The stellar disk of the Milky Way galaxy is approximately 100,000 light-years (9.5×10¹⁷ km) in diameter, and is believed to be, on average, about 1,000 ly (9.5×10¹⁵ km) thick.^[8] It is estimated to contain at least 200 billion stars^[9] and possibly up to 400 billion stars,^[10] the exact figure depending on the number of very low-mass stars, which is highly uncertain. Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12,000 ly (1.1×10¹⁷ km)—twice the previously accepted value.^[11] As a guide to the relative physical scale of the Milky Way, if it were reduced to 130 km in diameter, the Solar System would be a mere 2 mm in width.

The Galactic Halo extends outward, but is limited in size by the orbits of the two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~180,000 ly (1.7×10¹⁸ km).^[12] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky way.