Orion (constellation)

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy

ABSTRACT: The constellation Orion is easily found in the night sky. It is named for a figure in Greek mythology and is said to be in the shape of a hunter. Its most recognizable feature is a line of three bright stars that make up the hunter’s belt.

Finding the Hunter

A constellation is an area of space defined by group of stars that people have interpreted as a pattern. Humans have identified and studied constellations for tens of thousands of years. Modern astronomers of the International Astronomical Union (IAU) recognize eighty-eight official constellations, of which Orion is one.

Though Orion can be seen from every part of the world due to its position on the celestial equator, it is only visible during the winter months in the Northern Hemisphere. The later months of the year are often referred to as the hunting season, which makes Orion, named after a mythological hunter, particularly appropriate for this time of year. Ancient peoples recognized that some stars and constellations were only visible at certain times of year and used that information to keep track of the seasons. Throughout history, many people also found spiritual meaning in the patterns of the stars. Today, astronomers study constellations such as Orion to name and track stars and deep-space objects and to learn more about their origins.

Attributes of Orion

Orion can be clearly seen in the night sky from October to March in either hemisphere. It is most easily observed between latitudes 85 degrees north and 75 degrees south. It has a declination of +4.58 degrees and a right ascension of 5.59 hours. Orion is recognizable by the arrangement and intensity of its stars.

Many budding astronomers have found Orion easy to locate because of a distinctive row of bright stars known as Orion’s Belt, which is the constellation’s most prominent feature. The three stars of Orion’s Belt are epsilon Orionis (also known as Alnilam), delta Orionis (Mintaka), and zeta Orionis (Alnitak). All three belong to a spiral arm of the Milky Way galaxy. Orion’s shoulders are also stars: alpha Orionis (Betelgeuse) is the right shoulder, and gamma Orionis (Bellatrix) the left. Lambda Orionis (Meissa) forms Orion’s head; iota Orionis (Na’ir al Saif), the tip of his sword; kappa Orionis (Saiph), the right knee or foot; and beta Orionis (Rigel), the left knee. The Greek letters in star names usually rank a constellation’s stars in order of brightness. However, the brightest star in Orion is not Betelgeuse but beta Orionis (Rigel).

One element of the constellation appears to be a star but is really made of dust and ionized gases, including helium and hydrogen. This is the Orion Nebula, the midpoint of the hunter’s sword.

Most of the major stars of Orion are young blue giants. Though they appear to be grouped together, most are only in the same field of vision when viewed from Earth. Bellatrix is roughly 252 light-years away, while Alnilam is much farther away, at 1,340 light-years. The Orion Nebula is very far away, at about 1,600 light-years.

Betelgeuse is different from the other stars of Orion. It is a red giant three hundred times wider than the sun and about twenty times as massive. Its distance from Earth is 522 light years. A red giant is a massive star nearing the end of its lifespan. When Betelgeuse does eventually die, it will go out in a blaze of cosmic glory in a gigantic explosion known as a supernova. In 2021, the star’s brightness mysteriously dimmed by more than two-thirds, leading some scientists to believe the supernova was imminent. However, later observations found the dimming was cause by the star releasing a cloud of dust that temporarily obscured its light.

The Hunter’s History

Myths from many world cultures surround this constellation and its stars. Stories about Orion are found in the most ancient Greek works, indicating that it was among the earliest constellations recognized by early humans. Even before the Greeks, the Sumerians saw the figure as another hero, the demigod Gilgamesh. In these tales, Gilgamesh fights and slays the Bull of Heaven, a neighboring constellation that the Greeks reimagined as Taurus the bull.

According to several well-known Greek myths about the constellation, Orion’s parents were the sea god Poseidon and the Cretan princess Euryale. In one such tale, Orion claimed that he was the world’s mightiest hunter and would kill all the beasts. This angered Gaia, the earth mother, who sent a scorpion after him. The scorpion killed Orion, after which Zeus, king of the gods, lifted the hunter into the sky. Though many myths about Orion’s death have been told, most agree that he was killed by a scorpion. This constellation, Scorpius, is located far from Orion. According to some lore, as the scorpion rises in the eastern sky, Orion escapes by disappearing over the western horizon.

Many stories say that Orion is doomed to chase his prey across the sky but never catch it. Two other constellations, Canis Major and Canis Minor, are closely associated with Orion in Greek mythology. These nearby groupings of stars are known as the hunter’s dogs. They follow their master in pursuit of Lepus, the hare.

Another myth about Orion claims he pursued the Pleiades, the seven daughters of the Titan Atlas. Zeus rescued the maiden nymphs by placing them in the sky as the Pleiades star cluster, in the constellation Taurus, which Orion follows nightly in pursuit. Yet another says that Orion was killed by his lover, the goddess Artemis, when her brother, Apollo, tricked her into shooting him during an archery demonstration. The bereaved goddess put her beloved among the stars.

Arab astronomers saw Orion’s stars as part of a larger female figure that included the Gemini constellation as well. The name of this figure, al-Jauza, loosely translates as "central one," possibly because of its location on the celestial equator. Betelgeuse is one of her hands.

Chinese astronomers viewed Orion much as the Greeks did, as a warrior or hunter figure. The Chinese name for the constellation is Shen, which means "three stars." They saw this grouping as part of a vast hunting scene.

Orion Nebula

Though Orion’s Belt is easily locatable by even the most casual observers, it is another area of the constellation that draws the most intense scrutiny. Scientists study the Orion Nebula, formally called Messier 42 (M42), because they are interested in learning about the birth of stars and the origins of other celestial bodies and even the universe. The nebula is described as a birthplace of stars, and scientists have observed the creation of stars there.

The Orion Nebula is one of the most photographed areas of space. Scientists have found many nebulae, which are clouds of interstellar dust and gas, in the Milky Way galaxy. However, the Orion Nebula is one of only a handful visible from Earth with the naked eye on dark, moonless nights. Backyard astronomers can get a good look at this nebula with binoculars or a basic telescope, especially if they are far from well-lit areas such as cities.

The Orion Nebula is about 1,600 light-years from Earth and about 15 light-years across. Thousands of stars are being born there. Scientists have observed the development of an open star cluster, which some call the Orion Nebula Star Cluster. Gravity loosely ties these young stars together.

Early astrophysicists developed a theory of star formation that was fairly orderly. It theorized that stars formed one by one in isolation when the delicate balance of gravity and pressure in a gas cloud was disturbed. Late in the twentieth century, infrared images revealed a less tidy birth story. After the Spitzer Space Telescope was launched in 2003, the resulting images revealed that stars are created in chaos and through violent reactions. Most stars develop in groups, as is happening in the Orion Nebula. In 2016, astronomers using the Very Large Telescope in Chile were able to examine the Orion Nebula in more detail than ever before. They found the cloud of gas and dust is producing a multitude of young stars and planetary objects.

Another discovery in the Orion Nebula is a series of gas clumps that scientists have dubbed Orion Bullets. These gas clumps were first detected in 1983. Researchers say the clumps of gas contain iron atoms and are about ten times the size of Pluto’s orbit around the sun. More detailed images taken by the Gemini South Observatory in Chile suggest that the bullets were ejected from the nebula’s star-forming areas at hundreds of kilometers per second.

The Orion Nebula is significant for scientists and amateur astronomers alike. Many people are able to study the four brightest stars in the nebula, which they call the Trapezium, with basic telescopes. These infant stars are about a million years old.

PRINCIPAL TERMS

• celestial equator: the imaginary line above Earth’s equator that halves the celestial sphere.
• constellation: a region of space defined by a pattern of stars 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 angular distance from the vernal equinox, measured eastward along the celestial equator and expressed in hours, minutes, and seconds.

Bibliography

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