Orion (constellation)
Orion is a prominent set of stars visible during winter in the northern celestial hemisphere. It is one of the 88 modern constellations; it was among the 48 constellations listed by the 2nd-century AD/CE astronomer Ptolemy. It is named after a hunter in Greek mythology.
Orion is most prominent during winter evenings in the Northern Hemisphere, as are five other constellations that have stars in the Winter Hexagon asterism. Orion's two brightest stars, Rigel and Betelgeuse, are both among the brightest stars in the night sky; both are supergiants and slightly variable. There are a further six stars brighter than magnitude 3.0, including three making the short straight line of the Orion's Belt asterism. Orion also hosts the radiant of the annual Orionids, the strongest meteor shower associated with Halley's Comet, and the Orion Nebula, one of the brightest nebulae in the sky.
Characteristics
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 26th of the 88 constellations in size. The constellation boundaries, as set by Belgian astronomer 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 and, while the declination coordinates are between and. The constellation's three-letter abbreviation, as adopted by the International Astronomical Union in 1922, is "Ori".Orion is most visible in the evening sky from January to April, winter in the Northern Hemisphere, and summer in the Southern Hemisphere. In the tropics, the constellation transits at the zenith.
From May to July, Orion is in the daytime sky and thus invisible at most latitudes. However, for much of Antarctica in the Southern Hemisphere's winter months, the Sun is below the horizon even at midday. Stars are then visible at twilight for a few hours around local noon, just in the brightest section of the sky low in the North where the Sun is just below the horizon. At the same time of day at the South Pole itself, Rigel is only 8° above the horizon, and the Belt sweeps just along it. 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, Orion appears overhead in December around midnight and in the February evening sky.
Navigational aid
Orion is very useful as an aid to locating other stars. By extending the line of the Belt southeastward, Sirius can be found; northwestward, Aldebaran. A line eastward across the two shoulders indicates the direction of Procyon. A line from Rigel through Betelgeuse points to Castor and Pollux. Additionally, Rigel is part of the Winter Circle asterism. Sirius and Procyon, which may be located from Orion by following imaginary lines, also are points in both the Winter Triangle and the Circle.Features
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, at the center of which lie the three stars of Orion's Belt—Alnitak, Alnilam, and Mintaka. His head is marked by an additional eighth star called Meissa, which is fairly bright to the observer. Descending from the Belt is a smaller line of three stars, Orion's Sword, also known as the hunter's sword.Many of the stars are luminous hot blue supergiants, with the stars of the Belt and Sword forming the Orion OB1 association. Standing out by its red hue, Betelgeuse may nevertheless be a runaway member of the same group.
Bright stars
- Betelgeuse, also designated Alpha Orionis, is a massive M-type red supergiant star nearing the end of its life. It is the second-brightest star in Orion, and is a semiregular variable star. It serves as the right shoulder of the hunter. It is generally the 11th-brightest star in the night sky, but this has varied between being the tenth to the 23rd-brightest by the end of 2019. The end of its life is expected to result in a supernova explosion that will be highly visible from Earth, possibly outshining the Moon and being visible during the day. This is most likely to occur within the next 100,000 years.
- Rigel, also known as Beta Orionis, is a B-type blue supergiant that is the seventh-brightest star in the night sky. Similar to Betelgeuse, Rigel is fusing heavy elements in its core and will pass its supergiant stage soon, possibly eventually ending in a supernova explosion. It serves as the left foot of the hunter.
- Bellatrix is designated Gamma Orionis by Johann Bayer. It is the 27th-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 a large radius. Bellatrix marks Orion's left shoulder. Its name means "the female warrior", and is sometimes known colloquially as the "Amazon Star". It is the closest major star in Orion at only 244.6 light-years from the Solar System.
- Mintaka is designated Delta Orionis, despite being 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 and is named for the Arabic phrase meaning "string of pearls". It is the middle and brightest of the three stars of Orion's Belt. Alnilam is a B-type blue supergiant; despite being nearly twice as far from the Sun as the other stars on the Belt, its luminosity makes it nearly equal in magnitude. Alnilam is losing mass quickly, a consequence of its size. It is the farthest major star in Orion, at 1,344 light-years.
- Alnitak, meaning "the girdle", is designated Zeta Orionis, and is the easternmost star in Orion's Belt. It is a triple star system, with the primary star being a hot blue supergiant and the brightest O-type star in the night sky.
- Saiph is 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. It name means "the sword of the giant".
- Meissa is designated Lambda Orionis, forms Orion's head, and is a multiple star with a combined apparent magnitude of 3.33. Its name means "the shining one".
Belt
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 lying at a distance of 1150 light-years. Southwest of Mintaka lies the quadruple star Eta Orionis.
Sword
Orion's Sword contains the Orion Nebula, the Messier 43 nebula, Sh 2-279, and the stars Theta Orionis, Iota Orionis, and 42 Orionis.Head
Three stars comprise a small triangle that marks the head. The apex is marked by Meissa, 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 young star FU Orionis.Club
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 star U Orionis.Shield
West from Bellatrix lie six stars all designated Pi Orionis which make up Orion's shield.Meteor showers
Around 20 October each year, the Orionid meteor shower 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.Deep-sky objects
Hanging from Orion's Belt is his sword, consisting of the multiple stars θ1 and θ2 Orionis, called the Trapezium and the Orion Nebula. This is a spectacular object that can be clearly identified with the naked eye as something other than a star. Using binoculars, its clouds of nascent stars, luminous gas, and dust can be observed. 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 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. It can easily be mistaken for a comet in the eyepiece of a telescope. 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 Alnitak. 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 with a small telescope will reveal a wealth of interesting deep-sky objects, including M43, M78, and multiple stars including Iota Orionis and Sigma Orionis. A larger telescope may reveal objects such as the Flame Nebula, as well as fainter and tighter multiple stars and nebulae. Barnard's Loop can be seen on very dark nights or using long-exposure photography.
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. Due to its proximity, it is one of the most intense regions of stellar formation visible from Earth. The Orion molecular cloud complex forms the eastern part of an even larger structure, the Orion–Eridanus Superbubble, which is visible in X-rays and in hydrogen emissions.