The Near 3 kpc Arm (formerly also called Expanding 3 kpc Arm or simply 3 kpc Arm) was discovered in the 1950s by astronomer van Woerden and collaborators through 21-centimeter radio measurements of HI (atomic hydrogen).[1] [2] It was found to be expanding away from the center of the Milky Way at more than 50 km/s. This spiral arm contains about 10 million solar masses of gas, mostly hydrogen atoms and molecules.[3] It is named for its distance (more accurately, 3.3 kpc) from the Galactic Center.[3] It is about 5.2 kpc from the Sun, and is located in the fourth galactic quadrant.[2]
The last attempt to search for star forming regions in the Near 3 kpc Arm (in 1980) showed little star formation occurring in the numerous molecular clouds of this arm.[4] [5]
Along with the Far 3 kpc Arm discovered in 2008, these inner arms establish the Milky Way galaxy's simple symmetry.[4]