The milky way galaxy

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In 2008, it was announced that infrared images from NASA's Spitzer Space Telescope have shown that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to have four major arms. Image credit: NASA/JPL-Caltech

Vital statistics diameter 100,000 light-years thickness of disk 2,300 to 2,600 light-years thickness of bulge 16,000 light-years mass 1–2 × 1012 Msun proportion of gas and dust 5–10% of stellar mass mean density 0.1 Msun per cubic parsec total luminosity ~ 1044 erg/s magnetic field 3–5 × 10-6 gauss age (oldest star) 13.2 billion years

An older map of the Milky Way Galaxy showing four major arms, with the Sun located in a region called the Orion Arm or Local Spur. Image credit: NASA

The Milky Way Galaxy, our home galaxy, is a large barred spiral galaxy containing some 200 to 400 billion stars (possibly many more if brown dwarfs are included). Its barred structure, and the fact that its spiral arms are quite loosely wrapped, suggests that it is of type Sbc or SBbc in the Hubble galaxy classification scheme. Its main components are a disk, a central bulge, and a halo.

Recent estimates put the total mass of the Milky Way Galaxy in the range one to two trillion solar masses, including a large but uncertain amount of dark matter in the dark halo. The Milky Way is the second largest member of the Local Group, after the Andromeda Galaxy.