the mother goddess Don. Of all these diverse interpretations, the most famous one, and the one from which the name Cassiopeia is derived, originated in ancient Greece.

In Greek mythology, Cassiopeia was a queen in the legendary realm of Aethiopia. A beautiful but arrogant woman, she ruled the kingdom with her husband, King Cepheus. One day Cassiopeia boasted that she and her daughter, Andromeda, were more beautiful than the sea nymphs. This claim upset the sea god Poseidon, who decided to take revenge on the entire kingdom for its queen’s imprudent words. Poseidon cast a disastrous flood upon Aethiopia. Worse, he also unleashed a sea monster called Cetus to terrorize its people.

Cassiopeia and Cepheus were helpless against the wrath of the sea god. They consulted an oracle, an all-knowing source of wisdom, which advised them that only a human sacrifice could appease Poseidon. The king and queen agreed and had their daughter Andromeda chained to a rock by the sea as an offering to Cetus. Andromeda’s doom seemed assured until the hero Perseus, the son of Zeus and Danae, discovered and rescued her, thus confounding the plans of the gods and monarchs.

Poseidon felt that he had been denied his vengeance against the boastful queen, and so devised a new, more personal punishment. He banished both Cassiopeia and Cepheus into the sky to live forever among the stars. Just as Andromeda had been chained to a rock, Cassiopeia was chained to her royal throne. To humiliate the arrogant queen, Poseidon arranged for the throne to move in the sky so that it frequently turned upside-down, leaving the queen helplessly hanging in an undignified stance. Meanwhile, Cepheus also became a constellation, but the king and queen were positioned so they could never see or talk to one another again.

Long ago, astronomers referred to the constellation as Cassiopeia’s Chair. In the 1930s, a group of scientists at the International Astronomical Union decided to change the name to Cassiopeia the Queen. This name change, however small, deemphasized the ancient myth of Poseidon’s vengeance.

The myth of Cassiopeia fits into the larger scheme of constellation mythology. The constellation of the queen is part of the Perseus constellation family, a group of constellations sharing thematic connections. This family includes constellations representing the hero Perseus, Queen Cassiopeia, King Cepheus, and Andromeda. It also includes the sea monster Cetus, Perseus’s flying horse (Pegasus), a charioteer (Auriga), and a lizard (Lacerta).

Cassiopeia’s Importance Today

In ancient times when astronomers and storytellers first imagined the constellations, the stars were of great importance to everyday life. People looked to the night sky to find answers to their questions about spirituality and nature. Stars and constellations also served a practical purpose of helping farmers mark the changing of the seasons and decide when crops should be planted and harvested. In modern times, most people enjoy stargazing just for the beauty of it, and scientists use the constellations to help organize the stars and learn more about the always-changing universe.

Cassiopeia has many features of interest to amateur and professional astronomers. It is generally a fairly bright constellation, and some of its features may be seen with the naked eye. Once stargazers find Cassiopeia, they have numerous features to examine. The stars of the constellation include many types such as double stars, triple stars, and variable stars. Other sky objects are also of interest to astronomers, such as nebulae and star clusters. The astronauts of the Apollo missions used gamma Cassiopeiae as one of the thirty-six stars by which they navigated. Astronaut Gus Grissom (1926–67) nicknamed the star Navi, a backward spelling of his own middle name, Ivan.

For the earthbound observer, Cassiopeia is also a valuable reference point for locating other constellations in the sky. Some of the neighboring constellations include Perseus, Andromeda, Cepheus, and Lacerta, as well as Camelopardalis (the giraffe).

PRINCIPAL TERMS

• constellation: a region of space defined by a pattern of stars that can be seen in the night sky from Earth.
• celestial equator: the imaginary line above Earth’s equator that halves the celestial sphere; it is equally distant from the celestial poles.
• circumpolar constellation: a constellation that is always visible in the night sky; the Northern and Southern Hemispheres have different sets of circumpolar stars.
• declination: the north-south position of a celestial body relative to the celestial equator expressed in degrees of arc.
• International Astronomical Union: an association of professional astronomers from all over the world who define astronomical constants while promoting research, education, and discussion on important astronomical topics.
• right ascension: the east-west position of a celestial body relative to the celestial equator and expressed in hours and minutes, not degrees of arc.

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