Crux
Crux, commonly known as the Southern Cross, is a prominent constellation located in the Southern Hemisphere, recognized for its distinctive cross shape formed by five main stars. Despite its fame, Crux occupies the smallest area among the eighty-eight recognized constellations as defined by the International Astronomical Union. It is visible from nearly any location in the Southern Hemisphere throughout the year, appearing to change position in the sky with the seasons. While this constellation was once visible in parts of the Northern Hemisphere, a gradual shift in Earth's axis has confined it largely to southern latitudes.
The brightest star in Crux is Alpha Crucis, or Acrux, with other notable stars including Beta Crucis (Becrux) and Gamma Crucis (Gacrux), both of which are binary stars. Crux also encompasses deep-sky objects like the Coalsack nebula, which holds cultural significance in Australian Aboriginal astronomy, and the colorful star cluster known as the Jewel Box. Historically, the Southern Cross has been utilized by sailors for navigation, particularly in regions where traditional northern constellations like Polaris are not visible. Different cultures have interpreted the stars of Crux in various ways, seeing them as different animals or shapes, which reflects the rich tapestry of human observation of the night sky.
Crux
FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy
ABSTRACT: Crux is a cross-shaped constellation that appears in the Southern Hemisphere. Because of its location, Crux was not identified by many cultures in the Northern Hemisphere, but it was identified by various Aboriginal Australian and Pacific Islander peoples, among others. Europeans first saw the constellation in the sixteenth century. Scientists study Crux and other constellations to learn more about stars, the galaxy, and the universe.
The Southern Sky
Crux, also known as the Southern Cross, is a constellation of five stars that form the shape of a cross. Even though Crux is probably the most famous constellation in the Southern Hemisphere, its apparent area is also the smallest of all eighty-eight constellations recognized by the International Astronomical Union. This popular constellation pattern appears on flags and postage stamps of a number of countries in the Southern Hemisphere. Crux can be seen from anywhere in the Southern Hemisphere at almost any time of year. It appears to change positions in the sky during the year; to viewers on Earth, it can look as if it standing straight up, lying on its side, or standing upside down. At latitudes between 23.5 degrees south and the South Pole, Crux is a circumpolar constellation, meaning that it can be seen all year long at any time of night.
Attributes of Crux
Centuries ago, Crux was visible in parts of the Northern Hemisphere, including ancient Greece and what is now the United States. However, the subtle shifting of Earth’s axis over time caused it to appear mostly in the Southern Hemisphere. During spring in the Northern Hemisphere, Crux can be seen at some lower latitudes, starting at about 25 degrees north. Potential viewing locations include Hawaii and some parts of southern Texas and the Florida Keys.
Some novice stargazers looking for Crux may be fooled by a pattern that has become known as the False Cross. The False Cross is made up of four different stars, two from the constellation Vela and two from the constellation Carina. Because these four stars appear to make an X shape, they are easy to mistake for the Crux constellation. One way to tell the difference is to remember that the stars of the real Crux always point toward the celestial South Pole.
The brightest star in the Crux constellation is alpha Crucis, also known as Acrux. Beta Crucis, or Becrux, is the constellation’s second-brightest star, sometimes called Mimosa. The third-brightest star is gamma Crucis, or Gacrux. Both Becrux and Gacrux are binary stars. Binary stars are star systems in which two stars orbit a common center of mass. Acrux, Becrux, Gacrux, and delta Crucis are the four main stars that form the cross shape. Crux also contains several Cepheid variables, including R Crucis, S Crucis, and T Crucis. A Cepheid variable is a star can be used to calculate the distances between Earth and other galaxies.
Crux contains some deep-sky objects as well. One such object is the Coalsack, a large dark nebula in the Milky Way. The Coalsack is a significant object in Australian Aboriginal astronomy. For a number of indigenous Australian peoples, it forms part of the quasi-constellation known as the "emu in the sky." Another deep-sky object in Crux is the star cluster New General Catalog 4755 (NGC 4755), commonly called the Jewel Box for its bright, colorful stars. Three of the stars in Crux are known to have exoplanets—planets orbiting stars outside our solar system. These exoplanets are all gas giants.
Historical Observation
Although many of the classic constellations were named by ancient Greek or Roman astronomers, Crux was not one of them, as these civilizations were too far north to see it. However, Crux and its stars were visible to the ancient cultures of the Southern Hemisphere, including Aboriginal Australians and Pacific Islanders. Different cultures have seen different patterns in the constellation. While some saw a cross, others saw a bird or a fish.
For centuries, sailors have used the stars to help them navigate. When European sailors began exploring the Northern Hemisphere, they learned that they could use Polaris, also called the North Star, and the Big Dipper, a famous pattern of stars that makes up part of the constellation Ursa Major, to navigate while sailing. When Europeans first entered the Southern Hemisphere, they realized that Polaris and the Big Dipper vanished from view. They learned, however, that the formation that would later be called the Southern Cross could serve the same purpose. In the early sixteenth century, during his third voyage to South America, Italian explorer Amerigo Vespucci (1454–1512) noticed Crux in the sky. Vespucci called the pattern "the four stars." Other European sailors learned to use the constellation to find the celestial South Pole.
Studying Crux and Other Constellations
Modern astronomers use the various constellations to identify and name the stars found inside them. Because there are so many stars in the sky, this makes it easier to study and track them. Modern astronomers use constellations to help locate stars and other space objects. For instance, the stars of Crux can be found using its declination (about −60 degrees) and right ascension (12.45 hours) relative to the celestial equator, the projection of Earth’s equator onto the night sky. Astronomers study the stars and other objects inside the constellations to learn more about the galaxy and the universe.
PRINCIPAL TERMS
- 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 above the horizon of the night sky.
- constellation: a pattern of stars identified by humans that can be seen in the night sky from Earth.
- declination: a space object’s angular distance north or south of 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: a space object’s longitudinal arc along the celestial equator, measured eastward from the vernal equinox and expressed in hours.
Bibliography
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Privett, Grant, and Kevin Jones. The Constellation Observing Atlas. New York: Springer, 2013. Print
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Sessions, Larry. "Star of the Week: Mimosa Second-Brightest in Southern Cross." EarthSky. Earthsky Communications, 1 Apr. 2015. Web. 7 Apr. 2015.