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Figure 1.The observed structure of the Milky Way's spiral arms [1]. The Orion Arm, also known as the Orion–Cygnus Arm, is a minor spiral arm within the Milky Way Galaxy spanning 3,500 light-years (1,100 parsecs) in width and extending roughly 20,000 light-years (6,100 parsecs) in length. [2]
The spiral arms were first discovered in the Whirlpool Galaxy (M51), in which Lord Rosse identified the spiral structure in 1850. [42] In 1896, the problem of twisting was formulated. If spiral arms were material entities, due to differential rotation, they would twist very rapidly to the point where they would be impossible to observe.
The Milky Way is a barred spiral galaxy with a D 25 isophotal diameter estimated at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years), [10] but only about 1,000 light-years thick at the spiral arms (more at the bulge).
Observed structure of the Milky Way's spiral arms. The Carina–Sagittarius Arm (also known as the Sagittarius Arm or Sagittarius–Carina Arm, labeled -I [clarification needed]) is generally thought to be a minor spiral arm of the Milky Way galaxy. [1] Each spiral arm is a long, diffuse curving streamer of stars that radiates from the Galactic ...
The Scutum–Centaurus Arm, also known as Scutum-Crux arm, is a long, diffuse curving streamer of stars, gas and dust that spirals outward from the proximate end of the Milky Way's central bar. The Milky Way has been posited since the 1950s to have four spiral arms; numerous studies contest or nuance this number. [1]
The Milky Way is a barred spiral galaxy with two major arms and a number of minor arms or spurs. [1] The Perseus Spiral Arm, with a radius of approximately 10.7 kiloparsecs, is located between the minor Cygnus and Carina–Sagittarius Arms. [1] It is named after the Perseus constellation in the direction of which it is seen from Earth.
The Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s. [38] [39] Their suspicions were confirmed by Spitzer Space Telescope observations in 2005, [40] which showed that the Milky Way's central bar is larger than what was previously suspected.
The density wave theory also explains a number of other observations that have been made about spiral galaxies. For example, "the ordering of H I clouds and dust bands on the inner edges of spiral arms, the existence of young, massive stars and H II regions throughout the arms, and an abundance of old, red stars in the remainder of the disk".