Alnilam (ε Orionis)
The central and brightest of the three stars in → Orion's Belt and the fourth brightest in the whole of → Orion. Alnilam is a blue-white → supergiant of → spectral type B0 Iae with a → visual magnitude of 1.70 and a → luminosity of 375,000 times the → solar luminosity. It lies at about 1,340 → light-years.
Alnilam, from Ar. An-Nizam al-Jawza'
Alnitak (ζ Orionis)
The left hand or easternmost star in → Orion's Belt, which is the fifth brightest in the whole of → Orion with a → visual magnitude of 2.05. Alnitak is a → close binary system comprising Alnitak Aa and Alnitak Ab. Aa is a hot → blue supergiant of → spectral type O9.5 Iab with an → absolute magnitude of -6.0 and an → apparent magnitude of 2.0. Its mass is estimated as being up to 33 times as massive as the Sun and to have a diameter 20 times greater. It is some 250,000 times more luminous than the Sun, with a surface temperature of about 30,000 K. It is the brightest star of class O in the night sky. Alnitak Ab is a blue → subgiant of spectral type B1 IV with an absolute magnitude of -3.9 and an apparent magnitude of 4.3. Ab revolves around Ab with a period of 2,687 days. The system has a 4th magnitude companion, Alnitak B, nearly 3 arc-seconds distant. It is a B0 III type star which orbits Alnitak A every 1,500 years. Alnitak is associated with the → emission nebula → IC 434 containing the → Horsehead Nebula (C. A. Hummel et al., 2013, A&A 554, A52, arXiv:1306.0330).
Alnitak, from Ar. An-Nitaq al-Jawza'
Bellatrix (γ Orionis)
A bright, blue → giant star ( → spectral type B2 III), one of the main stars of the constellation → Orion. With a visual magnitude of 1.64, it is about 1000 times more luminous than the Sun, and lies at a distance of 243 → light-years.
From L. bellatrix "a female warrior," fem. of bellator, from bellum "war."
Merzam, Ar. name of the star; its other name is Nâjed.
Belt of Orion
kamarband-e Šekârgar, ~ Orion
Fr.: Baudrier d'Orion
Betelgeuse (α Orionis)
The → red supergiant that is the second brightest star in the constellation → Orion. Betelgeuse is one of the biggest stars known with a size of almost 1,000 times larger than the Sun, corresponding to an angular diameter of 43.76 ± 0.12 milli-arcseconds (Perrin et al. 2004, A&A 418, 675). It is a → semiregular variable whose → apparent visual magnitude varies between 0.2 and 1.2 shining very rarely more brightly than its neighbor → Rigel. The energy released by Betelgeuse is estimated to be only 13% in the form of visible light, with most of its radiation being at → infrared wavelengths. The distance of Betelgeuse is 643±146 → light-years (Harper et al. 2008, AJ 135, 1430), while its luminosity is about 140,000 times that of the Sun (→ solar luminosity). Its → spectral type is M2 Iab, its → surface temperature about 3,600 K, and its → initial mass 10 to 20 → solar masses (Msun). Neilson & Lester (2011, arXiv:1109.4562) recently proposed a mass of 11.6 (+5.0, -3.9) Msun for Betelgeuse, while Dolan et al. (2008, BAPS 53, APR.S8.6) obtained about 21 Msun. Its → rotation period is estimated to be about 17 years (Uittenbroek et al. 1998, AJ 116, 2501). Recent observations with the → Very Large Telescope resolve not only the apparent surface of Betelgeuse, but also reveal a large and previously unknown plume of gas extending into space from the surface of the star (Kervella et al. 2009, A&A 504, 115). The plume extends to at least six times the diameter of the star, corresponding to the distance between the Sun and Neptune. This detection suggests that the whole outer shell of Betelgeuse is not shedding matter evenly in all directions. More recently, an image of the surface of the star was obtained using long → baseline → interferometry at infrared wavelengths (Haubois et al. 2009, A&A 508, 923). It shows the presence of an irregular flux distribution possibly caused by enormous → convective cells. A very large dusty envelope has also been observed at larger distances from the star (Kervella et al. 2011, A&A 531, A117).
Betelgeuse, from Ar. Ibt al-Jauza' (
Ebtoljowzâ, from Ar. Ibt al-Jauza'.
Fr.: delta Orionis
Same as → Mintaka.
FU Orionis object
barâxt-e FU Šekârgar
Fr.: objet FU Orionis
A member of a class of → pre-main sequence stars that experience dramatic changes in magnitude and → spectral type. During an outburst the luminosity of such an object can increase by several orders of magnitude on short time-scales (few months to few years). The phenomenon is explained by abrupt mass transfer from an → accretion disk to a young, low mass → T Tauri star (accretion rates 10-4 to 10-3 solar masses per year). → EX Lupi; → Z CMa.
Mintaka (δ Orionis)
The faintest and the westernmost of the three stars which appear in a row and make up the → Orion's Belt. It is a blue star of magnitude 2.23 lying 915 light-years away. Mintaka is in fact an → eclipsing binary with a period of 5.7 days. The main star has a → spectral type of O9.5 and radiates near 90,000 times the → solar luminosity. Mintaka is remarkable as regards the discovery of the → interstellar medium. The ISM was discovered by the German astronomer Johannes Hartmann (1855-1936) through the study of δ Orionis. He remarked that the calcium line at 3934 Å did not share in the periodic displacements of the lines caused by the orbital motion of the star. This suggested that the calcium line was not from the stars but from an intervening interstellar absorbing cloud.
Mintaka, from al-Mintaqah "the belt," from al-Mintaqah al-Jauzâ'
Šekârgar (#), Orion (#)
The Hunter. A prominent constellation, one of the largest in the sky, located on the celestial equator around 5h 30m right ascension, 0° declination. This constellation is rich in bright stars and nebulae. The brightest star is Rigel (β Orionis), visual magnitude 0.2. The second brightest star is → Betelgeuse (α Orionis), magnitude between 0.2 and 1.0. A key feature of Orion's constellation is his Belt of three bright stars that form a nearly straight line across its central parts. It contains also the → Orion Nebula, the only region of massive star formation visible to the unaided eye.
In Gk. mythology, Orion was a giant hunter and the enemy of Artemis the huntress, who according to some tales was responsible for his death. Other stories, though, tell how he pursued the Pleiades and with them was turned into a constellation to chase them forever across the sky.
Šekârgar, "→ hunter."
bâzu-ye Šekârgar, ~ Orion
Fr.: bras d'Orion
A minor → spiral arm of the → Milky Way Galaxy close to which the → Sun is located. It is some 3,500 → light-years across and approximately 10,000 light-years in length. The solar system lies close to the inner rim of this spiral arm, about halfway along its length. Its name derives from the fact that the stars closest to the Sun which actually lie within the arm are in the constellation → Orion. Its other designations are → Local Arm, → Local Spur, → Orion Bridge, → Orion Spur, and → Orion-Cygnus Arm.
âhazeš-e Šekârgar, ~ Orion
Fr.: association d'Orion
mile-ye Šekârgar, ~ Orion
Fr.: barre d'Orion
A part of a → molecular cloud toward the → Orion Nebula viewed edge-on. It is the surface of interaction between the → H II region and its → associated molecular cloud. Same as the → Orion Bright Bar.
Fr.: pont d'Orion
Same as → Orion Arm.
Orion Bright Bar
mile-ye deraxšân-e Šekârgar, ~ ~ Orion
Fr.: barre brillante d'Orion
A prominent emission ridge in the → Orion Nebula located approximately 2' southeast of the → Trapezium cluster. Various observations have suggested that it is an escarpment in the main → ionization front of the Nebula seen almost edge-on. The Orion Bar is one of the nearest and best-studied → photodissociation regions.
Orion correlation theory
negare-ye hambâzâneš-e Oryon
Fr.: théorie de la corrélation d'Orion
A controversial proposition according to which a coincidence would exist between the mutual positions of the three stars of → Orion's Belt and those of the main Giza pyramids. More specifically, Khufu, Khafre, and Menkaure would be the monumental representation of → Alnitak, → Alnilam, and → Mintaka, respectively.
Orion molecular cloud
abr-e molekuli-ye Šekârgar, ~ ~ Orion
Fr.: nuage moléculaire d'Orion
A giant cloud, or complex of clouds, of interstellar gas and dust associated with the Orion nebula (M42). It is about 1,500 light-years away and measures about 240 light-years across. Besides M42 and M43 it contains a number of famous objects, including Barnard's Loop, the Horsehead Nebula, and the reflection nebulae around M78. Within this cloud, stars have formed recently, and are still in the process of formation.
miq-e Šekârgar, ~ Orion
Fr.: Nébuleuse d'Orion
The best known ionized nebula and one of the nearest regions to the Sun in which stars are presently being formed. It is visible to the naked eye in the constellation → Orion south of Orion's Belt as a fuzzy patch. It lies about 1,500 light-years away and measures about 30 light-years across. The Orion Nebula is ionized and made visible by a small group of O and B type stars known as the Trapezium cluster. Other designations: M 42, NGC 1976. → Huygens Region ; → Orion Bright Bar.
Fr.: éperon d'Orion
Same as → Orion Arm.
kamarband-e Šekârgar, ~ Orion
Fr.: Ceinture d'Orion
Three prominent stars in the central regions of the constellation → Orion that align to form the "belt" of the mythological Hunter. They are → Alnitak, → Alnilam, and → Mintaka. The easternmost star Alnitak is separated from the middle one, Alnilam, by 1°.36, and the westernmost Mintaka has an angular distance of 1°.23 from Alnilam. Their distance is between 800 and 1,300 light-years from Earth. They probably formed inside the same molecular cloud less than 10 million years ago.
Fr.: bras Orion-Cygne
Same as → Orion Arm.