<P> Two spiral arms, the Scutum--Centaurus arm and the Carina--Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way . If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point . Two surveys of near - infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum--Centaurus arm but not in the Carina--Sagittarius arm: the Scutum - Centaurus Arm contains approximately 30% more red giants than would be expected in the absence of a spiral arm . This observation suggests that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum--Centaurus arm . The rest of the arms contain excess gas but not excess old stars . In December 2013, astronomers found that the distribution of young stars and star - forming regions matches the four - arm spiral description of the Milky Way . Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars . The explanation for this apparent discrepancy is unclear . </P> <P> 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). It was found to be expanding away from the central bulge at more than 50 km / s . It is located in the fourth galactic quadrant at a distance of about 5.2 kpc from the Sun and 3.3 kpc from the Galactic Center . The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame (Harvard - Smithsonian CfA). It is located in the first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from the Galactic Center . </P> <P> A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy . </P> <P> It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound . In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer pseudoring, and the two patterns would be connected by the Cygnus arm . </P>

Where did the term milky way come from