| Orion's Belt |
Orion's Belt is an asterism in the constellation of Orion. Other names include the Belt of Orion, the Three Kings, and the Three Sisters.[1] The belt consists of three bright and easily identifiable collinear star systems – Alnitak, Alnilam, and Mintaka – nearly equally spaced in a line, spanning an angular size of ~ (2.3°).[2]
Owing to the high surface temperatures of their constituent stars, the intense light emitted are blue-white in color, aiding in the recognition of the familiar astronomical motif. In spite of their spot-like appearance, only Alnilam is a single star; Alnitak is a triple star system, and Mintaka a hextuple. Each of the three owe their luminosity to the presence of one or more blue supergiants. The brightest as viewed from Sol is Alnilam, with an apparent magnitude of 1.69,[3] followed by Alnitak at 1.74[4] and Mintaka at 2.25.[5] Together, the 10 stars of the three systems has a combined luminosity of nearly one million times (~970,000) that of the Sun's.
Orion's Belt has been called various names and mentioned in various cultures and literature. It has played a central role in astral navigation in the Northern hemisphere since prehistoric times, being readily pointed out by even the most unstarwise wayfarer. As a matter of fact, it is often considered by astronomers to be among the clearest constellations in the winter sky,[6] although it is not visible during summer, when the Sun is too visually close.
The discredited archeological Orion correlation theory postulated a connection between the positions of the Giza pyramids and those of the belt, with the linkage shown to be spurious when placed within the proper historical context.[7]
The names of the three stars that comprise the belt derive from Arabic. All three were once known as Arabic: Al Niṭhām (Arabic: النظام) meaning "string of pearls" with spelling variants that include Arabic: Alnihan and Arabic: Ainilam,[8] which was suggested by Knobel to be mistakes in transliteration or copy errors.[9]
See main article: Alnitak. Alnitak (ζ Orionis) is a triple star system at the eastern end of Orion's belt and is 1,260 light-years from the Earth. Alnitak B is a 4th-magnitude B-type star which orbits Alnitak A every 1,500 years. The primary (Alnitak A) is itself a close binary, comprising Alnitak Aa (a blue supergiant of spectral type O9.7 Ibe and an apparent magnitude of 2.0) and Alnitak Ab (a blue subgiant of spectral type B1IV and an apparent magnitude of about 4). Alnitak Aa is estimated to be up to 28 times as massive as the Sun and have a diameter 20 times greater. It is the brightest star of class O in the night sky.[10]
See main article: Alnilam. Alnilam (ε Orionis) is a singular B0 supergiant, approximately 2,000 light-years away from Earth and magnitude 1.69. It is the 29th-brightest star in the sky and the fourth-brightest in Orion. It is 375,000 times more luminous than the Sun.[11] Its spectrum serves as one of the stable anchor points by which other stars are classified.
See main article: Mintaka. Mintaka (δ Orionis) is a six-star system at the western end of the Belt, and the star system closest to the celestial equator.[8] It is the nearest massive multiple stellar system, composed of three spectroscopic components.[12] The most luminous individual star is a O9.5 II blue giant. Together, the system has a combined ~250,000 solar luminosity. Mintaka is 1,200 light-years distant, with a visual magnitude of 2.25. The innermost binary has a period of 5.732 days and a semi-major axis of approximately 32 million kilometers (0.22 AU), with the two massive stars eclipsing each other twice per completed orbit as viewed from Sol, from which regular minor dips in brightness arise.[13]
Richard Hinckley Allen lists many folk names for the Belt of Orion. English ones include: Jacob's Rod or Jacob's Staff; Peter's Staff; the Golden Yard-arm; The L, or Ell; The Ell and Yard; the Yard-stick, and the Yard-wand; the Ellwand; Our Lady's Wand; the Magi / the Three Kings; the Three Marys; or simply the Three Stars.[14]
The passage "Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion?" is found in the Bible's Book of Job[15] and Book of Amos.[16] Tennyson's poem The Princess describes Orion's belt as:
In China's Classic of Poetry, the asterism, under the name "Shen" (参), was paired with Antares, which is known as "Shang" (商), to be a metaphor for two people who could never unite.[17] This might have stemmed from the observation that both Orion's Belt and Antares rise in the east and set in the west, but Antares only rises once Orion's Belt has set and vice versa.
The three stars of the belt are known in Portugal, South America, and the Philippines as Las Tres Marías in Spanish, and as "As Três Marias" in Portuguese. They also mark the northern night sky when the Sun is at its lowest point, and were a clear marker for ancient timekeeping. In Mexico they are called the Los Tres Reyes Magos.[18]
In Finnish mythology, the Belt of Orion is called Väinämöisen vyö (Väinämöinen's Belt). The stars which appear to "hang" off the belt form an asterism called Kalevanmiekka (Kaleva's sword). In pre-Christian Scandinavia, the belt was known as Frigg's Distaff (Friggerock) or Freyja's distaff.[19] Similarly Jacob's Staff and Peter's Staff were European biblical derived terms, as were the Three Magi, or the Three Kings. Väinämöinen's Scythe (Kalevala) and Kalevan Sword are terms from Finnish mythology.[20]
The Seri people of northwestern Mexico call the three belt stars Hapj (a name denoting a hunter) which consists of three stars: Hap (mule deer), Haamoja (pronghorn), and Mojet (bighorn sheep). Hap is in the middle and has been shot by the hunter; its blood has dripped onto Tiburón Island.[21]
The Māori people of New Zealand refer to the belt as Tautoru (literally "string of three"), and it is often seen as the stern of the constellation Te Waka o Rangi (the canoe of Rangi), which extends to its prow at Matariki (The Pleiades). The rising of Matariki in the dawn sky marks the Māori New Year in late May or early June.[22]