The Carina–Sagittarius Arm (also known as Sagittarius Arm or Sagittarius–Carina Arm, labeled -I) is generally thought to be a minor spiral arm of the Milky Way galaxy. Each spiral arm is a long, diffuse curving streamer of stars that radiates out from the galactic center. These gigantic structures are often composed of billions of stars and thousands of gas clouds. The Carina–Sagittarius Arm is one of the most pronounced arms in our galaxy as a large number of HII regions, young stars and giant molecular clouds are concentrated in it.
The Milky Way is a barred spiral galaxy, consisting of a central crossbar from which two major and several minor spiral arms radiate outwards. The Carina–Sagittarius Arm lies between two major spiral arms—Scutum–Centaurus Arm inside and the Perseus Arm outside. It is named for its proximity to the Sagittarius and Carina constellations as seen in the night sky from Earth, in the direction of the galactic center.
The Sagittarius Arm is divided into two parts. Curving outward from the galaxy's central bar is the Sagittarius Arm (Sagittarius bar), which further outward becomes the Carina Arm.
In 2008, infrared observations with the Spitzer Space Telescope showed that the Carina–Sagittarius Arm has a relative paucity of young stars, in contrast with the Sagittarius and Norma spiral arms. This suggests that the Sagittarius Arm is a minor arm, along with the Norma Arm (Outer Arm). These two appear to be mostly concentrations of gas, sparsely sprinkled with pockets of newly formed stars.
A number of Messier objects and other objects visible through an amateur's telescope or binoculars are found in the Sagittarius Arm:
- M8, the Lagoon Nebula
- M11, the Wild Duck Cluster
- M16, the Eagle Nebula
- M17, the Omega Nebula
- Open Cluster M18
- M20, the Trifid Nebula
- Open Cluster M21
- M24, the Sagittarius Star Cloud
- Open Cluster M26
- Globular Cluster M55
- NGC 3372, the Carina Nebula
- Churchwell, Ed; Babler, Brian L.; Meade, Marlin A. (2009). "The Spitzer/GLIMPSE Surveys: A New View of the Milky Way" (pdf). Publications of the Astronomical Society of the Pacific. 121: 213–230. Bibcode:2009PASP..121..213C. doi:10.1086/597811.
- Steiman-Cameron, T. Y.; Wolfire, M.; Hollenbach, D. (2010). "Cobeand the Galactic Interstellar Medium: Geometry of the Spiral Arms from Fir Cooling Lines". The Astrophysical Journal. 722 (2): 1460. Bibcode:2010ApJ...722.1460S. doi:10.1088/0004-637X/722/2/1460.
- Shiga, David (2008-06-03). "Two of the Milky Way's spiral arms may be 'demoted'". NewScientist. Retrieved 2009-09-08.