Orion's Sword is an astronomical asterism in the constellation Orion. It comprises three stars (42 Orionis, Theta Orionis, and Iota Orionis) and M42, the Orion Nebula, which together are thought to resemble a sword or a scabbard. This group is situated under the prominent asterism, Orion's Belt, where it points in a southerly direction. Origins behind Orion's Sword are based in mostly Greco-Roman tradition, though this group of stars is referenced as a weapon in multiple cultural contexts (see below).
The Orion Nebula consists of a massive molecular cloud (30 - 40 light years in diameter) within the Milky Way Galaxy, though approximately 1,300 light years from the solar system. This makes the Orion Nebula one of the closest HII regions to Earth, a mass of hydrogen that has been ionized by nearby, hot, young stars. Regions like this are called stellar nurseries, nurturing the birth of multiple young stars such as the Orion Nebula Star Cluster. These stars are a significant feature of the Sword asterism.
42 Orionis, also called c Ori, is a B1V magnitude star in the northern half of the Orion nebula. Theta Orionis has a more central position in the nebula, and is actually composed of a multi-star system. Iota Orionis is one of the brightest in the collection, located in the bottom of the Orion nebula. Iota Orionis is a spectroscopic binary system, with a variable magnitude of O9III.
Given the scientific significance of M42, Orion's Sword is a popular spot for stellar and protostellar studies. Using the Hubble Space Telescope, O'dell et al. focused on identifying previously unseen features of the nebula, such as high-ionization shocks, compact sources, and protoplanetary disks. Some studies have focused on the sword region overall. Gomez & Leda found that less than half of the OB and Hα stars in this region are associated with well-defined stellar clusters. This positional similarity, as well as the high star formation rates and gas pressure in the nearby molecular cloud, confirms the previous notion that old, foreground OB stars triggered star formation in this cloud.
References in history and culture
In his De Astronomia, Hyginus describes the constellation Orion having three faint stars where the sword is depicted. Aratus goes into significant detail about the Orion constellation as well, proclaiming: "Should anyone fail to catch sight of him (Orion) up in the heavens on a clear night, he should not expect to behold anything more splendid when he gazes up at the sky. Cicero and Germanicus, the translators of Aratus's Phaenomena, expressed it as ensis, Latin for "sword". Arabic astronomers also saw this asterism as a sword (سيف saif), calling it Saif al Jabbār, Sword of the Powerful One or Sword of the Giant. Orion is one of the few constellations to have parallel identities in European and Chinese culture, given the name Shen, the hunter and warrior. Chinese astronomers made the sword a sub-constellation within Shen called Fa.
This asterism is not always associated with swords. In the myths of the Namaqua, Orion's sword was the arrow of the husband of the Pleiades, daughters of the sky god, who was represented by the star Aldebaran. When he fired his arrow at three zebras (Orion's belt) and missed, he was too afraid to retrieve the arrow due to its proximity to a fierce lion, represented by Betelgeuse. Therefore, he sits in the cold, suffering from hunger but too ashamed to return home. The Tswana called Orion's sword dintsa le Dikolobe, referring to three dogs which chase the three pigs of Orion's Belt. This serves as an etiological myth for why pigs have their litters in the same season Orion is prominent in the sky.
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- O'dell, C.R.; Wen, Zheng; Hu, Xihai (1993). "Discovery of new objects in the Orion nebula on HST images - Shocks, compact sources, and protoplanetary disks". Astrophysical Journal. 410 (2): 696–700. Bibcode:1993ApJ...410..696O. doi:10.1086/172786.
- Gomez, Mercedes; Lada, Charles (1998). "From Head to Sword: The Clustering Properties of Stars in Orion". The Astronomical Journal. 115 (4): 1524–1535. Bibcode:1998AJ....115.1524G. doi:10.1086/300276.
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