An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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; → bright; → bar.

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; → correlation; → theory.

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.

→ Orion; → molecular cloud.

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-type and → B-type stars known as the → Trapezium cluster. Other designations: M 42, NGC 1976. See also: → Orion molecular cloud; → Huygens Region ; → Orion association; → Orion Bar; → Orion Bright Bar.

→ Orion; → nebula.

An → OB association consisting of several dozen → hot stars of → spectral types O and B (→ O star, → B star). The Orion OB1 association consists of several subgroups, first divided by Blaauw (1964) into four subgroups. The subgroups differ in age and gas and dust content:
Orin OB1a, which contains the stars to the northwest of → Orion's Belt stars. Within this group lies another subgroup, known as the → 25 Orionis group;
Orion OB1b, containing the group of stars located in and around the Belt;
Orion OB1c, including the stars around → Orion's Sword; and
Orion OB1d, which contains the stars in and close to the → Orion Nebula (including the → Trapezium cluster).

→ Orion; → OB association.

Same as → Orion Arm.

→ Orion; → → spur.

Three prominent stars in the central regions of the constellation → Orion that align to form the "belt" of the mythological Hunter. They are → Alnitak (ζ Ori), → Alnilam (ε Ori), and → Mintaka (δ Ori). 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.

→ Orion; → belt.

An astronomical → asterism in the constellation → Orion forming an almost vertical line beneath → Orion's Belt. From north to south, the most prominent objects in the Sword are the cluster NGC 1981, the star → 42 Orionis, the famous → Orion Nebula, and the Sword's brightest star → Iota Orionis (→ Hatsya). None of these objects is particularly bright in itself, but their proximity to one another and the nebulosity across much of this region makes the Sword stand out clearly in the night sky.

→ Orion; → sword.

Same as → Orion Arm.

→ Orion; → Cygnus; → arm.

A meteor shower that appears to emanate from the constellation → Orion. It peaks between October 14 and 20 with about 20 meteors per hour.

→ Orion + → -ides.

A → supergiant star of visual magnitude 2.06 and → spectral type B0.5 Ia marking the right knee of Orion. It is about 700 light-years away.

Saiph "sword," from Ar. as-saiph al-jabbâr (سیف الجبار) "the Sword of the Giant."

Same as → Alnitak.

Zeta (ζ), Gk. letter in the → Bayer designation scheme.