How big is the Milky Way Galaxy?

One light year is about 5.9 x 10¹² miles, so the diameter of the Milky Way galaxy is about 5.9 x 10¹⁷ miles ( 9.5 x 10¹⁷ km) in diameter.

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The Milky Way is a large galaxy comprising an estimated 200 billion stars (some estimates range as high as 400 billion) arrayed in the form of a disk, with a central elliptical bulge (some 12,000 light years in diameter) of closely packed stars lying in the direction of Sagittarius. It is surrounded by a flat disk marked by six spiral arms, four major and two minor, which wind out from the nucleus like a giant pinwheel. Because of these arms, the Milky Way was classified as a spiral galaxy. However, increasing evidence indicates that the Milky Way probably has a bar or bar like structure of new, bright stars in its central region. This would modify its classification to a barred spiral or an intermediate type between barred and "normal" spiral.

Our sun is situated in one of the smaller arms, called the Local or Orion Arm, that connect the more substantial next inner arm and the next outer arm. The sun lies roughly two thirds of the way from the center of the disk, which is some 28,000 light years distant, and in the galactic plane. When we look in the plane of the disk we see the combined light of its stars as the Milky Way.

See link for more information including pictorial.

First answer by Emdrgreg. Last edit by Cosmospup. Contributor trust: 285 [recommend contributor]. Question popularity: 45 [recommend question].