Gemini

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy

ABSTRACT: A part of the zodiac, the constellation Gemini is located between fellow zodiac constellations Taurus and Cancer. The stars Castor, Pollux, and Alhena are the brightest and most easily identified in the constellation. The Gemini constellation has been identified by many different cultures, but it is most often associated with the Greek myth about twins named Castor and Pollux. The constellation Gemini is important to modern astronomers for its pulsating Cepheid variable stars, which have helped determine the relationship between a star’s period and its luminosity.

Gemini and the Zodiac

Gemini, which means "twin" in Latin, is a constellation, or area of the night sky defined by pattern of stars, has been seen as representing twin boys from Greek mythology. Gemini, like other constellations, has been identified by cultures around the world since ancient times. The International Astronomical Union recognizes Gemini among its eighty-eight official constellations.

Gemini is part of the zodiac. The sun appears to pass through Gemini and the other twelve constellations of the zodiac. This path, called the ecliptic, is an illusion caused by Earth’s orbit of the sun. However, the constellations, including Gemini, are noted by their distance relative to not to the ecliptic, but to the celestial equator. Gemini, for instance, has a declination of 20 degrees north of the celestial equator and a right ascension of about seven hours.

Attributes of Gemini

When drawn, the constellation Gemini looks like twin boys standing or sitting side by side. The stars in the constellation look similar to two stick figures. Two bright stars represent the heads; faint stars represent the bodies and arms; and a few bright stars represent the twins’ feet. Gemini is one of the few constellations that has an outline that looks similar to what the constellation represents.

Gemini is located between Taurus to the west and Cancer to the east. It is northeast of Orion. The Milky Way is visible at the southwestern tip of the constellation. Stargazers in the Northern Hemisphere have their best view of Gemini during February when it is located high in the winter sky. By April and May, the constellation is far to the west and can be seen after sunset.

The Gemini constellation is made up of a number of stars and other objects. Castor and Pollux are two of the main stars in the constellation. They are named after figures from Greek mythology and are the "heads" of the two twins.

The star Pollux, also known as Beta Geminorum, is the seventeenth brightest star in the sky and the brightest star in the Gemini constellation. Pollux is an orange giant located about thirty-five light-years away from Earth. Castor, also known as Alpha Geminorum, is actually a sextuplet star system, meaning it is made up of six stars bound together by gravity. Castor is about fifty light-years from Earth. Castor appears less luminous than Pollux in the night sky. Another bright star in the constellation is Alhena, also known as Gamma Geminorum. It is the foot of one of the twins. In places where pollution is an issue, Castor, Pollux, and Alhena are often the only stars of the constellation visible to the naked eye. The Eskimo Nebula and the Medusa Nebula are also located inside the Gemini constellation.

The Myth of Gemini

For tens of thousands of years, humans have looked to the sky and found objects in the patterns of the stars. Scholars believe that humans used constellations to represent religious stories. They also used them to tell the time of year in order to plant and harvest at the right times. Since constellations appear in about the same place in the sky at the same time year every year, farmers could track the time of year by the position of the constellations.

Gemini has been identified by humans for thousands of years. The constellation is one of the forty-eight discussed by the early astronomer Ptolemy (ca. 100–170 CE). A number of different stories have been told about the constellation. The most enduring story is of the twins Castor and Pollux.

In Greek mythology, Castor and Pollux were twin brothers. They had the same mother—Leda, the queen of Sparta—but different fathers. According to the story, the god Zeus disguised himself as a swan and seduced Leda. Leda had four children, two of whom were Zeus’s, while two were those of her husband, Tyndarus. Castor’s father was Tyndarus, but Pollux’s father was Zeus. Because his father was a god, Pollux was immortal. When Castor eventually died, Pollux became distraught. He asked Zeus to allow him to die so he could be with his brother. Zeus then put Castor and Pollux in the sky so they could always be together.

The Gemini constellation has been an important symbol in other cultures as well. Ancient sailors believed that the Gemini twins protected those who sailed the ocean. People in Elizabethan England considered the twins protectors of the entire sea.

Studying Gemini and Other Constellations

Scientists study constellations to learn more about the universe. Scientists sometimes use constellations to the name stars, so that the stars can be easily categorized and located. For example, the stars Castor and Pollux were named by astronomers many years ago; however, as scientists identified more stars in the constellation, they wanted to give all the stars easily recognizable names. So scientists gave the stars in the Gemini constellation names such as alpha Geminorum and beta Geminorum. The Greek letters alpha, beta, and so forth indicate the luminosity of each star in the constellation.

One star of interest in Gemini is Mekbuda, also known as zeta Geminorum. This star is a supergiant with a radius of about 220,000 times the size of the sun. This star is important to scientific research because it is a Cepheid variable. Cepheid variables help scientists better measure the distances between the stars and Earth. These large stars have used up their hydrogen fuel supplies, so they begin to pulsate. Scientists have found the periods of the stars are related to their luminosity. Scientists can determine how luminous a Cepheid variable is based on the period. Then, they can see how luminous the stars appear in the sky. With these two pieces of information, scientists can determine how far away stars are from Earth. This measurement method has been refined over the years and has become integral to stellar research.

As of 2022, astronomers have detected more than five thousand planets outside of our solar system. Of these exoplanets, nineteen have been found in Gemini. One planet, a massive world more than twice the size of Jupiter, was found to be orbiting Pollux.

PRINCIPAL TERMS

• celestial equator: the imaginary line above Earth’s equator that halves the celestial sphere; it is equally distant from the celestial poles.
• declination: the north-south position of a celestial body relative to the celestial equator expressed in degrees of arc.
• constellation: a section of the night sky identified by patterns of stars seen from Earth.
• International Astronomical Union: a worldwide association of professional astronomers that sets the rules for naming celestial bodies and features on them and defines scientific constants of importance to astronomy.
• right ascension: the east-west position of a celestial body defined in relation to the celestial equator and expressed in hours and minutes, not degrees of arc.

Bibliography

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