An Etymological Dictionary of Astronomy and Astrophysics
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A blue star of → apparent visual magnitude 4.96 lying in the → Orion constellation. It has other designations, including: ψ¹ Orionis, BD+01 1005, HR 1789, and HIP 25302. 25 Ori is a → main sequence star of → spectral type B1 V. It lies at a distance of 1,040 → light-years and has a → luminosity of 9,300 MSun, a radius of 5.5 Rsun, and a mass of about 10 Msun. 25 Ori is in fact a → Be star.

A group of nearly 200 low-mass → pre-main-sequence stars, concentrated within ~ 1 of the early → B star  → 25 Orionis, in the component a of the → Orion OB1 Association. The group also harbors the → Herbig Ae/Be star V346 Ori and a dozen other early-type stars. The velocity distribution for the low-mass stars shows a narrow peak at 19.7 km s^-1, offset ~ 10 km s^-1 from the velocity characterizing the younger stars of the Ori OB1b subassociation, and 4 km s^-1 from the velocity of widely spread young stars of the Ori OB1a population. This indicates that the 25 Ori group is a distinct kinematic entity. The low-mass members follow a well-defined band in the → color-magnitude diagram, consistent with an age of ~ 7-10 Myr (Briceno et al., 2007, ApJ 661, 1119).

→ 25 Orionis; → group.

A blue star with → apparent visual magnitude 4.59 in → Orion's Sword. Also known as → c Orionis, HD 37018, HR 1892, and HIP 26237. More specifically, it is a → main sequence star of → spectral type B1 V, lying in the → H II region→ NGC 1977. 42 Orionis is approximately 900 → light-years away based on parallax. It is the major source for ionizing photons in NGC 1977. 42 Ori is a → mutiple star system. The primary star, Aa, of magnitude 6.3 has a → companion companion Ab at a separation of 0.16'', and a more distant companion B of 7.5 magnitude at 1.6'' separation. An irradiated → circumstellar disk near 42 Ori has been detected by Bally et al. (2012) in the → HST image using Hα filter (F658N). They identified a bent → protostellar jet HH1064 from Parenago 2042 (the Spindle) in NGC 1977 with numerous → bow shock features. They argue that the arc feature in the Hα Spindle is centered on the star and its brightened side of the arc is facing toward 42 Ori, suggesting that it may be a → proplyd (Kim et al., 2016, arXiv::1606.08271).

Number 42 in → Flamsteed designation; Orionis, genitive of → Orion.

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' (النظام‌الجوزاء) "the Orion's arrangement (of pearls, beads)," from nizam "arrangement" + Jawza' "Orion".

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 1.79. Alnitak is a wide visual binary system consisting of components ζ Ori A (HR 1948) and ζ Ori B (HR 1949), currently separated by ~ 2''.4. ζ Ori A 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 2.08. 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, an apparent magnitude of 4.3, and a mass of 14 Msun. 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.A much fainter fourth component, ζ Ori C, is located about 57'' away from ζ Ori Aa (C. A. Hummel et al., 2013, A&A 554, A52, arXiv:1306.0330).

Alnitak, from Ar. An-Nitaq al-Jawza' (النطاق‌الجوزاء) "Orion's Belt," from nitaq "belt" + Jawza "Orion."

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.

→ Orion's Belt.

→ belt; → Orion.

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 (M_sun). Neilson & Lester (2011, arXiv:1109.4562) recently proposed a mass of 11.6 (+5.0, -3.9) M_sun for Betelgeuse, while Dolan et al. (2008, BAPS 53, APR.S8.6) obtained about 21 M_sun. 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' (ابط‌الجوزاء) "the armpit of Jauza'," from ibt "armpit" + Jauza' "Orion."

Ebtoljowzâ, from Ar. Ibt al-Jauza'.

Same as → 42 Orionis.

Star name in the → Bayer designation scheme.

Same as → Mintaka.

→ delta; → Orion.

A segment of the → zodiac extending 2.5 degrees, as considered in Babylonian and Hellenistic astrology; plural: dodekatemoria. Dodekatemoria result from a subdivision of each → zodiacal sign into twelve equal parts, each given the name of a → sign, beginning with the name of the sign being divided and continuing throughout the other eleven sequentially. Each zodiacal sign therefore contained a micro-zodiac within its own 30° span. Textual evidence for the micro-zodiac does not antedate the sixth century BC (F. Rochberg, 2010, In the Path of the Moon, BRILL).

From Gk. dodekatemorion "twelfth part," from dodekate "twelfth" (from dodeka "twelve") + morion "part."

Davâzdahân, from Mid.Pers. dwâzdahân "the twelve ones," from dwâzdah (Mod.Pers. davâzdah) "twelve;" Av. dvadasa, from dva "→ two" + dasa "→ ten."

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.

F and U, alphabet letters; Orionis, → Orion; → object.

A → multiple star system in the → Orion constellation. Also known as → Hatsya, → Na'ir al-Saif, and HR 1899. It is the brightest star of → Orion's Sword, located at the sword's tip, with an → apparent visual magnitude of 2.8. From parallax measurements, it is located at a distance of roughly 1,330 → light-years (410 parsecs) from the Sun. The system has three components designated Iota Orionis A, B and C. Iota Orionis A is itself a massive spectroscopic binary, with components Iota Orionis Aa and Ab.

Iota, Greek letter ι used in the → Bayer designation of star names; Orionis, genitive of → Orion.

Same as → Meissa.

Lambda (λ), a Greek letter used in the → Bayer designation of star names.

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' (المنطقه‌الجوزاء) "the belt of the central one (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."

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.

→ Orion; → arm.

A large OB stellar association centered on the → Orion Nebula. Lying some 1500 → light-years away, it is about 400 light-years across and contains the main stars of Orion, except → Betelgeuse.

→ Orion; → association.

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.

→ Orion; → bar.

Same as → Orion Arm.

→ Orion; → bridge.