Arms on a Spiral Galaxy

    A spiral galaxy is a very common space body, and is the easiest to recognize; having angular momentum and a central bulge with a disk surrounding it. The arms spread out logarithmically, and orbit around the bulge in a roughly circular or oval path, called an ellipse. Our galaxy is long supposed to be spiral, but recent research shows a possibility of it being a barred spiral. When two galaxies collide, one being a spiral (the only kind that moves) and the other of any type, an elliptical galaxy, is formed.

Orion Arm: Named as such due to the close proximity to the Orion constellation, and is also known as "local arm." The two neighboring arms are the inner Sagittarius Arm, and the outer Perseus Arm. It is unknown whether or not the Local Arm is a branch of the Perseus or Sagittarius arm, or a whole arm by itself. Our solar system is located on the inner side of this celestial appendage.

Sagittarius Arm: Also known as Sagittarius-Carina, this arm is one of the two main spiral trails is our galaxy. Named after the near-by Sagittarius constellation, this area of space contains the messier objects M8, M11, M16, M17, M18, M20, M21, M24, M26, M55. The radius of this arm is unknown.

Perseus Arm: Formally named after the Perseus constellation, this arm has a radius of 10.7 ± 1.0 kpc, and is located between the Cygnus Arm and the Orion Arm. There are six Messier objects found in this area: M1, M36, M37, M38, M52, and M103.

Centaurus Arm: This arm is also known as the Scutum-Crux Arm, and is located between the Sagittarius Arm and the Norma Arm. There is not much known about this area of space.

Cygnus Arm: Also known as the outer arm, this appendage is named after the Cygnus constellation, and has a radius of 15.5 ± 2.8 kpc.  Not much is known about it, due perhaps to it being the farthest away from the bulge at the center of the Milky Way. This celestial trail is one of the two major arms; the Sagittarius arm being the other.

Norma Arm: Also known as the 3-kpc Arm, the Norma arm is a little known inner spiral appendage at the center of the Milky Way. The arm is from the joining of the Perseus Arm and the Cygnus Arm, and, as you might have guessed, it is named after the proximity to the Norma constellation.

Below is a diagram of the spiral arms in the Milky Way.

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