Orion's Belt Asterism Distinctively Dominates Winter Skies

Summary: With three fairly evenly spaced stars, Orion's Belt asterism distinctively dominates winter skies December to February in the Northern Hemisphere.

Orion's Belt asterism with reddish arc of Barnard's Loop (center); “Orion Head to Toe,” October 2010: Rogelio Bernal Andreo, CC BY SA 3.0 Unported, via Wikimedia Commons

Orion's Belt asterism, which comprises three bright, fairly evenly spaced, massive stars, distinctively dominates winter skies December to February in the Northern Hemisphere.

The recognizable pattern of stars that is known as an asterism may comprise participants in an official constellation or borrowings from more than one constellation. Orion's Belt asterism lies about midway down the length of the hourglass-shaped constellation. Its conspicuous location dramatizes Orion the Hunter as the dominant constellation in the Northern Hemisphere's winter skies.

The three bright, second-magnitude stars that define the Orion's Belt asterism appear in a fairly evenly spaced alignment that dips diagonally from west to east. The trio's bluish-white color emphasizes their nearly precise alignment against the dark backdrop of naked eye astronomy as well as against telescopic astronomy's colorful deep sky objects, such as the gigantic arc of emission nebula Barnard's Loop.

Mintaka is a double star in a multiple star system. Bluish-white variable star Mintaka marks the westernmost position in Orion's Belt. Because it lies only a quarter of a degree south of the celestial equator, which is the projection of Earth's equator onto the imaginary celestial sphere that surrounds Earth, Mintaka is the first of the asterismal belt's trio to rise and to set.

Mintaka's advantageous placement qualifies it for equatorial star status. Because the celestial equator's intersections with the horizon line always occur at the due east and due west points of the compass, an equatorial star's rising and setting indicate due east and due west, respectively.

Mintaka ranks seventh among Orion the Hunter constellation's seven brightest main stars. It is the dimmest of the trio in Orion's Belt asterism.

Mintaka's Bayer designation, devised by stellar designator and celestial cartographer Johann Bayer (1572-March 7, 1625), is Delta Orionis (δ Orionis; Delta Ori; δ Ori). Its popular name derives from an Arabic word, al mintaqa, referencing the belt worn by al-Jauza (“the Central One”), a mysterious female figure discerned by Arab astronomy in western astronomy's constellation of Orion.

Blue-white, slightly variable supergiant Alnilam occupies the middle of Orion's Belt asterism. It is the brightest of the belt trio and ranks fourth among Orion the Hunter's seven brightest stars. Unlike its two belt neighbors, Alnilam is only a single star.

Alnilam's Bayer designation is Epsilon Orionis (ε Orionis; Epsilon Ori; ε Ori). The popular name for the belt's central star derives from an Arabic word translated as “string of pearls.”

Alnitak marks the easternmost point of the Orion's Belt asterism. Alnitak is the second brightest of the belt trio and holds fifth place in the ranking of the equatorial constellation's seven brightest stars.

As with Mintaka, Alnitak is actually a multiple star system. The primary star is a hot blue-white supergiant that is the brightest of the rare class of O-type main-sequence stars. Its close binary companion is a blue dwarf. A third star, shining at fourth magnitude, orbits the primary star every 1,500 years. A fourth star, shining at ninth magnitude, remains unconfirmed as gravitationally bound to the Alnitak system.

Alnitak's Bayer designation is Zeta Orionis (ζ Orionis; Zeta Ori; ζ Ori). Its popular name derives from an Arabic word translated as “belt,” referencing, as with Mintaka, the mysterious female figure discerned by Arab astronomy in the stars of Orion, in combination with some stars from western astronomy's Gemini the Twins constellation.

The takeaway for the Orion's Belt asterism is its distinctive dominance of the Northern Hemisphere's winter skies from December to February as an aesthetically pleasing, easily findable and immediately recognizable trio of blue whiteness.

Precisely spaced diagonal alignment of three bright stars, Mintaka, Alnilam and Alnitak, define Orion's Belt asterism; credit Davide De Martin & the ESA/ESO/NASA Photoshop FITS Liberator: CC BY 4.0 International, via ESA Hubble

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Orion's Belt asterism with reddish arc of Barnard's Loop (center); “Orion Head to Toe,” October 2010: Rogelio Bernal Andreo, CC BY SA 3.0 Unported, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Orion_Head_to_Toe.jpg;
Astronomy Picture of the Day 2010 October 23, via NASA APOD @ https://apod.nasa.gov/apod/ap101023.html

Precisely spaced diagonal alignment of three bright stars, Mintaka, Alnilam and Alnitak, define Orion's Belt asterism; credit Davide De Martin & the ESA/ESO/NASA Photoshop FITS Liberator: CC BY 4.0 International, via ESA Hubble @ https://www.spacetelescope.org/projects/fits_liberator/fitsimages/davidedemartin_12/

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