Orion is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous and recognizable constellations in the night sky. It was named after Orion, a hunter in Greek mythology. Orion is bordered by Taurus to the northwest, Eridanus to the southwest, Lepus to the south, Monoceros to the east, and Gemini to the northeast. Covering 594 square degrees, Orion ranks twenty-sixth of the 88 constellations in size. The constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 26 sides. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 04h 43.3m and 06h 25.5m, while the declination coordinates are between 22.87° and −10.97°. Orion is most visible in the evening sky from January to March, winter in the Northern Hemisphere, and summer in the Southern Hemisphere. In the tropics (less than about 8° from the equator), the constellation transits at the zenith. In the Southern Hemisphere’s summer months, when Orion is normally visible in the night sky, the constellation is actually not visible in Antarctica because the sun does not set at that time of year south of the Antarctic Circle. In countries close to the equator (e.g. Kenya, Indonesia, Colombia, Ecuador), Orion appears overhead in December around midnight and in the February evening sky.
Orion’s seven brightest stars form a distinctive hourglass-shaped asterism, or pattern, in the night sky. Four stars—Rigel, Betelgeuse, Bellatrix and Saiph—form a large roughly rectangular shape, in the centre of which lie the three stars of Orion’s Belt—Alnitak, Alnilam and Mintaka. Descending from the ‘belt’ is a smaller line of three stars, Orion’s Sword (the middle of which is in fact not a star but the Orion Nebula), also known as the hunter’s sword. The seven brightest stars are:
- Betelgeuse, alternatively by its Bayer designation Alpha Orionis, is a massive M-type red supergiant star nearing the end of its life. When it explodes it will even be visible during the day. It is the second brightest star in Orion and is a semiregular variable It serves as the “right shoulder” of the hunter it represents (assuming that he is facing the observer), and is the eighth brightest star in the night sky.
- Rigel, also known as Beta Orionis, is a B-type blue supergiant that is the sixth brightest star in the night sky. Similar to Betelgeuse, Rigel is fusing heavy elements in its core and will pass its supergiant stage soon (on an astronomical timescale), either collapsing in the case of a supernova or shedding its outer layers and turning into a white dwarf. It serves as the left foot of Orion, the hunter.
- Bellatrix was designated Gamma Orionis by Johann Bayer but is known colloquially as the “Amazon Star. It is the twenty-seventh brightest star in the night sky. Bellatrix is considered a B-type blue giant, though it is too small to explode in a supernova. Bellatrix’s luminosity is derived from its high temperature rather than its radius, a factor that defines Betelgeuse. Bellatrix serves as Orion’s left shoulder.
- Mintaka garnered the name Delta Orionis from Bayer, even though it is the faintest of the three stars in Orion’s Belt. Its name means “the belt.” It is a multiple star system, composed of a large B-type blue giant and a more massive O-type main-sequence star. The Mintaka system constitutes an eclipsing binary variable star, where the eclipse of one star over the other creates a dip in brightness. Mintaka is the westernmost of the three stars of Orion’s Belt, as well as the northernmost.
- Alnilam is designated Epsilon Orionis, a consequence of Bayer’s wish to name the three stars in Orion’s Belt (from north to south) in alphabetical order. Also called Al Nathin, Alnilam is named for the Arabic phrase meaning “string of pearls.” Alnilam is a B-type blue supergiant; despite being nearly twice as far from the Sun as Mintaka and Alnitak, the other two belt stars, its luminosity makes it nearly equal in magnitude. Alnilam is losing mass quickly, a consequence of its size; it is approximately four million years old.
- Alnitak, meaning “the girdle,” was designated Zeta Orionis by Bayer, and is the easternmost star in Orion’s Belt. It is a triple star some 800 light years distant, with the primary star being a hot blue supergiant and the brightest class O star in the night sky.
- Saiph was designated Kappa Orionis by Bayer and serves as Orion’s right foot. It is of a similar distance and size to Rigel, but appears much fainter, as its hot surface temperature (46,000 °F / 26,000 °C) causes it to emit most of its light in the ultraviolet region of the spectrum.
Orion’s Belt or The Belt of Orion is an asterism within the constellation. It consists of the three bright stars Zeta (Alnitak), Epsilon (Alnilam), and Delta (Mintaka). Alnitak is around 800 light years away from earth and is 100,000 times more luminous than the Sun; much of its radiation is in the ultraviolet range, which the human eye cannot see. Alnilam is approximately 1340 light years away from Earth, shines with magnitude 1.70, and with ultraviolet light is 375,000 times more luminous than the Sun. Mintaka is 915 light years away and shines with magnitude 2.21. It is 90,000 times more luminous than the Sun and is a double star: the two orbits each other every 5.73 days. Looking for Orion’s Belt in the night sky is the easiest way to locate the constellation. In the Northern Hemisphere, Orion’s Belt is best visible in the night sky during the month of January around 9:00 pm, when it is approximately around the local meridian. Just southwest of Alnitak lies Sigma Orionis, a multiple star system composed of five stars that have a combined apparent magnitude of 3.7 and lies 1150 light years distant. Southwest of Mintaka lies the quadruple star Eta Orionis. Orion’s Sword contains the Orion Nebula, the Messier 43 nebula, the Running Man Nebula, and the stars Theta Orionis, Iota Orionis, and 42 Orionis. Three stars compose a small triangle that marks the head. The apex is marked by Meissa (Lambda Orionis), a hot blue giant of spectral type O8 III and apparent magnitude 3.54, which lies some 1100 light years distant. Phi-1 and Phi-2 Orionis make up the base. Also nearby is the very young star FU Orionis. Stretching north from Betelgeuse are the stars that make up Orion’s club. Mu Orionis marks the elbow, Nu and Xi mark the handle of the club, and Chi1 and Chi2 mark the end of the club. Just east of Chi1 is the Mira-type variable red giant U Orionis. West from Bellatrix lies six stars all designated Pi Orionis (π1 Ori, π2 Ori, π3 Ori, π4 Ori, π5 Ori and π6 Ori) which make up Orion’s shield.
Hanging from Orion’s belt is his sword, consisting of the multiple stars θ1 and θ2 Orionis, called the Trapezium and the Orion Nebula (M42). This is a spectacular object that can be clearly identified with the naked eye as something other than a star. The Trapezium cluster has many newborn stars, including several brown dwarfs, all of which are at an approximate distance of 1,500 light-years. Named for the four bright stars that form a trapezoid, it is largely illuminated by the brightest stars, which are only a few hundred thousand years old. Observations by the Chandra X-ray Observatory show both the extreme temperatures of the main stars—up to 60,000 kelvins—and the star-forming regions still extant in the surrounding nebula. M78 (NGC 2068) is a nebula in Orion. With an overall magnitude of 8.0, it is significantly dimmer than the Great Orion Nebula that lies to its south; however, it is at approximately the same distance, at 1600 light-years from Earth. M78 is associated with the variable star V351 Orionis, whose magnitude changes are visible in very short periods of time. Another fairly bright nebula in Orion is NGC 1999, also close to the Great Orion Nebula. It has an integrated magnitude of 10.5 and is 1500 light-years from Earth. The variable star V380 Orionis is embedded in NGC 1999. Another famous nebula is IC 434, the Horsehead Nebula, near ζ Orionis. It contains a dark dust cloud whose shape gives the nebula its name. NGC 2174 is an emission nebula located 6400 light-years from Earth. Besides these nebulae, surveying Orion will reveal a wealth of interesting deep-sky objects, including M43, M78, as well as multiple stars including Iota Orionis, Sigma Orionis, Barnard’s Loop and the Flame Nebula (NGC 2024). All of these nebulae are part of the larger Orion Molecular Cloud Complex, which is located approximately 1,500 light-years away and is hundreds of light-years across. It is one of the most intense regions of stellar formation visible within our galaxy.
Around 20 October each year, the Orionid meteor shower (Orionids) reaches its peak. Coming from the border with the constellation Gemini as many as 20 meteors per hour can be seen. The shower’s parent body is Halley’s Comet. Credit: Wikipedia.