Artist's impression of the Milky Way (updated - annotated)

The Near 3 kpc Arm (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 A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen constit ...

).A New Spiral Arm of the Galaxy: The Far 3-Kpc Arm
T. M. Dame, P. Thaddeus, ''ApJ Letters'', 2008 It was found to be expanding away from the center of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

at more than 50

km/s The metre per second is the unit of both speed (a scalar quantity) and velocity (a vector quantity, which has direction and magnitude) in the International System of Units (SI), equal to the speed of a body covering a distance of one metre in ...

. This

spiral arm Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es of gas, mostly hydrogen atoms and molecules.Milky Way's Inner Beauty Revealed
Press Release Harvard-Smithsonian Center for Astrophysics, 2008 It is named for its distance (more accurately, 3.3 kpc) from the

Galactic Center The Galactic Center or Galactic Centre is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact rad ...

. It is about 5.2 kpc from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, and is located in the fourth

galactic quadrant A galactic quadrant, or quadrant of the Galaxy, is one of four circular sectors in the division of the Milky Way Galaxy. Quadrants in the galactic coordinate system In actual astronomical practice, the delineation of the galactic quadrants ...

. The last attempt to search for star forming regions in the Near 3 kpc Arm (in 1980) showed little

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

occurring in the numerous molecular clouds of this arm.The Far 3kpc Arm
NASA/Astronomy Picture of the Day, 2008Star formation and ionization in the 3 kiloparsec arm
''Astrophysical Journal'', Part 1, vol. 241, Oct. 1, 1980, p. 200–207 Along with the Far 3 kpc Arm discovered in 2008, these inner arms establish the Milky Way galaxy's simple symmetry.

References

{{Portal bar, Stars, Spaceflight, Outer space, Solar System Milky Way arms Galactic astronomy Spiral galaxies

See also

References